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Wednesday, January 12, 2011

写作圣经On Writing Well 精华摘选

纽约时报评价On Writing Well 是一本指导英文写作的圣经,任何想让要自己文章简洁的人都应该没事拿出来读一读,膜拜膜拜。

Library Journal 说,在这本书里,你可以看到A love and respect for the language is evident on every page.”

读 这本书的时候,我随时都把William Zinsser这个小老头恪守的标准和我写好的personal statement交叉对比,每每不禁面红耳赤,内牛满面。咱申请的是新闻专业,未来这两年好歹也得写出几箩筐的英文。但是按照现在的水准,我每一行每一 段都可以被轰至渣。不过庆幸我手头有这本书,还一口气把它看完了。作者对Cliché和Clutter由衷且无时无刻地表达鄙视之情,以至于我现在看到这 俩词就虎躯一震。

这本书适合谁看?如果你想提高自己英文写作的水准,特别是挖掘自己的风格; 如果你即将去米国读研究生,未来两年经常要面对一茬一茬的各种paper; 如果你以后是一个英文写作从业者,经常要写news release & report. 这本书就是你的圣经。作者对写作标准的严苛和尊敬,作者的智慧和幽默感让我随时流着哈喇子拿出各种颜色的笔圈点勾画。这种经典看一遍是绝对不够的,我认为 有些句子是完全可以打印出来,贴在自己的写作台上,时时鞭策警示自己。这样才能多写出一些人类看得懂的句子,少留下一些Cliché和Clutter,让 自己看到脸红。

以下的笔记摘选自本书第一章Principles和第二章Methods. 这两章的篇幅只占全书的三分之一,但信息量非常之大,看完你就知道了。

摘选的内容包括:

  • Simplicity & Clutter 怎样把文章写的简洁
  • Style 风格只有把“人”写出来,才会有自己的风格
  • The audience 你的文章为谁而写
  • Words 措辞怎样的用词会把你的文章搞坏,什么又是好的措辞
  • Unity 整体性如何写出牛逼的开头和结尾,怎么寻找素材
  • Bits & Pieces 动词,副词,形容词,缩写,that/which等等用法


Φ

1. Simplicity & Clutter

怎么样把文章写简洁?

Zinsser痛恨兜圈子,任何模棱两可的措辞,表意不明的句子在他看来都是灾难。他对简洁如此执着,以至于Zinsser这个名字成了文风简洁的代名词。美国有些老师会让学生Zinsser一下他们的文章,Zinsser成了一个清除文中clutter的动词。

什么是所谓的clutter呢? 放到中文语境里,遍地都是,比如说“有关部门”,比如说“某某领导高度重视.” 比如说胡版2011年新年贺词的全部。

美版:
Clutter is the disease of American writing. We are a society strangling in unnecessary words, circular constructions, pompous frills and meaningless jargon.

Fighting clutter is like fighting weeds—the writer is always slightly behind. New varieties sprout overnight, and by noon they are part of American speech. Consider what President Nixon's aide John Dean accomplished in just one day of testimony on television during the Watergate hearings. The next day everyone in America was saying "at this point in time" instead of "now."

Take the adjective “personal,” as in “a personal friend of mine,” “his personal feeling.” It’s typical of hundreds of words that can be eliminated. The personal friend has come into the language to distinguish him or her from the business friend, thereby debasing both language and friendship. Someone’s feeling is that person’s personal feeling­—that’s what “his” means. Friends are friends, the rest is clutter.

Clutter is the ponderous euphemism that turns a slum into a depressed socioeconomic area, garbage collectors into waste disposal personnel and the town dump into the volume reduction unit.

Clutter is the official language used by corporations to hide their mistakes. When General Motors had a plant shutdown, that was a “volume-related production-schedule adjustment.” When an Air Force missile crashed, it “impacted with the ground prematurely.” Companies that go belly-up have “a negative cash-flow position.”

“Experiencing” is one of the worst clutters. Instead of “it is raining”, there is no way to say “At the present time we are experiencing precipitation.” Even your dentist will ask if you are experiencing any pain. If he had his own kid in the chair he would say,” Does it hurt?”

The point of raising these examples is to serve notice that clutter is the enemy. Beware, then, of the long word that's no better than the short word: "assistance"(help), "numerous" (many), "facilitate" (ease), "individual"(man or woman), "remainder" (rest), "initial" (first), "implement"(do), "sufficient" (enough), "attempt" (try), "referred to as"(called) and hundreds more. Beware of all the slippery new fad words: paradigm and parameter, prioritize and potentialize. They are all weeds that will smother what you write.

How can the rest of us achieve such enviable freedom from clutter? The answer is to clear our heads of clutter. Clear thinking becomes clear writing; one can't exist without the other. It'simpossible for a muddy thinker to write good English. He may get away with it for a paragraph or two, but soon the reader will be lost, and there's no sin so grave, for the reader will not easily be lured back.

作者的一个tip,“括号剔除法”.经我的PS测试,发现非常好用

Is there any way to recognize clutter at a glance? Here's a device my students at Yale found helpful. I would put brackets around every component in a piece of writing that wasn't doing useful work. Often just one word got bracketed: the unnecessary preposition appended to a verb ("order up"), or the adverb that carries the same meaning as the verb ("smile happily"), or the adjective that states a known fact ("tall skyscraper"). Often my brackets surrounded the little qualifiers that weaken any sentence they inhabit ("a bit," "sort of), or phrases like "in a sense," which don't mean anything. Sometimes my brackets surrounded an entire sentence—the one that essentially repeats what the previous sentence said, or that says something readers don't need to know or can figure out for themselves. Most first drafts can be cut by 50 percent without losing any information or losing the authors voice.

My reason for bracketing the students' superfluous words, instead of crossing them out, was to avoid violating their sacred prose. I wanted to leave the sentence intact for them to analyze. I was saying, "I may be wrong, but I think this can be deleted and the meaning won't be affected. But you decide. Read the sentence without the bracketed material and see if it works." In the early weeks of the term I handed back papers that were festooned with brackets. Entire paragraphs were bracketed. But soon the students learned to put mental brackets around their own clutter, and by the end of the term their papers were almost clean. Today many of those students are professional writers, and they tell me, "I still see your brackets—they're following me through life."

You can develop the same eye. Look for the clutter in your writing and prune it ruthlessly. Be grateful for everything you can throw away. Reexamine each sentence you put on paper. Is every word doing new work? Can any thought be expressed with more economy? Is anything pompous or pretentious or faddish? Are you hanging on to something useless just because you think it's beautiful?

Simplify, simplify.

2. Style

以下对写PS挠头的同学颇为有用

Few people realize how badly they write. Nobody has shown them how much excess or murkiness has crept into their style and how it obstructs what they are trying to say. If you give me an eight-page article and I tell you to cut it to four pages, you'll howl and say it can't be done. Then you'll go home and do it, and it will be much better. After that comes the hard part: cutting it to three.

The point is that you have to strip your writing down before you can build it back up. You must know what the essential tools are and what job they were designed to do. Extending the metaphor of carpentry, it's first necessary to be able to saw wood neatly and to drive nails. Later you can bevel the edges or add elegant finials, if that's your taste. But you can never forget that you are practicing a craft that's based on certain principles. If the nails are weak, your house will collapse. If your verbs are weak and your syntax is rickety, your sentences will fall apart.

为什么必须要有自己的风格

I'll admit that certain nonfiction writers, like Tom Wolfe and Norman Mailer, have built some remarkable houses. But these are writers who spent years learning their craft, and when at last they raised their fanciful turrets and hanging gardens, to the surprise of all of us who never dreamed of such ornamentation, they knew what they were doing. Nobody becomes Tom Wolfe overnight, not even Tom Wolfe.

First, then, learn to hammer the nails, and if what you build is sturdy and serviceable, take satisfaction in its plain strength. But you will be impatient to find a "style"—to embellish the plain words so that readers will recognize you as someone special. You will reach for gaudy similes and tinseled adjectives, as if "style" were something you could buy at the style store and drape onto your words in bright decorator colors. (Decorator colors are the colors that decorators come in.) There is no style store; style is organic to the person doing the writing, as much a part of him as his hair, or, if he is bald, his lack of it. Trying to add style is like adding a toupee. At first glance the formerly bald man looks young and even handsome. But at second glance—and with a toupee there's always a second glance—he doesn't look quite right. The problem is not that he doesn't look well groomed; he does, and we can only admire the wigmaker's skill. The point is that he doesn't look like himself. This is the problem of writers who set out deliberately to garnish their prose. You lose whatever it is that makes you unique. The reader will notice if you are putting on airs. Readers want the person who is talking to them to sound genuine. Therefore a fundamental rule is: be yourself.

怎样写文章才会有自己的style----把自己放进去,找到那个“人”

Assume that you are the writer sitting down to write. You think your article must be of a certain length or it won't seem important. You think how august it will look in print. You think of all the people who will read it. You think that it must have the solid weight of authority. You think that its style must dazzle. No wonder you tighten; you are so busy thinking of your awesome responsibility to the finished article that you can't even start. Yet you vow to be worthy of the task, and, casting about for grand phrases that wouldn't occur to you if you weren't trying so hard to make an impression, you plunge in. Paragraph 1 is a disaster—a tissue of generalities that seem to have come out of a machine. No person could have written them. Paragraph 2 isn't much better. But Paragraph 3 begins to have a somewhat human quality, and by Paragraph 4 you begin to sound like yourself. You've started to relax. It s amazing how often an editor can throw away the first three or four paragraphs of an article, or even the first few pages, and start with the paragraph where the writer begins to sound like himself or herself. Not only are those first paragraphs impersonal and ornate; they don't say anything—they are a self-conscious attempt at a fancy introduction. What I'm always looking for as an editor is a sentence that says something like "I'll never forget the day when I . . . "
I think, "Aha! A person!"

3. The audience

写作悦己

"Who am I writing for?

It s a fundamental question, and it has a fundamental answer: You are writing for yourself. Don't try to visualize the great mass audience. There is no such audience—every reader is a different person. Don't try to guess what sort of thing editors want to publish or what you think the country is in a mood to read. Editors and readers don't know what they want to read until they read it. Besides, they're always looking for something new.

Don't worry about whether the reader will "get it" if you indulge a sudden impulse for humor. If it amuses you in the act of writing, put it in. (It can always be taken out, but only you can put it in.) You are writing primarily to please yourself, and if you go about it with enjoyment you will also entertain the readers who are worth writing for. If you lose the dullards back in the dust, you don't want them anyway.

如果你平时说话不是文邹邹的,那写东西的时候也不要尽是之乎者也。

Whatever your age, be yourself when you write. Many old men still write with the zest they had in their twenties or thirties; obviously their ideas are still young. Other old writers ramble and repeat themselves; their style is the tip-off that they have turned into garrulous bores. Many college students write as if they were desiccated alumni 30 years out. Never say anything in writing that you wouldn't comfortably say in conversation. If you're not a person who says "indeed" or "moreover," or who calls someone an individual ("he's a fine individual"), please don't write it.


4. Words 措辞

日常写作避免新闻笔调(journalese)

What is "journalese"? It's a quilt of instant words patched together out of other parts of speech. Adjectives are used as nouns ("greats," "notables"). Nouns are used as verbs ("to host"), or they are chopped off to form verbs ("enthuse," "emote"), or they are padded to form verbs ("beef up," "put teeth into"). This is a world where eminent people are "famed" and their associates are "staffers," where the future is always "upcoming" and someone is forever "firing off" a note. Nobody in America has sent a note or a memo or a telegram in years. Famed diplomat Henry Kissinger, who hosted foreign notables to beef up the morale of top State Department staffers, sat down and fired off a lot of notes. Notes that are fired off are always fired in anger and from a sitting position.(囧) What the weapon is I've never found out.

案例:作者眼里失败界的翘楚

Here's an article from a famed newsmagazine that is hard to match for fatigue:

Last February, Plainclothes Patrolman Frank Serpico knocked at the door of a suspected Brooklyn heroin pusher. When the door opened a crack, Serpico shouldered his way in only to be met by a .22-cal. pistol slug crashing into his face. Somehow he survived, although there are still buzzing fragments in his head, causing dizziness and permanent deafness in his left ear. Almost as painful is the suspicion that he may well have been set up for the shooting by other policemen. For Serpico, 35, has been waging a lonely, four-year war against the routine and endemic corruption that he and others claim is rife in the New York City police department. His efforts are now sending shock waves through the ranks of New York's finest.. . . Though the impact of the commissions upcoming report has yet to be felt, Serpico has little hope that. . .

为什么这篇文章的用词很纱布?

The upcoming report has yet to be felt because it's still upcoming, and as for the permanent deafness, it's a little early to tell. And what makes those buzzing fragments buzz? By now only Serpico's head should be buzzing. But apart from these lazinesses of logic, what makes the story so tired is the failure of the writer to reach for anything but the nearest cliché. "Shouldered his way," "only to be met," "crashing into his face," "waging a lonely war," "corruption that is rife," "sending shock waves," "New York's finest"—these dreary phrases constitute writing at its most banal. We know just what to expect. No surprise awaits us in the form of an unusual word, an oblique look. We are in the hands of a hack, and we know it right away. We stop reading.

要写好文章,首先学会模仿。但能印在报纸杂志上的不一定就牛逼到哪去了.要选好对象。

Make a habit of reading what is being written today and what has been written by earlier masters. Writing is learned by imitation. If anyone asked me how I learned to write, I'd say I learned by reading the men and women who were doing the kind of writing I wanted to do and trying to figure out how they did it. But cultivate the best models. Don't assume that because an article is in a newspaper or a magazine it must be good. Sloppy editing is common in newspapers, often for lack of time, and writers who use clichés often work for editors who have seen so many clichés that they no longer even recognize them.

用字典, 考过GRE的同学表示对以下引用的单词毫无压力。

Also get in the habit of using dictionaries. My favorite for handy use is Webster's New World Dictionary, Second College Edition, although, like all word freaks, I own bigger dictionaries that will reward me when I'm on some more specialized search. If you have any doubt of what a word means, look it up. Learn its etymology and notice what curious branches its original root has put forth. See if it has any meanings you didn't know it had. Master the small gradations between words that seem to be synonyms. What's the difference between "cajole," "wheedle," "blandish" and "coax"? Get yourself a dictionary of synonyms.

Thesaurus,这玩意儿是GRE Verbal的大杀器

And don't scorn that bulging grab bag Roget's Thesaurus. It's easy to regard the book as hilarious. Look up "villain," for instance, and you'll be awash in such rascality as only a lexicographer could conjure back from centuries of iniquity, obliquity, depravity, knavery, profligacy, frailty, flagrancy, infamy, immorality, corruption, wickedness, wrongdoing, backsliding and sin. You'll find ruffians and riffraff, miscreants and malefactors, reprobates and rapscallions, hooligans and hoodlums, scamps and scapegraces, scoundrels and scalawags, Jezebels and jades. You'll find adjectives to fit them all (foul and fiendish, devilish and diabolical), and adverbs and verbs to describe how the wrongdoers do their wrong, and cross-references leading to still other thickets of venality and vice. Still, there's no better friend to have around to nudge the memory than Roget. It saves you the time of rummaging in your brain—that network of overloaded grooves—to find the word that's right on the tip of your tongue, where it doesn't do you any good. The Thesaurus is to the writer what a rhyming dictionary is to the songwriter—a reminder of all the choices—and you should use it with gratitude. If, having found the scalawag and the scapegrace, you want to know how they differ, then go to the dictionary.

一定要看the Elements of Style这本神作

E. B. White makes the case cogently in The Elements of Style, a book every writer should read once a year, when he suggests trying to rearrange any phrase that has survived for a century or two, such as Thomas Paine s "These are the times that try men's souls":

Times like these try men's souls.
How trying it is to live in these times!
These are trying times for men's souls.
Soulwise, these are trying times.

Paine s phrase is like poetry and the other four are like oatmeal— which is the divine mystery of the creative process. Good writers of prose must be part poet, always listening to what they write. E. B. White is one of my favorite stylists because I'm conscious of being with a man who cares about the cadences and sonorities of the language. I relish (in my ear) the pattern his words make as they fall into a sentence. I try to surmise how in rewriting the sentence he reassembled it to end with a phrase that will momentarily linger, or how he chose one word over another because he was after a certain emotional weight. It's the difference between, say, "serene" and "tranquil"—one so soft, the other strangely disturbing because of the unusual n and q.

好文章悦耳,因为有韵律感。

Such considerations of sound and rhythm should be woven through everything you write. If all your sentences move at the same plodding gait, which even you recognize as deadly but don't know how to cure, read them aloud. (I write entirely by ear and read everything aloud before letting it go out into the world.) You'll begin to hear where the trouble lies. See if you can gain variety by reversing the order of a sentence, or by substituting a word that has freshness or oddity, or by altering the length of your sentences so they don't all sound as if they came out of the same mold. An occasional short sentence can carry a tremendous punch. It stays in the reader's ear.

Remember that words are the only tools you've got. Learn to use them with originality and care. And also remember: somebody out there is listening.

5. Unity

一篇文章什么都写了就等于什么都没写。好的文章一定要有自己非常清晰的point.一个牛逼哄哄的point足够亮瞎双眼.

Nobody can write a book or an article "about" something. Tolstoy couldn't write a book about war and peace, or Melville a book about whaling. They made certain reductive decisions about time and place and about individual characters in that time and place— one man pursuing one whale. Every writing project must be reduced before you start to write.

Therefore think small. Decide what corner of your subject you're going to bite off, and be content to cover it well and stop. Often you'll find that along the way you've managed to say almost everything you wanted to say about the entire subject. This is also a matter of energy and morale. An unwieldy writing task is a drain on your enthusiasm. Enthusiasm is the force that keeps you going and keeps the reader in your grip. When your zest begins to ebb, the reader is the first person to know it.

As for what point you want to make, every successful piece of nonfiction should leave the reader with one provocative thought that he or she didn't have before. Not two thoughts, or five—just one. So decide what single point you want to leave in the reader s mind. It will not only give you a better idea of what route you should follow and what destination you hope to reach; it will affect your decision about tone and attitude. Some points are best made by earnestness, some by dry understatement, some by humor.

The Lead and the Ending 开头和结尾

什么是好的开头

The most important sentence in any article is the first one. If it doesn't induce the reader to proceed to the second sentence, your article is dead. And if the second sentence doesn't induce him to continue to the third sentence, it's equally dead. Of such a progression of sentences, each tugging the reader forward until he is hooked, a writer constructs that fateful unit, the "lead."

How long should the lead be? One or two paragraphs? Four or five? There's no pat answer. Some leads hook the reader with just a few well-baited sentences; others amble on for several pages, exerting a slow but steady pull. Every article poses a different problem, and the only valid test is: does it work? Your lead may not be the best of all possible leads, but if it does the job it's supposed to do, be thankful and proceed. Sometimes the length may depend on the audience you're writing for. Readers of a literary review expect its writers to start somewhat discursively, and they will stick with those writers for the pleasure of wondering where they will emerge as they move in leisurely circles toward the eventual point. But I urge you not to count on the reader to stick around. Readers want to know— very soon—what's in it for them.

Therefore your lead must capture the reader immediately and force him to keep reading. It must cajole him with freshness, or novelty, or paradox, or humor, or surprise, or with an unusual idea, or an interesting fact, or a question. Anything will do, as long as it nudges his curiosity and tugs at his sleeve.

Next the lead must do some real work. It must provide hard details that tell the reader why the piece was written and why he ought to read it. But don't dwell on the reason. Coax the reader a little more; keep him inquisitive.

Continue to build. Every paragraph should amplify the one that preceded it. Give more thought to adding solid detail and less to entertaining the reader. But take special care with the last sentence of each paragraph—it's the crucial springboard to the next paragraph. Try to give that sentence an extra twist of humor or surprise, like the periodic "snapper" in the routine of a standup comic. Make the reader smile and you've got him for at least one more paragraph.



什么是坏的开头

Speaking of everybody else's lead, there are many categories I'd be glad never to see again. One is the future archaeologist: "When some future archaeologist stumbles on the remains of our civilization, what will he make of the jukebox?" I'm tired of him already and he's not even here. I'm also tired of the visitor from Mars: "If a creature from Mars landed on our planet he would be amazed to see hordes of scantily clad earthlings lying on the sand barbecuing their skins." I'm tired of the cute event that just happened to happen "one day not long ago" or on a conveniently recent Saturday afternoon: "One day not long ago a small button-nosed boy was walking with his dog, Terry, in a field outside Paramus, N.J., when he saw something that looked strangely like a balloon rising out of the ground." And I'm very tired of the have-in-common lead: "What did Joseph Stalin, Douglas MacArthur, Ludwig Wittgenstein, Sherwood Anderson, Jorge Luis Borges and Akira Kurosawa have in common? They all loved Westerns." Let's retire the future archaeologist and the man from Mars and the button-nosed boy. Try to give your lead a freshness of perception or detail.

在生活中寻找素材

One moral is that you should always collect more material than you will use. Every article is strong in proportion to the surplus of details from which you can choose the few that will serve you best—if you don't go on gathering facts forever. At some point you must stop researching and start writing.

Another moral is to look for your material everywhere, not just by reading the obvious sources and interviewing the obvious people. Look at signs and at billboards and at all the junk written along the American roadside. Read the labels on our packages and the instructions on our toys, the claims on our medicines and the graffiti on our walls. Read the fillers, so rich in self-esteem, that come spilling out of your monthly statement from the electric company and the telephone company and the bank. Read menus and catalogues and second-class mail. Nose about in obscure crannies of the newspaper, like the Sunday real estate section—you can tell the temper of a society by what patio accessories it wants. Our daily landscape is thick with absurd messages and portents. Notice them. They not only have social significance; they are often just quirky enough to make a lead that's different from everybody else's.


结尾要有惊喜,这就像一根诱人口水的大骨头,首先它要有嚼头,然后它得告诉读者,这是最后一根了。


The positive reason for ending well is that a good last sentence—or last paragraph—is a joy in itself. It gives the reader a lift, and it lingers when the article is over. The perfect ending should take your readers slightly by surprise and yet seem exactly right. They didn't expect the article to end so soon, or so abruptly, or to say what it said. But they know it when they see it. Like a good lead, it works. It's like the curtain line in a theatrical comedy. We are in the middle of a scene (we think), when suddenly one of the actors says something funny, or outrageous, or epigrammatic, and the lights go out. We are startled to find the scene over, and then delighted by the aptness of how it ended. What delights us is the playwrights perfect control.

For the nonfiction writer, the simplest way of putting this into a rule is: when you're ready to stop, stop. If you have presented all the facts and made the point you want to make, look for the nearest exit.

Something I often do in my own work is to bring the story full circle—to strike at the end an echo of a note that was sounded at the beginning. It gratifies my sense of symmetry, and it also pleases the reader, completing with its resonance the journey we set out on together.

But what usually works best is a quotation. Go back through your notes to find some remark that has a sense of finality, or that's funny, or that adds an unexpected closing detail. Sometimes it will jump out at you during the interview—I've often thought, "That's my ending!"—or during the process of writing.

In the mid-1960s, when Woody Allen was just becoming established as Americas resident neurotic, doing nightclub monologues, I wrote the first long magazine piece that took note of his arrival. It ended like this:

"If people come away relating to me as a person," Allen says, "rather than just enjoying my jokes; if they come away wanting to hear me again, no matter what I might talk about, then I'm succeeding." Judging by the returns, he is. Woody Allen is Mr. Related-To, and he seems a good bet to hold the franchise for many years.

Yet he does have a problem all his own, unshared by, unrelated to, the rest of America. "I'm obsessed," he says, "by the fact that my mother genuinely resembles Groucho Marx."

There's a remark from so far out in left field that nobody could see it coming. The surprise it carries is tremendous. How could it not be a perfect ending? Surprise is one of the most refreshing elements in nonfiction writing. If something surprises you it will also surprise—and delight—the people you are writing for, especially as you conclude your story and send them on their way.

6. Bits & Pieces 细节才是王道

This is a chapter of scraps and morsels—small admonitions on many points that I have collected under one, as they say, umbrella.

VERBS.
不到万不得已,不要用被动动词

Use active verbs unless there is no comfortable way to get around using a passive verb. The difference between an active verb style and a passive-verb style—in clarity and vigor—is the
difference between life and death for a writer.

"Joe saw him" is strong. "He was seen by Joe" is weak. The first is short and precise; it leaves no doubt about who did what. The second is necessarily longer and it has an insipid quality: something was done by somebody to someone else. It’s also ambiguous. How often was he seen by Joe? Once? Every day? Once a week? A style that consists of passive constructions will sap the readers energy. Nobody ever quite knows what is being perpetrated by whom and on whom.

用词要精确

Verbs are the most important of all your tools. They push the sentence forward and give it momentum. Active verbs push hard; passive verbs tug fitfully. Active verbs also enable us to visualize an activity because they require a pronoun ("he"), or a noun ("the boy"), or a person ("Mrs. Scott") to put them in motion. Many verbs also carry in their imagery or in their sound a suggestion of what they mean: glitter, dazzle, twirl, beguile, scatter, swagger, poke, pamper, vex. Probably no other language has such a vast supply of verbs so bright with color. Don't choose one that is dull or merely serviceable. Make active verbs activate your sentences, and try to avoid the kind that need an appended preposition to complete their work. Don't set up a business that you can start or launch. Don't say that the president of the company stepped down. Did he resign? Did he retire? Did he get fired? Be precise. Use precise verbs.

If you want to see how active verbs give vitality to the written word, don't just go back to Hemingway or Thurber or Thoreau. I commend the King James Bible and William Shakespeare.

ADVERBS

大多数副词是可以被省略的

Most adverbs are unnecessary. You will clutter your sentence and annoy the reader if you choose a verb that has a specific meaning and then add an adverb that carries the same meaning. Don't tell us that the radio blared loudly; "blare" connotes loudness. Don't write that someone clenched his teeth tightly; there's no other way to clench teeth. Again and again in careless writing, strong verbs are weakened by redundant adverbs. So are adjectives and other parts of speech: "effortlessly easy," "slightly spartan," "totally flabbergasted." The beauty of "flabbergasted" is that it implies an astonishment that is total; I can't picture someone being partly flabbergasted. If an action is so easy as to be effortless, use "effortless." And what is "slightly spartan"? Perhaps a monk's cell with wall-to-wall carpeting. Don't use adverbs unless they do necessary work. Spare us the news that the winning athlete grinned widely.

ADJECTIVES

大多数形容词也是可以被省略的

Most adjectives are also unnecessary. Like adverbs, they are sprinkled into sentences by writers who don't stop to think that the concept is already in the noun. This kind of prose is littered with precipitous cliffs and lacy spiderwebs, or with adjectives denoting the color of an object whose color is well known: yellow daffodils and brownish dirt. If you want to make a value judgment about daffodils, choose an adjective like "garish." If you're in a part of the country where the dirt is red, feel free to mention the red dirt. Those adjectives would do a job that the noun alone wouldn't be doing.

Most writers sow adjectives almost unconsciously into the soil of their prose to make it more lush and pretty, and the sentences become longer and longer as they fill up with stately elms and frisky kittens and hard-bitten detectives and sleepy lagoons. This is adjective-by-habit—a habit you should get rid of. Not every oak has to be gnarled. The adjective that exists solely as decoration is a self-indulgence for the writer and a burden for the reader.

Again, the rule is simple: make your adjectives do work that needs to be done. "He looked at the gray sky and the black clouds and decided to sail back to the harbor." The darkness of the sky and the clouds is the reason for the decision. If it's important to tell the reader that a house was drab or a girl was beautiful, by all means use "drab" and "beautiful." They will have their proper power because you have learned to use adjectives sparsely.

LITTLE QUALIFIERS.

删掉那些让你的文章看上去不靠谱的口水词

Prune out the small words that qualify how you feel and how you think and what you saw: "a bit," "a little," "sort of," "kind of," "rather," "quite," "very," "too," "pretty much," "in a sense" and dozens more. They dilute your style and your persuasiveness.

Don't say you were a bit confused and sort of tired and a little depressed and somewhat annoyed. Be confused. Be tired. Be depressed. Be annoyed. Don't hedge your prose with little timidities. Good writing is lean and confident.

Don't say you weren't too happy because the hotel was pretty expensive. Say you weren't happy because the hotel was expensive. Don't tell us you were quite fortunate. How fortunate is that? Don't describe an event as rather spectacular or very awesome. Words like "spectacular" and "awesome" don't submit to measurement. "Very" is a useful word to achieve emphasis, but far more often it's clutter. There's no need to call someone very methodical. Either he is methodical or he isn't.

The larger point is one of authority. Every little qualifier whittles away some fraction of the reader's trust. Readers want a writer who believes in himself and in what he is saying. Don't diminish that belief. Don't be kind of bold. Be bold.


The Dash 破折号的用法

The dash is used in two ways. One is to amplify or justify in the second part of the sentence a thought you stated in the first part. "We decided to keep going—it was only 100 miles more and we could get there in time for dinner."By its very shape the dash pushes the sentence ahead and explains why they decided to keep going. The other use involves two dashes, which set apart a parenthetical thought within a longer sentence. "She told me to get in the car—she had been after me all summer to have a haircut—and we drove silently into town." An explanatory detail that might otherwise have required a separate sentence is dispatched along the way.

MOOD CHANGERS

谁说but不能用来开头了?

Many of us were taught that no sentence should begin with "but." If that's what you learned, unlearn it—there's no stronger word at the start. It announces total contrast with what has gone before, and the reader is thereby primed for the change. If you need relief from too many sentences beginning with "but," switch to "however." It is, however, a weaker word and needs careful placement. Don't start a sentence with "however"—it hangs there like a wet dishrag. And don't end with "however"—by that time it has lost its howeverness. Put it as early as you reasonably can, as I did three sentences ago. Its abruptness then becomes a virtue.

CONTRACTIONS.

I’ll 会比I will显得更有爱

Your style will be warmer and truer to your personality if you use contractions like "I'll" and "won't" and "can't" when they fit comfortably into what you're writing. "I'll be glad to see them if they don't get mad" is less stiff than "I will be glad to see them if they do not get mad." (Read that aloud and hear how stilted it sounds.) There's no rule against such informality—trust your ear and your instincts. I only suggest avoiding one form—"I'd," "he'd," "we'd," etc.—because "I'd" can mean both "I had" and "I would," and readers can get well into a sentence before learning which meaning it is. Often it's not the one they thought it was. Also, don't invent contractions, like "could've." They cheapen your style. Stick with the ones you can find in the dictionary.

THAT AND WHICH.

能用that 就不要用which

Anyone who tries to explain "that" and "which" in less than an hour is asking for trouble. Fowler, in his Modern English Usage, takes 25 columns of type. I'm going for two minutes, perhaps the world record. Here (I hope) is much of what you need to bear in mind:

Always use "that" unless it makes your meaning ambiguous. Notice that in carefully edited magazines, such as The NewYorker, "that" is by far the predominant usage. I mention this because it is still widely believed—a residue from school and college—that "which" is more correct, more acceptable, more literary. It's not. In most situations, "that" is what you would naturally say and therefore what you should write.

If your sentence needs a comma to achieve its precise meaning, it probably needs "which." "Which" serves a particular identifying function, different from "that." (A) "Take the shoes that are in the closet." This means: take the shoes that are in the closet, not the ones under the bed. (B) "Take the shoes, which are in the closet." Only one pair of shoes is under discussion; the "which" usage tells you where they are. Note that the comma is necessary in B, but not in A.

A high proportion of "which" usages narrowly describe, or identify, or locate, or explain, or otherwise qualify the phrase that preceded the comma:

The house, which has a red roof,
The store, which is called Bob s Hardware,
The Rhine, which is in Germany,
The monsoon, which is a seasonal wind,
The moon, which I saw from the porch,

That's all I'm going to say that I think you initially need to know to write good nonfiction, which is a form that requires exact marshaling of information.

CONCEPT NOUNS.

尽量少用抽象的概念词,因为里头没有“人”

Nouns that express a concept are commonly used in bad writing instead of verbs that tell what somebody did. Here are three typical dead sentences:

The common reaction is incredulous laughter.
Bemused cynicism isn't the only response to the old system.
The current campus hostility is a symptom of the change.

What is so eerie about these sentences is that they have no people in them. They also have no working verbs—only "is" or "isn't." The reader can't visualize anybody performing some activity; all the meaning lies in impersonal nouns that embody a vague concept: "reaction," "cynicism," "response," "hostility." Turn these cold sentences around. Get people doing things:

Most people just laugh with disbelief.
Some people respond to the old system by turning cynical; others say. . .
It’s easy to notice the change—you can see how angry all the students are.

My revised sentences aren't jumping with vigor, partly because the material I'm trying to knead into shape is shapeless dough. But at least they have real people and real verbs. Don't get caught holding a bag full of abstract nouns. You'll sink to the bottom of the lake and never be seen again.

CREEPING NOUNISM.

用人类的语言交流

This is a new American disease that strings two or three nouns together where one noun—or, better yet, one verb—will do. Nobody goes broke now; we have money problem areas. It no longer rains; we have precipitation activity or a thunderstorm probability situation. Please, let it rain.

Today as many as four or five concept nouns will attach themselves to each other, like a molecule chain. Here's a brilliant specimen I recently found: "Communication facilitation skills development intervention." Not a person in sight, or a working verb. I think it's a program to help students write better.

THE SUBCONSCIOUS MIND.

写不出东西来怎么办?也许睡一觉就好了

Your subconscious mind does more writing than you think. Often you'll spend a whole day trying to fight your way out of some verbal thicket in which you seem to be tangled beyond salvation. Frequently a solution will occur to you the next morning when you plunge back in. While you slept, your writers mind didn't. A writer is always working. Stay alert to the currents around you. Much of what you see and hear will come back, having percolated for days or months or even years through your subconscious mind, just when your conscious mind, laboring to write, needs it.

REWRITING.

好文章很少一遍促成,不要吝惜使用Delete键。

Rewriting is the essence of writing well: it's where the game is won or lost. That idea is hard to accept. We all have an emotional equity in our first draft; we can't believe that it wasn't born perfect. But the odds are close to 100 percent that it wasn't. Most writers don't initially say what they want to say, or say it as well as they could. The newly hatched sentence almost always has something wrong with it. It's not clear. It's not logical. It’s verbose. It's klunky. It's pretentious. It's boring. It's full of clutter. It's full of clichés. It lacks rhythm. It can be read in several different ways. It doesn't lead out of the previous sentence. It doesn't... The point is that clear writing is the result of a lot of tinkering.

Many people assume that professional writers don't need to rewrite; the words just fall into place. On the contrary, careful writers can't stop fiddling. I've never thought of rewriting as an unfair burden; I'm grateful for every chance to keep improving my work. Writing is like a good watch—it should run smoothly and have no extra parts. Students, I realize, don't share my love of rewriting. They think of it as punishment: extra homework or extra infield practice. Please—if you're such a student—think of it as a gift. You won't write well until you understand that writing is an evolving process, not a product. Nobody expects you to get it right the first time, or even the second time.

What do I mean by "rewriting"? I don't mean writing one draft and then writing a different second version, and then a third. Most rewriting consists of reshaping and tightening and refining the raw material you wrote on your first try. Much of it consists of making sure you've given the reader a narrative flow he can follow with no trouble from beginning to end. Keep putting yourself in the reader's place. Is there something he should have been told early in the sentence that you put near the end? Does he know when he starts sentence B that you've made a shift—of subject, tense, tone, emphasis—from sentence A?

When you read your writing aloud with these connecting links in mind you'll hear a dismaying number of places where you lost the reader, or confused the reader, or failed to tell him the one fact he needed to know, or told him the same thing twice: the inevitable loose ends of every early draft. What you must do is make an arrangement—one that holds together from start to finish and that moves with economy and warmth.

Learn to enjoy this tidying process. I don't like to write (I like to have written). But I love to rewrite. I especially like to cut: to press the DELETE key and see an unnecessary word or phrase or sentence vanish into the electricity. I like to replace a humdrum word with one that has more precision or color. I like to strengthen the transition between one sentence and another. I like to rephrase a drab sentence to give it a more pleasing rhythm or a more graceful musical line. With every small refinement I feel that I'm coming nearer to where I would like to arrive, and when I finally get there I know it was the rewriting, not the writing, that won the game.

Go WITH YOUR INTERESTS.

爱啥写啥,只要你有兴趣,再邪门的东西你都能写好。

There's no subject you don't have permission to write about. Students often avoid subjects close to their heart—skateboarding, cheerleading, rock music, cars—because they assume that their teachers will regard those topics as "stupid." No area of life is stupid to someone who takes it seriously. If you follow your affections you will write well and will engage your readers.

I've read elegant books on fishing and poker, billiards and rodeos, mountain climbing and giant sea turtles and many other subjects I didn't think I was interested in. Write about your hobbies: cooking, gardening, photography, knitting, antiques, jogging, sailing, scuba diving, tropical birds, tropical fish. Write about your work: teaching, nursing, running a business, running a store. Write about a field you enjoyed in college and always meant to get back to: history, biography, art, archeology. No subject is too specialized or too quirky if you make an honest connection with it when you write about it.


Φ

【太长了? 一个简化版的DOs & DON'Ts,来自The Spinozanator,Amazon.com】

1. Do - prune out every word that does not perform a necessary function. Strip each sentence to its cleanest components. A clear sentence is no accident.

2. Do - use the thesaurus liberally. Learn the small gradations between words that seem to be synonyms.

3. Do - try to improve the rhythm by reversing the order of a sentence, substituting a word that has freshness or oddity, and by varying the lengths of sentences.

4. Do - make your first sentence the best one - your lead must capture the reader.

5. Do - make each sentence lead into the next. Readers think linearly.

6. Do - Take special care with the last sentence in your paragraph - its the springboard to the next paragraph.

7. Do - make your paragraphs short. Readers think in segments.

8. Do - pay special attention to the last sentence. The perfect ending should take your reader slightly by surprise and yet seem exactly right.

9. Do - Read it aloud to see how it sounds and re-edit - then do it again. Clear writing is the result of lots of tinkering.


On the other hand:

1. Don't - use passive verbs unless there is no comfortable way to use an active verb.

2. Don't - use adverbs that convey the same meaning as your strong active verb - prune it out.

3. Don't - use adjectives when the concept is already in your carefully chosen noun - prune it out.

4. Don't - use small words that qualify how you feel: "a bit," "a little," "sort of," and dozens more. Good writing is lean and confident.

5. Don't - use concept nouns:
Instead of - "The common reaction is incredulous laughter."
Write - "Most people just laugh with disbelief."

6. Don't - use the exclamation point unless you must, do use the period more frequently, don't forget the versatile dash, and cut down on the use of semi-colons and colons. If you don't know how to punctuate, get a grammar book.

On Writing Well的第三、四章分门别类描述了怎么写Nonfiction as Literature, People, Place, yourself, Science and Technology, Business, Sports, Arts and Humor. 每一节都自成一体,无需引用。 总之,这本书基本可以应付日常英文写作的各个环节。是居家旅行模仿必备良品。为什么说模仿呢?

Zinsser写道, “Never hesitate to imitate another writer. Imitation is part of the creative process for anyone learning an art or a craft. Bach and Picasso didn’t spring full-blown as Bach and Picasso; they needed models. This is especially true and writing. Find the best writers in the fields that interest and read their work aloud. Get their voice and their taste into your ear—their attitude toward language. Don’t worry that imitating them you’ll lose your own voice and your own identity. Soon enough you will shed those skins and become who you are supposed to become.”

Monday, August 23, 2010

LED简介

简介
( c. p/ T c3 iLED(发光二极体)光源具有节省能源,环保与坚固耐用等优点,具有取代灯源的潜力,也因此LED相关研究在2000年获得诺贝尔奖的肯定.2000年诺贝尔奖物理项得主Herbert Kroemer就是因为异质结构半导体界面的研究而获奖,而他的研究正是LED发展的基础.2000年诺贝尔奖化学项得主之一Alan J. Heeger则是因为研究高分子导电材料而获奖,而他正是OLED (有机发光二极体)的先驱.在市场方面,根据Strategies Unlimited的统计,2002年LED全球的产值高达38亿美元,其中高亮度LED产值为18亿美元,较2001年的12亿美元成长了50%,预估2003年会达23亿美元.未来五年高亮度LED年复合成长率应可达到20%,产值并将在2007年达到47亿美元.台湾在2002年LED的产值已经跃居全世界第二位.2002年高亮度LED的主要应用为:显示器背光模组应用(40%),讯号指示与显示器(23%),汽车(18%),照明(5%),交通号志(2%). 1907年美国Round 首次研发出SiC LED(发光二极体),在10 V偏压下发现微弱的黄光,绿光与橘光在阴极出现,其中SiC是研磨沙纸上常用的材料.1923年俄国Losseve则将电流注入意外形成的SiC p-n接面,并使组件发出蓝光.1936年法国Destriau发现了注入电流可以让ZnS粉末发光.1962年任职于美国GE公司N. Holoyak Jr (现任University of Illinois, Urbana-Champaign电子电算工程系与物理系教授)等人制作并发表首颗GaAsP红光LED,但直到1970年LED的发光原理才 被进一步了解,1971年夏天美国RCA公司Pankove等人制作出第一个电激发光MIS结构GaN LED.有机半导体材料LED (OLED)则在1980年中期到末期开始发展.1990年初期美国Hewlett-Packard公司的Kuo与日本Toshiba公司的 Sugawara等人使用AlInGaP材料发展高亮度红光与琥珀色LED.
, Z8 k- x: T7 T3 X( B0 s6 b( }% K1 a1986年Amano等人(Isamu Akasaki-赤崎勇教授研究团队,Nagoya
6 e7 \. o9 I. s/ `' qUniversity-日本名古屋城大学)利用MOCVD磊晶低温AlN缓冲层,成功地成长透明,没有表面崩裂的GaN薄膜. 稍后Akasaki等人进一步由X-ray绕射光谱,光激光谱-PL等量测结果,验证了加入低温AlN缓冲层后所磊晶的GaN薄膜,具有完美的晶格排列, 此外本质缺陷所形成的施体浓度,也因此减少到1×1015 cm-3,电子移动率则提高了一个次级(10倍)以上,而低温缓冲层的加入也改善了GaN薄膜的电特性.1989年使用CP2Mg掺杂源已经可以在低温缓 冲层上,成功地磊晶出p-GaN薄膜,日本Akasaki研究团队利用低能量电子束照射(Low-Energy Electron-Beam Irradiation, LEEBI)GaN薄膜,并藉此获得低电阻特性,同时他们也成功地制作出具有p-n接面之蓝光GaN LED.1992年日本Nichia公司的Nakamura (中村修二博士),使用热退火技术成功地活化磊晶在低温缓冲层上的GaN薄膜,并在1995年研制出高亮度GaN蓝光与绿光LED.1996年Nakamura又提出利用InGaN蓝光LED (波长460 nm ~ 470 nm)激发钇铝石榴石: 铈黄色萤光物质之白光LED.
: d( d0 F. s/ |+ X, l/ P/ q7 C( i fLED的 发光波长范围包含紫外光到红外光,目前主要的应用市场仍以可见光为主,在材料方面目前则以应用在红光,黄光的AlGaInP(磷化铝镓铟)与应用在绿光, 蓝光的AlGaInN(氮化铝镓铟)可以达到较高的发光效率.此外有机材料LED近几年在显示器的应用上亦有相当大的进步.在本篇论文中,我们将针对 AlGaInP与AlGaInN LED发展的瓶颈与方向作详细的介绍,希望可以让专业领域的读者可以藉此建立研究地图,而非专业领域的读者也可以因此了解LED发展的轮廓.
8 f7 x4 Q3 |% @% r! C; F p, M* C8 w1 d3 w8 j0 k. D# d

+ M# K% K7 A5 y& _一,磷化物材料 4 l4 n X2 ^5 E: K9 y* [
因为AlP与GaP的晶格常数相 当接近,因此当AlInGa中In的组成固定在048时,AlGaInP材料的晶格常数几乎与Ga,Al的组成比例无关,并可以与GaAs基板达到晶格匹 配.当AlGa中Al的比例低于0.58时(相当于2.25 eV的能隙与550 nm波长),AlGaInP材料具有直接能隙.0 E6 E. S8 J [, a& w6 L! n
AlGaInP材料与有机金属化学气相沈积设备(MOCVD) 的发展息息相关,因为在MOCVD技术成熟发展前,LED多采用液相沈积法(LPE)或氢化物气相磊晶法(HVPE),然而AlGaInP材料并无法用 LPE或HVPE获得高品质的薄膜.1990年美国Hewlett-Packard公司与日本Toshiba公司的研发团队,才分别利用MOCVD设备成 功地研制出高效率AlGaInP LED,主要波长应用在560 nm (黄绿光)~650 nm (红光).
- j* i( l: r8 ^5 M" m1.主要的瓶颈
1 ^2 ?( R0 @" j% z! M高 效率AlGaInP LED最主要面临的瓶颈为光取出效率低,这是因为内部光损耗机制与高折射系数(n=3~3.5)的缘故.AlGaInP的高折射系数导致LED与外面的介 质间有很大的折射系数差,因此大部分的光在LED表面与外面介质的接面处产生全反射(全反射角约只有16°),因此尽管LED的内部量子效率接近 100%,其外部量子效率可能仅剩下2%.内部光损耗机制包括:自由载子的吸收,主动区的再吸收与欧姆金属电极反射等. 2 w4 X9 H3 E* |" M" _& {
2. 目前主要发展方向
+ v, K. n6 G0 S( y以下将针对AlGaInP LED目前的研究方向进行说明,主要包括:窗户层,电流阻障层,布拉格反射层,芯片键合技术,透明或网状电极,表面糙化与晶粒外观改变. ) F4 Q( I# S! Q
2.1 窗户层(Window Layer)
& \3 i6 U0 Q( f7 o9 K9 e' c9 N% B# z1992年 美国Hewlett-Packard公司的Huang等人提出,以MOCVD磊晶技术成长AlGaInP LED后,再以HVPE磊晶9~63 μm的GaP窗户层,将可以使波长范围在555~620 nm的AlGaInP LED,外部量子效率达到26%,能量转换效率则可以达到20 lm/W.除了GaP材料常被采用为窗户层外,具有大能隙的AlGaAs也被Toshiba公司采用为窗户层.窗户层除了可以增加光从LED侧面取出的效 率外,尚具有改善电流分散的效果,因此已成为近来专利诉讼的焦点.
( O( O( _, ^- n* }: C7 y' L- f/ p
) A4 L; I+ A7 N# U. D* h9 g2.2 电流阻障层(Current Blocking Layer)
7 b6 _2 Q; h: V+ t l/ A5 ]AlInGaP LED大 部分的注入电流多集中在正电极的下方,然而当主动区产生的光抵达正电极时,大部分却被反射最后被半导体吸收转为热,因此要达到低金属接触电阻(电极够 厚),又要避免电极过份阻挡光输出,在1989年Gaw等人在其美国专利中揭露,于正面电极的正下方加入电流阻障层,如此将可以减少正电极下方电流路径, 提高光输出效率.1991年日本Toshiba公司的Sugawara等人则在视窗层与上夹层(Cladding layer)间加入整流pn接面,但由于需要两次MOCVD成长,因此会有成本高与良率低的问题.此外,其它研究团队也分别提出使用萧特基或异质整流接 面,当作电流阻档层.
; J3 L+ l. Z- Y" J; z2 f8 g5 D9 ~2.3 布拉格反射镜面(DBR) ( J( l( j3 h5 i! e b
由于AlGaInP LED都使用GaAs基板,然而GaAs材料因为能隙小,因此会吸收AlInGaP LED所发出的光,并将吸收光转换为热能,容易导致组件特性劣化.利用AlGaAs或AlGaInP材料所制作的布拉格反射镜面(利用折射系数不同的材 料,重复堆叠结构),常被安置在发光区与基板间,使朝向基板放射的光线,可以大部分被反射再利用.譬如波长630 nm之LED可以使用20周期的AlGaAsAlAs DBR,其整体反射率可以达到50%.
9 I+ r* a w2 ?; O8 n4 A9 K! W2.5 芯片键合(Wafer Bonding) . U C; g1 H5 ?% [2 `& P+ r
因为GaP在AlGaInP LED放射光波长范围具有良好的光穿透性,因此Hewlett-Packard公司的Kish等人发展去除会吸光的GaAs基板技术,并将剩余的LED薄膜直接键合到透明GaP基板上,并成功地开发出2LED芯 片键合技术,然而在朝向更大面积发展的同时,缺乏大面积的GaP基板反而是该技术的主要限制.键合时芯片表面必须有相当严格的清洁度限制,并需要施予单一 轴向的压力,环境温度需升高到750 oC以上,此外芯片方向的对准与良率问题均是严格的考验,目前Hewlett-Packard公司持续在生产该产品,并称之为TS (Transparent Substrate) 型LED.
6 x+ S- x/ {9 {3 E# C9 v& e, o. n利 用金属层作为芯片键合介质在Si积体电路上已经普遍被采用,特别在微机电组件的制作.在AlGaInP LED方面,则是将LED磊芯片利用金属层(焊料,譬如:AuSn合金)先键合到新基板上,再将GaAs基板去除,利用金属与半导体介面的高反射率与欧姆 接触特性,提高LED的光取出效率与散热性.薄膜LED其制作温度仅需要350oC,远低于TS型LED直接芯片键合所需之温度.利用退火制程将可以降低 接触电阻,但所形成的合金也会使光反射率减少,因此可以将光反射金属与半导体接触金属分开,在导电金属上制作网状薄介电质材料(譬如:SiN或SiO 等),没有介电质覆盖的区域负责电流传输,有覆盖介电质的区域则具有高光反射功能.利用蚀刻技术也可以先在键合接口处,制作具有高光反射的平台或曲面,达 到增加光输出的效果,德国Osram公司已经利用此一技术量产四LED产品.
) @6 l8 i8 G" B! e国 内大叶大学洪瑞华教授(现任中兴大学教授)的研究团队,则将SiO沈积在低价的Si基板上,再利用Au与AuBe与AlGaInP LED(波长600~620 nm)键合,该LED具有不错的热导性,但正,负电极必须制作在同一面,在20 mA的操作电流下波长620 nm的LED具有90 mcd的光输出,50 mA的操作电流下则有2054 w" R0 o6 I7 D c
mcd的光输出.2003年该研究团队将原本绝缘的SiO以金属取代,并制作电极分别置放在LED上下的结构,在波长626 nm时操作电流20 mA下操作电压为2.1 V,光输出达到165 mcd.
- h( o( E: H) f9 k+ e+ J2.6透明电极与网状电极 5 a# `: Z% w: e$ K5 i R( A
在传统的LED结构 中,AuZn,AuBe常被使用为正面p型金属电极材料,然而为了减少金属-半导体接触电阻,金属的厚度通常让主动区所产生的光无法穿透,又为了达到均匀 电流分布的效果,正面金属电极的大小通常占据了LED大部分的正面面积,导致大部分主动区所产生的光,都在金属与半导体接面被反射或吸收,被反射的光也可 能因为在半导体内行进路径过长,而被半导体所吸收并产生剩余热.
. c: ], e; x& i7 i. X7 \& Z为 了减少正面金属电极阻挡过多的光输出,自1994年包括国内清华大学与工研院光电所,英国Wales大学与Cardiff大学等研究团队,开始采用 ITO(氧化铟锡)当作AlGaInP LED的透明电极.由于ITO电极无法直接与AlGaInP材料形成良好的欧姆金属接触,因此需要先在LED结构上先磊晶一层高掺杂之p+-GaAs (p~1019 cm-3)半导体接触层,才能获得具有低顺向偏压之操作特性,然而高掺杂的半导体层,过多的自由载子会吸收大量的输出光,因此必须限制高掺杂GaAs层的 厚度.我们的研究团队提出利用化学蚀刻法,将p+-GaAs层蚀刻为网状结构,接着沈积ITO薄膜,因此覆盖在p+-GaAs层上的ITO将可形成良好的 欧姆接触,但覆盖在AlGaInP上的ITO则会形成萧特基接触(Shcottky Contact),所以注入电流既可以透过p+-GaAs网状结构,均匀注入LED,主动区所产生的光也可以充分地由没有覆盖p+-GaAs的区域取出.
( s- K9 Q x: ~6 E* A6 ]2.7 表面糙化与晶粒外观改变 , @! d0 u0 m5 p; V* p% h& K
晶粒外观的改变包括半球面,圆锥 状等都曾被提出,以增加光取出效率,但实际上尚未被应用在量产上.1973年Bergh等人在美国专利中提出,将LED的表面糙化并在背面制作反射镜面, 将可以增加光取出效率.直到1993年美国加州大学Los Angels分校Schnitzer等人利用胶体材料当作光罩,实际进行LED表面的糙化蚀刻,并将LED的基板去除后镀上光反射层,制作出外部量子效率 高达30%的GaAs LED.1 f; M. L# r- l" V- Z, B: @ j
1999年美国Hewlett-Packard公司的Krames等人发表倒金字塔(TIP- Truncated# |0 L; T1 C1 v8 N" l* Z
Inverted Pyramid)结 构,使LED的光取出效率可以达到55%,主要是因为减少自由载子与主动层的再吸收.该结构的制作方式如下:将AlGaInP/GaP多层量子井结构磊晶 在GaAs基板上,再将GaAs基板以化学蚀刻的方式去除,并透过芯片键合技术贴到透明的GaP:S基板上,使用倾斜刀片划过晶粒侧边,使形成与垂直方向 具有35o夹角,最后在封装时 将p型材料朝下使发光区域得以接近散热底座,达到良好的散热效果.该组件主要操作在红光波段,中心波长为650 nm,在600 mA的直流注入电流操作下,最大光输出可以达到440 mW,比传统的组件大一个级次以上.在100 mA直流电流操作下,外部量子效率可以达到55%,转换效率可以达到100 lmW.最佳特性为橘光(610 nm),峰值转换效率为102 lm/W,琥珀色(波长598 nm)则具有68 lm/W的操作特性. 2 i( Q4 _ G. M' w+ n/ z- {3 p9 Y( r
) ]1 @0 R' u! q; C! v5 B3 Y2 Q
二,氮化物材料
" H% k2 C5 d$ ~, ~7 L根 据2003年六月Strategies Unlimited的统计,累积到2003年第一季全世界共有184家公司参与AlGaInN量产或研发(组件,材料与设备),约有293间学校,研究中 心参与AlGaInN的研发工作,相较于2000年五月的统计,分别增加了74%与24%.从1999年开始AlGaInN组件市场(主要为蓝光,绿光与 白光LED)共成长了221%,产值为13.5亿美元.AlGaInN LED在1995年开始量产后市场以每年年增率64.5%增加.GaN组件(包括电组件)应该可以在2007年前达到45亿美元的产值.
* q( X9 y* y1 o, |/ e, e; _, b目前市面上AlGaInN LED波长范围,主要包含近紫外光(380 nm)到绿光(530" s) X; b# p$ V
nm),尽管相较于AlGaInP材料LED,AlGaInN LED显得较不成熟,但在蓝光或较短波长的LED方面,其外部量子效率已经可以达到20%以上.
! \$ ]) \. b% V0 }: I+ t1. 主要的瓶颈
+ x+ Y. J- @: X$ `0 z8 CAlGaInN LED发展的主要瓶颈包括:缺乏低价之晶格匹配基板,低p-GaN掺杂浓度导致高金属半导体接触电阻,半透明电极大量吸收放射光,高温磊晶条件使发光区特性劣质化,缺陷密度过高导致短波长组件寿命短,组件散热特性不佳,材料硬度高导致低制程良率. . A7 E/ m! W. C* N
2. 目前主要发展方向
) x. t% F/ Q+ [( J5 c' [. K% W以下将针对AlGaInN LED目前主要的研究方向进行说明,主要包括:改善正极金属半导体接触电阻,改善正极金属光穿透度,芯片键合与覆晶(Flip-Chip)技术.
% K& ?+ K1 ~# {6 P, q2.1 改善正极金属半导体接触电阻
% @5 X- _9 F1 N/ Y& A m% b# A由 于p型掺杂Mg在p-GaN中的活化率很低,因此导致金属与p-GaN材料的接触电阻很大,LED也需要较高的操作电压.包括美国加州大学Santa Barbara分校,波士顿大学与日本NTT等研究团队发现,利用AlGaN/GaN或InGaN/GaN超晶格结构,将可以有效提高掺杂浓度,以达到较 低的金属半导体接触电阻.国内中央大学许进恭博士研究团队,将AlGaN/GaN超晶格结构应用在AlGaInN LED上,使20 mA操作电流条件下,操作电压由原先的3.8 V减少到3 V.而我们的研究团队则将InGaN/GaN短周期超晶格结构,应用在AlGaInN LED的正极金属接触上,当操作在20 mA时操作电压为3.78~2.94 V,串联电阻则由原本的41欧姆减少到10欧姆.
0 c; J% U" W8 ^6 x原 本与正极接触的p-GaN材料,可以利用穿透接面(p+-n+)将原本p型材料金属接触,转为利用n型材料与金属接触,如此就可以利用n型材料高掺杂的特 性,减少接触电阻,同时正负电极均可以采用相同的金属.韩国Chonbuk大学Jeon等人在发光区的上方加入穿透接面,由于减少半穿透电极的吸光作用, 光输出可以大幅提升,但也因为穿透接面的加入,使得操作电压增加1 V,因此对LED整体特性改善有限.研究团队提出p-down结构,并将穿透接面加在LED发光区的下方,但是此一组件同样面临操作电压过大的问题.
" T, {+ d8 _# B- p* x1 c/ w2.2改善正极金属光穿透度
4 I( {7 q$ g. u7 x* f, F1999年 美国加州大学Santa Barbara分校的Margalith研究团队,将ITO(氧化铟锡)透明电极应用在AlGaInN LED,在10mA的操作电压下操作电压为2 V,这是因为ITO与p-GaN间的接触电阻过大,但是ITO在波长420 nm时的光吸收低于金属材料,因此该LED组件仍有不错的光输出.2001年国内中兴大学洪瑞华教授团队,利用Ni改善ITO与p-GaN间的接触电阻, 并在空气中以600 °C退火,获得极低的特性接触电阻(8.6×10-4 Ωcm2)与高光穿透率(在波长450~550 nm范围超过80%).我们的研究团队利用Ni/Au/Ni/ITO制作透明电极,经过300 °C快速退火后,AlGaInN LED在20 mA的操作电流下顺向电压达到3.29 V,光输出较传统Ni/Au电极LED高出65%.韩国Pohang科技大学Kim的研究团队,将ITO覆盖在Ni/Au电极上,并在沈积ITO前先以 500 °C在氧气的环境中退火,获得2.0×10-4( c S) n+ d. R3 h# j2 H
Ωcm2极低的特性电阻,在波长 470 nm光穿透率可以高达90.3%.国内工研院光电所潘锡明博士研究团队,直接利用NiO改善ITO与AlInGaN LED的接触电阻,并不需要高温退火的步骤,在波长470 nm时光穿透率高达98%,在20 mA的操作电流下,操作电压为3.4 V,光输出约6.6 mW. , [3 q- i% T. F! y# {
2.3 芯片键合与覆晶(Flip-Chip)技术
- g3 F% ?6 I2 u2 HAlGaInN LED主 要磊晶在Sapphire基板上,由于Sapphire基板并不导电,因此LED的正负电极必须制作在组件的同一面,也因此大部分的光输出都被金属电极所 反射或吸收,其中又以NiAu半透明正电极对光输出的影响最大.在理想条件下半透明正电极的厚度必须要够厚(>500 ),如此才能在低p-GaN掺杂浓度条件下,增加电流的分散效果,然而当半透明电极厚度增加的同时,却又导致光损耗的大幅增加.透过覆晶技术可以让 AlGaInN LED透过透明基板射出,因此不再有放射光被半透明电极吸收的问题,此外正负电极的厚度也不再被严格限制,因此可以增加LED的散热特性,又利用高反射率 金属材料当作电极,则可以反射原本损耗在底座接面的光输出.5 c. P2 n1 A8 y! R- t
覆晶技术是在30年前首次由美国IBM公司所提出,并已成熟应用在Si积体电路制程上,透过覆晶技术可以解决不同材料与组件间积体化的问题,稍后IBM则将此技术授权给Motorola,AMD等公司,此外Delco与其它稍具规模的公司则自行发展相关技术.( z; \, A. |4 D/ G+ C) W/ M
1980年 日本Matsushita公司的Hiroyuki Kobayashim等人在美国专利(专利编号4316208,4396929,4476620)中揭露,在GaN LED的封装上利用覆晶技术取代传统的wire bonding,以提高组件操作的可靠度.2001年美国LumiLeds Lighting公司的Wierer等人利用覆晶技术,制作大面积(0.07 mm2)大功率的AlGaInN+ H4 d; J5 [% _1 k: M
LED,可 以在200~1000 mA大电流条件下操作,其操作电压在200 mA时仅2.8 V,外部量子效率在波长435 nm时可以达到21%,在1A操作电流时可以达到400 mW的光输出.2002年日本Toyda Gosei的Koike等人,发表以有机金属气相沈积AlN缓冲层并配合覆晶技术,将蓝光LED的光输出提高为两倍,并配合量产的AlGaInP LED发展出多彩封装技术.
- h; S2 ]3 B$ U; Y; N+ P 6 t3 ?' k5 q& q9 W/ q
结论 ) }+ M: \" Z4 e+ l6 \4 G
在本论文中,我们回顾了新近磷化 物与氮化物LED的一些研发方向,以为未来研究发展之参考.尽管LED产业看似已经相当成熟,但评估目前LED的特性,尚仍无法完全取代液晶萤幕背光的 CCFL(冷阴极萤光灯管)或一般照明用日光灯管,这也意味着LED组件的结构,材料特性等方面,仍有相当大的努力空间.国内的LED产业已具相当规模, 并开始威胁到拥有基础专利的一些国际大厂,因此将来面临的不仅是组件本身的瓶颈,也必须要正视专利诉讼问题,但除了必须加强国内LED产业的研发能力外, 结合产官学的研发计画也应该要持续推动,透过==的力量整合学术界与产业界的研发能量,才有希望持续立足于世界的舞台.

Wednesday, August 11, 2010

太阳能电池I-V特性

一般地说,太阳能电池由p-n结构成,其中的光能(光子)引起电子和空穴的重新组合,产生电流。因为p-n结的特性类似于二极管的特性,如图1所示的电路通常被用于简化太阳能电池的特性。


图1:简化的太阳能电池的电路模型。

电 流源IPH产生的电流正比于落在太阳能电池上的光量。在没有负载连接的时候,几乎所有产生的电流都流过二极管D,其正向电压决定太阳能电池的开路电压 (VOC)。该电压的变化严格地取决于每一种类型的太阳能电池。但是,对于大多数硅电池,其0.5V到0.8V之间的电压范围恰好就是p-n结二极管的正 向电压。

并联电阻(RP)代表实际太阳能电池中出现的微小泄漏电流,Rs代表连接损耗。随着负载电流增加,由太阳能电池所产生的大部分电流被分流到二极管并进入负载。对于大多负载电流的数值,这只对输出电压有很小的影响。

图 2所示为太阳能电池的输出特性,由于二极管的I-V特性存在微小的变化,串联电阻(Rs)上的电压降也存在微小的变化,但是,输出电压保持很大的恒定。然 而,在一些点通过内部二极管的电流是如此之小,以至于它变得偏置不够,并且,随着负载电流的增加,跨越它的电压快速减少。最后,如果所有产生的电流流过负 载并且不流过二极管的话,输出电压就为零。该电流被称为太阳能电池的短路电流(ISC),它与VOC一道是定义工作性能的主要参数之一。因此,太阳能电池 被认为是“电流受限”的电源。当输出电流增加的时候,其输出电压降低,直到最终减少为零,如果负载电流达到其短路电流的话。


图2:典型的太阳能电池I-V特性。

在 大多数应用中,人们期望从太阳能电池获取尽可能多的功率。因为输出功率是输出电压和电流的乘积,有必要确定电池的哪一部分的工作区域产生的输出电压和电流 的乘积的数值最大,这一点被称为最大功率点(MPP)。在一种极端情况下,输出电压为其最大数值(VOC),但是,输出电流为零;在其它极端情况下,输出 电流位其最大值(ISC),但是,输出电压为零。在两种情况下,输出电压和电流的乘积都是零。因此,MPP必须位于两种极端情况之间的某处。

可 以容易地证明:在任何应用中,MPP实际上出现在太阳能电池的输出特性(见图3)下半部的某个位置。实际上,问题在于太阳能电池的MPP的严格位置会根据 光线和环境温度变化。因此,所设计的系统要产生最大的太阳能,就必须动态地调节太阳能电池输出的电流,以便它在实际工作条件下位于或接近MPP工作。


图3:太阳能电池输出特性。

Monday, April 19, 2010

LED半导体发光二极管工作原理、特性及应用
半导体发光器件包括半导体 发光二极管(简称LED)、数码管、符号管、米字管及点阵式显示屏(简 称矩阵管)等。事实上,数码管、符号管、米字管及矩阵管中的每个发光单元都是一个发光二极管。

一、 半导体发光二极管工作原理、特性及应用
(一)LED发光原理
发光二极管是由Ⅲ-Ⅳ族化合物,如GaAs(砷化镓)、GaP(磷化镓)、GaAsP(磷砷化镓)等半导体制成的,其核心是PN结。因此它具有一般P-N 结的I-N特性,即正向导通,反向截止、击穿特性。此外,在一定条件下,它还具有发光特性。在正向电压下,电子由N区注入P区,空穴由P区注入N区。进 入对方区域的少数载流子(少子)一部分与多数载流子(多子)复合而发光,如图1所示。



假设发光是在P区中发生的,那么注入的电子与价带空穴直接复合而发光,或者先被发光中心捕获后,再与空穴复合发 光。除了这种发光复合外,还有些电子被非发光中心(这个中心介于导带、介带中间附近)捕获,而后再与空穴复合,每次释放的能量不大,不能形成可见光。发光 的复合量相对于非发光复合量的比例越大,光量子效率越高。由于复合是在少子扩散区内发光的,所以光仅在靠近PN结面数μm以内产生。 理论和实践证明,光的峰值波长λ与发光区域的半导体材料禁带宽度Eg有关,即λ≈1240/Eg(mm)式中Eg的单位为电子伏特(eV)。若能产生 可见光(波长在380nm紫光~780nm红光),半导体材料的Eg应在3.26~1.63eV之间。比红光波长长的光为红外光。现在已有红外、红、黄、 绿及蓝光发光二极管,但其中蓝光二极管成本、价格很高,使用不普遍。

(二)LED的特性
1. 极限参数的意义
(1)允许功耗Pm:允许加于LED两端正向直流电压与流过它的电流之积的最大值。超过此值,LED发热、损坏。
(2) 最大正向直流电流IFm:允许加的最大的正向直流电流。超过此值可损坏二极管。
(3)最大反向电压VRm:所允许加的最大反向电压。超过此 值,发光二极管可能被击穿损坏。
(4)工作环境topm:发光二极管可正常工作的环境温度范围。低于或高于此温度范围,发光二极管将不能正常工 作,效率大大降低。

2.电参数的意义
(1)光谱分布和峰值波长:某一个发光二极 管所发之光并非单一波长,其波长大体按图2所示。由图可见,该发光管所发之光中某一波长λ0的光强最大,该波长为峰值波长。
(2)发光强度 IV:发光二极管的发光强度通常是指法线(对圆柱形发光管是指其轴线)方向上的发光强度。若在该方向上辐射强度为(1/683)W/sr时,则发光1坎德 拉(符号为cd)。由于一般LED的发光二强度小,所以发光强度常用坎德拉(mcd)作单位。
(3)光谱半宽度Δλ:它表示发光管的光谱纯度. 是指图3中1/2峰值光强所对应两波长之间隔.
(4)半值角θ1/2和视角:θ1/2是指发光强度值为轴向强度值一半的方向与发光轴向(法向) 的夹角。半值角的2倍为视角(或称半功率角)。

图3给出的二只不同型号发光二极管发光强度 角分布的情况。中垂线(法线)AO的坐标为相对发光强度(即发光强度与最大发光强度的之比)。显然,法线方向上的相对发光强度为1,离开法线方向的角度越 大,相对发光强度越小。由此图可以得到半值角或视角值。



(5)正向工作电流If:它是指发光二极管正常发光时的正向电流值。在实际使用中应根据需要选择IF在 0.6·IFm以下。
(6)正向工作电压VF:参数表中给出的工作电压是在给定的正向电流下得到的。一般是在 IF=20mA时测得的。发光二极管正向工作电压VF在1.4~3V。在外界温度升高时,VF将下降。
(7)V-I特性:发光二极管的电压与电 流的关系可用图4表示。在正向电压正小于某一值(叫阈值)时,电流极小,不发光。当电压超过某一值后,正向电流随电压迅速增加,发光。由V-I曲线可以得 出发光管的正向电压,反向电流及反向电压等参数。正向的发光管反向漏电流IR<10μa以下。

(三)LED的分类
1.按发光管发光颜色分
按发光管发光颜色分,可分成红色、橙色、绿色(又细分黄绿、标准绿和 纯绿)、蓝光等。另外,有的发光二极管中包含二种或三种颜色的芯片。根据发光二极管出光处掺或不掺散射剂、有色还是无色,上述各种颜色的发光二极管还可分 成有色透明、无色透明、有色散射和无色散射四种类型。散射型发光二极管和达于做指示灯用。

2. 按发光管出光面特征分
按发光管出光面特征分圆灯、方灯、矩形、面发光管、侧向管、表面安装用微型管等。圆形灯按直径分为φ2mm、 φ4.4mm、φ5mm、φ8mm、φ10mm及φ20mm等。国外通常把φ3mm的发光二极管记作T-1;把φ5mm的记作T-1(3/4);把 φ4.4mm的记作T-1(1/4)。由半值角大小可以估计圆形发光强度角分布情况。从发光强度角分布图来分有三类:
(1)高指向性。一般为尖 头环氧封装,或是带金属反射腔封装,且不加散射剂。半值角为5°~20°或更小,具有很高的指向性,可作局部照明光源用,或与光检出器联用以组成自动检测 系统。
(2)标准型。通常作指示灯用,其半值角为20°~45°。
(3)散射型。这是视角较大的指示灯,半值角为45°~90°或更 大,散射剂的量较大。

3.按发光二极管的结构分
按发光二极管的结构分有全环氧 包封、金属底座环氧封装、陶瓷底座环氧封装及玻璃封装等结构。
4.按发光强度和工作电流分
按发光强度和工作电流分有普通亮度的 LED(发光强度<10mcd);超高亮度的led(发光强度>100mcd);把发光强度在10~100mcd间的叫高亮度发光二极管。一 般LED的工作电流在十几mA至几十mA,而低电流LED的工作电流在2mA以下(亮度与普通发光管相同)。

除上述分类方法外,还有按芯片材料分类及按功能分类的方法。

(四)LED 的应用
由于发光二极管的颜色、尺寸、形状、发光强度及透明情况等不同,所以使用发光二极管时应根据实际需要进行恰当选择。由于发光二极管具有 最大正向电流IFm、最大反向电压VRm的限制,使用时,应保证不超过此值。为安全起见,实际电流IF应在0.6IFm以下;应让可能出现的反向电压 VR<0。6VRm。LED被广泛用于种电子仪器和电子设备中,可作为电源指示灯、电平指示或微光源之用。红外发光管常被用于电视机、录像机等的遥 控器中。
(1)利用高亮度或超高亮度发光二极管制作微型手电的电路如图5所示。图中电阻R限流电阻,其值应保证电源电压最高时应使LED的电流 小于最大允许电流IFm。

(2)图6(a)、(b)、(c)分别为直流 电源、整流电源及交流电源指示电路。
图(a)中的电阻≈(E-VF)/IF;
图(b)中的R≈(1.4Vi-VF)/IF;
图(c)中的R≈Vi/IF式中,Vi——交流电压有效值。
(3)单LED电 平指示电路。在放大器、振荡器或脉冲数字电路的输出端,可用LED表示输出信号是否正常,如图7所示。R为限流电阻。只有当输出电压大于LED的阈值电压 时,LED才可能发光。
(4)单LED可充作低压稳压管用。由于LED正向导通后,电流随电压变化非常快,具有普通稳压管稳压特性。发光二极管 的稳定电压在1.4~3V间,应根据需要进行选择VF,如图8所示。

(5)电平表。目前,在音响设备中大量使用LED电平表。它是利用多只 发光管指示输出信号电平的,即发光的LED数目不同,则表示输出电平的变化。图9是由5只发光二极管构成的电平表。当输入信号电平很低时,全不发光。输入 信号电平增大时,首先LED1亮,再增大LED2亮……。

(五)发光二极管的检测
1. 普通发光二极管的检测
(1)用万用表检测。利用具有×10kΩ挡的指针式万用表可以大致判断发光二极管的好坏。正常时,二极管正向电阻阻值为几 十至200kΩ,反向电阻的值为∝。如果正向电阻值为0或为∞,反向电阻值很小或为0,则易损坏。这种检测方法,不能实地看到发光管的发光情况,因为 ×10kΩ挡不能向LED提供较大正向电流。

如果有两块指针万用表(最好同型号)可以较好 地检查发光二极管的发光情况。用一根导线将其中一块万用表的“+”接线柱与另一块表的“-”接线柱连接。余下的“-”笔接被测发光管的正极(P区),余下 的“+”笔接被测发光管的负极(N区)。两块万用表均置×10Ω挡。正常情况下,接通后就能正常发光。若亮度很低,甚至不发光,可将两块万用表均拨至 ×1Ω若,若仍很暗,甚至不发光,则说明该发光二极管性能不良或损坏。应注意,不能一开始测量就将两块万用表置于×1Ω,以免电流过大,损坏发光二极管。
(2) 外接电源测量。用3V稳压源或两节串联的干电池及万用表(指针式或数字式皆可)可以较准确测量发光二极管的光、电特性。为此可按图10所示连接电路即可。 如果测得VF在1.4~3V之间,且发光亮度正常,可以说明发光正常。如果测得VF=0或VF≈3V,且不发光,说明发光管已坏。


2.红外发光二极管的检测

由于红外发光二极管,它发射 1~3μm的红外光,人眼看不到。通常单只红外发光二极管发射功率只有数mW,不同型号的红外LED发光强度角分布也不相同。红外LED的正向压降一般为 1.3~2.5V。正是由于其发射的红外光人眼看不见,所以利用上述可见光LED的检测法只能判定其PN结正、反向电学特性是否正常,而无法判定其发光情 况正常否。为此,最好准备一只光敏器件(如2CR、2DR型硅光电池)作接收器。用万用表测光电池两端电压的变化情况。来判断红外LED加上适当正向电流 后是否发射红外光。其测量电路如图11所示。资料来源:电子技术网

Wednesday, March 31, 2010

美国GaAs晶圆厂激光划片系统表现不俗

尽管还存在不少问题,美国领先的GaAs晶圆厂已经开始享受使用激光划片系统更换以前的划线-折断设备所带来的好处了。而那些还没有更换使用激光划片工艺 的公司能从TriQuint半导体公司的经验中受益。

  化合物半导体制造商倾向于在他们引进新工艺和新技术的时候保持低调。不久前在 化合物半导体制造技术国际会议(CS Mantech)上,一些技术细节得到了披露,但此时这项工艺可能已经使用了至少一年以上。

 今 年在芝加哥北郊Wheeling举办的Mantech会议上,TriQuint表示他们已经使用激光划片切割GaAs RFIC晶片有相当长时间了。这在它自身而言并不出人意外,因为在过去几年里,这家俄勒冈州公司及其竞争者都在努力实现这项技术以提高晶片的产量。更令人 惊奇的事情,可能是TriQuint在实现激光划片的过程中所遇到的问题。

 Travis Abshere为Mantech会议撰写了一篇名为《激光划片的经验总结》的论文。尽管事实证明采用Advanced Laser Separation International (ALSI)公司提供的激光系统非常成功,但工艺的研发和改进过程显然不是一帆风顺的。

 改用激光划片 的主要动机是提高晶片的产量。TriQuint团队估计,使用激光切割比传统的金刚石工艺能提高5倍的产量,这将为投资带来极具吸引力的回报。激光切割不 仅加工速度快,还尽可能地减小了芯片的尺寸、有效地增加每块晶片乃至整个晶圆厂的产量。Abshere表示通过采用激光划片设备,能将晶片上相邻器件的间 隔从50祄减小到25祄,这样就能从每批次的加工中得到“额外的”晶片。

 在评价该工艺用于实际生产的效果之前,Abshere及其同 事提出了他们认为最有可能出现的问题:例如破坏芯片间区域的部分电介质材料,改变了芯片切口的形貌,可能导致芯片开裂等等。

成品率下降

 当TriQuint引进激光划片后成品率降低了。这不令人意外,原因很显然,基于GaAs的功率放大器芯片由于引线键合造成了短路失 效。最后证实TriQuint的一个转包组装厂(SCA)采用了一种工艺,使得用于芯片间互连的引线下垂并与芯片锯齿形切口边缘暴露的金属短接。 Abshere解释说,“使用激光划片的芯片,键合的引线可能下垂并与锯齿形切口边缘的金属接触,从而造成了短路”。

 通常这些下垂的 引线并不会造成任何问题,因为保护芯片免于划伤的SiN涂层同样会保护金属的切口。但是激光处理工艺会破坏芯片间区域的SiN保护层,而且破坏会一直延伸 到金属切口保护层,甚至深入芯片本身。为了避免出现这样的状况,TriQuint有选择性地除去了芯片上的SiN层。然而该工艺导致的金属暴露一直延伸到 了金属切口的中心,最终与下垂的金属引线相短接。

 ALSI的DCA 802自动激光划片系统事实上已影响了GaAs晶片切割市场。该系统采用的技术最早是由Philips开发的,并与ASML光刻系统具有部分相同的设计。

 为了解决这个问题,TriQuint对工艺进行了两项重要的改进。首先,它要求转包组装厂解决引线下垂的问题;其次,要保证SiN涂 层覆盖了整个金属切口,另外还要向芯片间区域延伸几个微米。

 另一个技术问题是激光切割的缝宽比金刚石切割刀的要窄。更窄的切口意味着 需要更小心地处理经过划片工艺后的晶片,以确保上面的芯片不会因为互相摩擦、碰撞而被破坏。一般套环(Hoop ring)能把晶片贴膜绷紧撑开,保证芯片之间的距离。而金刚石切割刀较宽的割缝就不会出现上述问题。

 TriQuint所有的三家转 包组装厂曾经都有过使用套环的经验,所以工艺改进能进行下去。唯一的问题就是他们在GaAs晶片上都没有使用套环的经验。于是当绷紧的晶片运达时,晶片的 套环与他们使用的刀架并不合适,而且他们也不能将晶片边缘的芯片都分拣出来。

经验之二

 Abshere说:“我们从 中学到的经验是尽快将加工好的晶片交给转包组装厂,但他们都没有6英寸那么小的套环”。解决方案很简单:TiQuint自己更换更大的刀架和套环,但这会 增加工艺成本,还拖延了时间。

 第三个问题是芯片的开裂,TriQuint在更换使用激光划片工艺之初就已被列为主要的风险之一。引入 新的工艺以后,他们开始注意到激光划片的芯片发生了开裂和高失效率。然而,这个问题还是与TriQuint的团队之前预料的情形不太一样。

  TriQuint最初的担忧主要是激光形成的切口的形状会开裂。所以在咨询ALSI和其他导入了激光划片工艺的GaAs制造商之后,TriQuint引入 了一种腐蚀-清洗的工艺步骤用来强化激光加工后芯片的边缘。

 但是事情没完全向计划的方向发展。尽管腐蚀清洗对GaAs晶片的顶部有 用,但它并没有强化边缘。因此,芯片开裂是不可避免的结果,而且TriQuint不得不停止所有的激光划片工艺直到将这个问题解决。

点击察看原图




 一个具有30祄 划线间距、100祄厚的GaAs晶片(左图),以及在切割并张紧后的同一块晶片显示出割缝为17祄(右图)。TriQuint早期使用者的经验应该对还没 有改用GaAs晶片激光划片系统的公司有所裨益。RFMD是另一个早期的使用者,它被认为已经采购了ALSI的设备用于GaAs厂的量产。

  Abshere承认,“我们起初认为腐蚀清洗是一个很好的机会,为了避免芯片出现脆弱的边缘,与其做大幅的变革,不如使用腐蚀清洗工艺,这样也许能避免使 用保护层涂覆工艺。与ALSI合作进行了芯片强度的测试后,该团队断定之前遇到的开裂问题很大程度上是由于正在使用的腐蚀清洗设备,并且晶片应该在套圈张 紧之后再进行腐蚀,以保证良好的清洗效果。通过采购自动化程度更高的设备以及套圈,这样在各个芯片间能有更多的空间容纳腐蚀溶液。如此处理过的芯片具有更 高强度的边缘,甚至比金刚石刀切割的还要高。

 TriQuint远不是激光划片的唯一使用者。最大的GaAs射频器件(RFIC)制造 商RFMD可能是尝试使用该技术的先行者,据业内人士估计它可能大约有8台激光划片机。

 还有另一家领先的制造商,自2007年夏季开 始在它的晶圆厂采用ALSI的设备,并进行了广泛的试验。与TriQuint类似,这家制造商对此非常满意,并计划为将来的产能扩大购买更多的设备。该公 司某匿名人士说,“我们几乎在划片工艺上全面采用了激光工艺,实际上已经淘汰了切割-折断设备”。

 第二家公司似乎并没有芯片开裂的问 题,很明显,虽然采用激光工艺的时间比其竞争对手要晚,但它从TriQuint早期使用者的经验中受益不少。

  这家未知名公司的晶片产量提高了五倍,这意味着需要外包出去加工的晶片数量大幅减少。他们说,“激光划切的速度更快,而且所得成品率也和金刚石切割的不相 上下”。对于一般具有平均芯片数量的典型6英寸晶片来说,该公司的两台ALSI设备每周能分别加工600片左右的晶片。据称“那比金刚石切割最大的产量多 得多”。现实的数据与ALSI提供的相当吻合,设备加工一块典型的6英寸晶片需要10分钟左右,相当于在晶圆厂内每周(24小时制)加工1000块以上的 晶片。

 ALSI的设备似乎要成为GaAs晶圆厂激光划片设备的首选。一家荷兰公司的商务主管向本刊表示,他们已经在其美国和亚洲 RFIC制造厂内全都部署了该设备。

 该技术最初是由Philips开发的,它的发源已成为一种优势。用户也对ALSI设备及其工艺方 式留下了深刻的印象,他们并不是简单地将传统的划片设备的金刚石刀换作大功率激光器,而是从零开始对整个激光划片系统进行了研究和优化。

  Hendriks解释道:“激光处理工艺由许多功率相对较小的独立激光光束来完成,这样可以将针对晶片的热效应和破坏减至最小,并且提高了总体生产效 率。”

 最后,Abshere相信虽然遇到了不少预料之外的问题,但一年之内TriQuint就能在这项投资(激光划片设备)上获得回 报。其他GaAs晶圆厂管理者能从TriQuint的经验中吸取教训,这样投资回报的周期就会变得更短些。只要有能力解决任何可能出现的问题,这个商业案 例是相当有吸引力的。