Illustration: John MacNeill Already and Future Process: Today’s semiconductor accomplishment uses a admixture of accelerated single-wafer processing accessories and slower batch-processing machines [left]. The best avant-garde action will use abandoned single-wafer accoutrement for authoritative the ICs [right]. Some processes will still use accumulation accessories for packaging. Others will about-face to wafer-level packaging application both accumulation and single-wafer machines [center]. In three to bristles years, the best avant-garde processes will use abandoned single-wafer equipment.
Since the apparatus of the dent ambit in 1958, the cardinal of processing accomplish appropriate to accomplish one has developed from beneath than 10 to several hundreds. At the aforementioned time, the silicon wafers on which the ICs are produced accept gone from actuality coin-sized to actuality dinner-plate-sized.
Today, one of these 300-millimeter wafers can crop added than 700 ICs. And that, for a growing cardinal of dent makers, is absolutely the problem. With such a ample cardinal of ICs advancing from a distinct dent and with wafers advancing off accomplishment curve at ante of tens of bags a month, companies can bound acquisition themselves adversity from dent glut, abnormally in agitated markets.
For the accomplished bristles years, aback the beginning of the dot-com balloon in 2000 beatific semiconductor sales into a tailspin, the industry has been disturbing to rid itself of balance inventory. In the additional division of 2001, the absolute accumulation chain, including dent makers, distributors, arrangement manufacturers, and consumer-product manufacturers, was blimp with an balance of chips account added than US $13 billion, according to some estimates. Companies chock-full hiring new advisers and laid off absolute ones. As a result, semiconductor industry jobs in the United States abandoned alone from 268 000 in 1999 to 235 100 in July 2003, according to the U.S. Department of Labor. As afresh as the third division of aftermost year, dent crowd was still at a awkward $1.6 billion, according to a basic assay by iSuppli Corp., a ysis close in El Segundo, Calif. [See graph, “Swimming in Chips.”]
Sources: iSuppli and DigiTimes.com Swimming in Chips: The semiconductor industry has been aggravating to rid itself of its balance account of chips for years. The oversupply, admitting not as bad as it already was, still stood at a amount of about US $1.5 billion at the end of aftermost year.
Clearly, the semiconductor industry is still adverse austere problems as it claws its way aback adjoin advantage and abiding application growth. And for bread-and-er and abstruse reasons, the adamant drive adjoin faster, cheaper, and abate chips is a growing problem. The solution, we believe, lies in a axiological change in the machines that action the wafers: a about-face from accumulation to single-wafer manufacturing.
The single-wafer access is a absolutely consecutive one, in which aloof one dent is candy at a time, all the way through the branch from alpha to finish. There is never a time aback the machines assignment on a ample accumulation of wafers at the aforementioned time, as they do today. The single-wafer address will break the crowd botheration by abridgement the time it takes to accomplish a finished, packaged dent to beneath than one month, rather than the three months or added that is archetypal today. Basically, with single-wafer manufacturing, semiconductor companies will be able to aftermath chips bound aback the orders appear in, in the exact quantities defined by those orders. There will be no charge to body up huge inventories that may aloof sit on shelves until they become obsolete.
And it isn’t aloof bazaar chips, which are bogus in baby quantities, that would account from the single-wafer approach. Alike t like changeless random-access anamnesis (SRAM) and microcontroller chips, which ache from alternate crowd and the consistent amount plunges and bargain profits, would account from a added active acknowledgment to alteration bazaar demands.
So what will it booty to about-face to single-wafer manufacturing? First, accede today’s archetypal semiconductor plant. It combines single- and batch-processing steps; some of the machines action wafers in groups, while others already action them singly. True single-wafer accomplishment eliminates all the accumulation processes and uses abandoned machines that action wafers one at a time. Today, abandoned a few semiconductor plants accept switched over absolutely to single-wafer manufacturing.
In 2001, Trecenti Technologies Inc. of Hitachinaka, Japan (now allotment of Renesas Technology Corp.), adopted 100 percent single-wafer processing for the artifact of avant-garde semiconductor ICs on 300-mm wafers. The company’s acquaintance with this address has been remarkable. It has begin that it can abate accomplishment time from 90 to 30 days, and the cardinal of canicule bare for anniversary dent band has alone from 2.25 to 0.25. Alike added arresting is the advance in the artifact time for a dent of SRAM chips bogus up of 130-nanometer structures. That time has alone from about 60 canicule to beneath than six days.
Several added IC manufacturers are additionally currently because 100 percent single-wafer processing. Freescale, Philips, and STMicroelectronics accept formed the Crolles2 Alliance. Its 300-mm dent facility, in Crolles, France, uses single-wafer processing for best steps. Tokyo-based Toshiba Corp.’s minifab, in Oita City, Japan, is addition archetype of IC accomplishment bedeviled by single-wafer processing.
The artifact of a finished, packaged IC has three key groups of processing steps, behindhand of whether it happens in a accepted or a single-wafer setup. First, the transistors are made; next, they’re active calm into circuits on the chip; and finally, the baby slivers of silicon, accepted as die, are packaged into accomplished chips [see illustration, “Once and Future Process”].
Making transistors, declared simply, is a consecutive action that builds the four aloft transistor components: the source, drain, channel, and gate. Basically, the aboriginal footfall in the accumulation of the transistor is to accoutrement a silicon dent with a photosensitive material, alleged a photoresist. The photoresist is apparent to ablaze in the arrangement of the areas in which transistors are to be built. Anniversary of these regions will eventually accommodate the transistor’s four components.
The arrangement forms aback the ablaze passes through a bottle affectation blooming with chrome. The acknowledgment of the photoresist to ablaze changes its solubility, so aback the dent is rinsed afterwards exposure, some of the photoresist washes away. What charcoal is the arrangement of transistor areas in photoresist on the wafer. This step, alleged lithography, is afresh at about every date of manufacturing, architecture up the transistors, and accordingly the chip, band by layer.
Next comes a footfall alleged ion implantation, which infuses the dent with assertive ions, about arsenic or boron, in the areas breadth the photoresist has been removed. These ions are alleged dopants, because their attendance changes the transistor’s electrical properties, for example, its attrition or the voltage that charge be activated to the aboideau to about-face it on.
The aing assignment is to body the gate. A attenuate band of silicon dioxide insulation is developed to electrically abstract the aboideau from the channel. This footfall about occurs in a ample boiler that heats the wafers in an oxygen-rich environment. The top of the band is alloyed with nitrogen to accommodate a concrete barrier to the access of algae from the aboideau into the silicon dioxide. Afresh polycrystalline silicon is deposited over the absolute wafer.
The aing footfall takes the dent aback to lithography, breadth the aforementioned action of acknowledgment of photoresist through a mask, followed by rinsing, leaves photoresist accoutrement the regions breadth the transistors’ gates are to reside. The lithographic footfall is followed by an carving step, in which ions, bombarding the polycrystalline silicon surface, abrasion abroad all the polycrystalline silicon actual not covered with photoresist. The polycrystalline silicon gates abide afterwards the careful band of photoresist is removed.
After aboideau accumulation comes addition ion-implanting step, this time to acclimatize the electrical backdrop of the antecedent and drain, and afresh annealing, in which the dent goes either aback into the boiler or into a single-wafer annealing machine, enabling the ions to move into the able locations in the material.
Making the affairs that affix the transistors into circuits is the final footfall in the accumulation of the ICs on the wafer. The dent may accept as abounding as nine levels of wires, which are all formed the aforementioned way. Aboriginal an careful actual is deposited assimilate the acme of the completed transistors. Afresh trenches are categorical into the insulator and abounding to overflowing with metal—usually tungsten or copper. Polishers bullwork bottomward the metal until it is akin with the top of the insulation. These metal structures anatomy the links, or vias, that will affix the base to the transistors. A additional band of insulation is deposited on top of the vias, and it too is categorical into the adapted wire pattern. Afresh nut is deposited and able bottomward to be alike with the additional band of insulation, basic the aboriginal akin of metal wiring. This process, alleged chemical-mechanical polishing, uses both concrete cutting and a band-aid of chemicals to bullwork the metal to the appropriate level.
Once the circuits are absolutely built, the dent moves on to testing and packaging. For packaging, the dent is broken up into alone die. The dent is amid in an closed package, with electrical pads on the die affiliated to the accustomed leads bulging from the package.
In today’s archetypal fab facility, the single-wafer accomplish accommodate lithography, chemical-mechanical polishing, and the actual degradation acclimated in base the transistors. But some thermal processing steps, like silicon dioxide growth, the beverage of nitride into the aboideau insulation, and annealing are done in batches of as abounding as 200 wafers in ample furnaces. Periodically, throughout manufacturing, the wafers charge a charwoman to bright abroad bits that may accept accumulated during antecedent accomplishment steps. Some of the charwoman accomplish dip the wafers into a wet bench, a backlog abounding with a charwoman liquid. Wet benches additionally handle wafers in ample batches.
The batch-processing accessories awfully increases the time it takes for a dent to canyon through the assembly line. The acumen for the adjournment is that the boiler or wet bank cannot action a distinct wafer—or alike two, or 20—when it arrives from a antecedent accomplishment step. The equipment’s temperature, gas-flow rate, or actinic absorption is set to handle abounding wafers at once. To handle beneath wafers would crave altered settings. So a dent charge delay until dozens of added wafers are accessible to go in. This coercion agency that some wafers may delay about for canicule afore they move into the accessories for the aing step.
The greatest abstruse advantage of single-wafer accomplishment is that it produces wafers with added acceptable chips than accumulation processing does. It additionally produces ICs that are faster and added reliable. There are several affidavit for these improvements, but the best important ones can be summed up in two words: tighter tolerances. Single-wafer accessories is abate than accumulation equipment, so it is accessible to accept bigger ascendancy over altitude like temperature and gas flow. In accumulation furnaces, for example, the annealing temperature or gas breeze may alter from abode to abode central the furnace. These differences actualize variations in the electrical backdrop of the circuits from dent to wafer, above a distinct wafer, and alike on an alone IC. And these properties, in turn, affect how the circuits work. For example, if the voltage at which the transistor turns on is too high, the transistor switches added boring than one with a lower beginning voltage. And slower transistors beggarly slower circuits.
If beginning voltages alter from dent to wafer, the circuits on those wafers will run at altered speeds. And, alike worse, if the beginning voltages devious too far from their nominal values, the ambit may not assignment at all.
The ambush in single-wafer accomplishment is to devise processes that handle a distinct dent calmly abundant to compensate—through speed, for example—for the economies of calibration that are absent with the aishment of accumulation processing. In the annealing step, for example, the single-wafer another to accumulation processing is accelerated thermal processing. It occurs in a alcove in which lamps calefaction the dent directly, so the temperature to which anniversary dent rises is added compatible over time, and the electrical backdrop of the circuits are added compatible as well.
To acceleration the wafers through a single-wafer facility, engineers array several accomplishment accomplish into a distinct unit, which additionally helps to ascendancy ambient altitude added precisely. For example, basic the layers that accomplish up the transistor’s aboideau takes three abstracted substeps: growing of the silicon dioxide aboideau insulation, abacus nitrogen to the silicon dioxide to assure the insulation adjoin the beverage of algae from the aboideau material, and depositing the polycrystalline silicon that will eventually anatomy the gates of alone transistors. In a single-wafer facility, all three of these substeps can action central a distinct machine, as they do at best of the 47 accessories in the apple breadth the best avant-garde bartering chips are fabricated.
These accumulated machines, accepted as array tools, accommodate a abstracted alcove for anniversary of the accomplish of the action [see photo, “Get It Together”]. The dent passes from one alcove to the aing through a exhaustion arena amid at the centermost of the unit. Connecting the action accommodation through a exhaustion prevents the apparent of the dent from actuality apparent to the atmosphere, which could alloy it, throughout the absolute gate-formation process.
The acute a accessible with this array apparatus allows the silicon dioxide array to be controlled ogously to a few monolayers of atoms—a ascendancy not accessible with accumulation machines. Such accord is ytical to the artifact of the best avant-garde circuits today—those with affairs abandoned 90 nm across—because the absolute array of the silicon dioxide band that insulates the aboideau from the access in these accessories is abandoned 1 or 2 nm thick. Array accessories will become alike added ytical in the aing semiconductor generation, which will aftermath affairs abandoned about 65 nm wide.
In some cases a single-wafer apparatus is fundamentally bigger than its accumulation ogue because of differences in the concrete attempt through which they work. For example, both the rapid-thermal-processing machines acclimated in single-wafer accomplishment and the furnaces acclimated for accumulation processing calefaction the wafers by irradiating them with photons. But the wavelengths of the photons in the two processes are different. The batch-processing boiler produces abandoned photons with wavelengths aloft 800 nm—in the bittersweet arena of the optical spectrum.
The single-wafer alternative, on the added hand, uses rapid-thermal-processing lamps that aftermath photons both aloft and able-bodied beneath 800 nm—even some photons in the arresting and ultraviolet genitalia of the spectrum. These shorter-wavelength photons are added active than the bittersweet photons and can accelerate the atoms of the dent electronically. As a result, they admittance a lower processing temperature while about abridgement the processing time from hours to minutes. The lower temperatures additionally accomplish it easier to acquaint new abstracts into the accomplishment process, for example, new aboideau insulators that will lower the arising accepted amid the aboideau and the access and cut bottomward appreciably on the adeptness that the ICs consume. The basal band is bigger performance, reliability, and crop for the chips.
Another adorable affection of single-wafer processing accessories is the use of sensors central the machines to admeasurement important parameters, such as dent temperatures, gas densities, and acknowledgment rates. These sensors broadcast advice to a database and acquiesce software to adjust, in absolute time, any ambit that may accept absent from their nominal values.
Batch equipment, on the added hand, uses adviser wafers that run either avant-garde of or alongside the ambit wafers. On these adviser wafers are circuits that acquiesce engineers to admeasurement band widths, band spacings, wire resistances, and added ambit features. Afterwards the action footfall ends, the adviser wafers are taken out and their assorted circuits are measured. Engineers use the advice to accomplish adjustments for the aing accumulation of wafers to move through that allotment of equipment.
Some single-wafer processing steps, conspicuously lithography, additionally use adviser wafers. But the sensors acclimated in added single-wafer accomplish let technicians accomplish adjustments to backdrop like dent temperatures and gas concentrations on the fly. It gives them bigger ascendancy of accomplishment altitude and yields wafers with beneath aberration from dent to dent and from dent to wafer.
The bread-and-er advantages of single-wafer processing extend above the chips themselves. Alike the accessories itself is abundant abate physically than that acclimated in accumulation processing. For example, a avant-garde adeptness that produces ICs on 300-mm wafers and produces 25 000 wafers per ages adeptness absorb 10 000 to 20 000 aboveboard meters of attic space. Of that area, batch-processing accessories would booty up 1200 m2. The breadth taken up by the agnate single-wafer processing accessories is abandoned 600 m2 [see graph, “Saving Space”]. The abate breadth agency that architecture the adeptness costs less, and abounding operating expenses—such as lights, air purification, and temperature control—also are decidedly lower. The accessories itself costs hardly beneath as well—about 6 percent less—for single-wafer processing than for accumulation processing, but that 6 percent represents millions of dollars. In an era aback it costs amid $2 billion and $3 billion to body a semiconductor accomplishment plant, alike a accumulation of a few percent matters.
Single-Wafer Processes are acceptable important in packaging chips as able-bodied as in architecture the wafers. Today abounding packaging operations still use ample wet benches to apple-pie debris—small particles of base metal or insulation—from the apparent of ample batches of wafers. But fab facilities—particularly avant-garde ones—are added axis to single-wafer charwoman accessories for the packaging process. The arch architect of single-wafer charwoman accessories is The Sez Group, Zurich, Switzerland. Activated Abstracts Inc., Santa Clara, Calif., additionally offers a apparatus for charwoman alone wafers during transistor formation.
A packaging address alleged wafer-level packaging is added accepted amid dent makers. It is a footfall adjoin single-wafer processing, because it makes it accessible to do the charwoman accomplish and to body access amid the amalgamation and the chip, and amid chips in the package, in a single-wafer artifact facility. It may assume accessible enough, but in actuality this bucks attitude in the industry; best wafers today are alien to alien locations for packaging, which can add weeks to the accomplishment process.
To sum up, the three capital business advantages of single-wafer processing—faster time to market, abate inventory, and lower accomplishment cost—are starting to advance dent makers’ adeptness to administer their accumulation chains and bigger accord with the bang and apprehension cycles that accept continued bedeviled the industry.
Faster time to bazaar is accessible because it takes beneath time for wafers to accomplish it through the artifact process. The three months it takes to accomplish a dent abounding of chips with batch-process accessories can be bargain to beneath than a ages with a pure, or about pure, single-wafer setup.
For dent makers, these advances are overdue. The balance of anamnesis and argumentation chips that alike now continues to affect the bazaar can be blamed, at atomic in part, on authoritative wafers in ample batches. This abiding dent glut has continued depressed dent prices. So while the cardinal of chips awash in the aboriginal three years of this decade climbed steadily from 300 billion in 2001 to 360 billion in 2003, revenues backward almost flat, at about $160 billion per year. In added words, manufacturers accept been affairs added chips, but they accept not been authoritative any added money [see graph, “Where It Hurts”].
This accomplished year of 2004 saw some acceptable account for the industry: record-breaking acquirement of $214 billion—28 percent college than that of 2003, according to the Semiconductor Industry Association, in San Jose, Calif. But the bad account is that revenues will not abound decidedly in 2005. Dent prices are afresh bottomward because of dent oversupply. In a address appear in November 2004, iSuppli ysts predicted that 2005 dent prices would abatement in best categories of ICs, from anamnesis chips to accepted argumentation circuits.
The complication of IC accomplish grows with anniversary new bearing of semiconductor technology. On-chip wire widths shrink. New techniques and new materials, both insulators and conductors, appear into the mix. The amount of architecture a semiconductor artifact adeptness continues to skyrocket. Afore too long, single-wafer accomplishment will not be alone an alternative, it will be a necessity. We accept that aural three to bristles years all manufacturers will be application single-wafer processing alone to accomplish transistors and aing wires. They will accordingly accept single-wafer packaging universally in the afterward years.
Rajendra Singh (F) is the D. Houser Banks Professor of Electrical and Computer Engineering and the administrator of the Centermost for Silicon Nanoelectronics at Clemson University, in South Carolina ([email protected]).
Randhir Thakur (SM) is accumulation carnality admiral and accepted administrator of Activated Abstracts Inc.’s Front End Products Business Group, in Santa Clara, Calif. ([email protected]).
“Special Section on Single-wafer Accomplishment in Nanochip Era,” by Rajendra Singh et al., in IEEE Transactions on Semiconductor Manufacturing, Vol. 16, May 2003, pp. 90-178, gives abstruse capacity of the single-wafer approach.
“Dominant Role of Distinct Dent Accomplishment in Providing Abiding Advance of the Semiconductor Industry,” by Rajendra Singh et al., in Semiconductor Fabtech, 19th copy (Henley Publishing, London), 2003, pp. 85-93, describes the accent of single-wafer manufacturing.
In “Limits of Dent Ambit Manufacturing,” Proceedings of the IEEE, Vol. 89, March 2001, pp. 375-93, Robert Doering and Yoshio Nishi yze the processing pitfalls of semiconductor manufacturing.
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