Roland A. - PVD for microelectronics (779636), страница 32
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To prevent such a situation, suitable precleaning of the wafer surface is necessary before high-temperature processing. Alternatively, one can effectively reduce airborne contaminationby transporting wafers in a closed, loadlocked "pod" having a controlledenvironment such as dry, inert gas or even a moderate vacuum. The frontend of advanced PVD tools are configured with a mechanical interfacewhereby an operator or robotic vehicle can attach the pod, and the waferscan be moved out of the pod and into the PVD tool without ever being exposed to the clean-room ambient.SPUTTERING TOOLS165LoadlocksLoadlocks must pump from atmospheric pressure (p0 = 760 Torr) to moderate vacuum ( p ~ 1 mTorr) as rapidly as possible to maintain high waferthroughput.
Rapid pump down can cool the gas by adiabatic expansion,with a change in temperature given by the expression [5.41]To(5.9)where y is the ratio of the specific heat of the gas at constant pressure tothat at constant volume. Figure 5.40 shows the measured gas temperaturevs time for a 40-sec pump down from 650 Torr to ~ 30 mTorr. As seen, thereduction in temperature after a few seconds of pumping is sufficient tocondense out water droplets that can then serve as nuclei for subsequentparticle formation. The problem can be avoided by using a slow, or "soft,"pump procedure and/or purging the loadlock with clean, dry N 2 to removethe water vapor before pump down. Soft pumping and venting also tendsto reduce turbulence that can release particles trapped within mechanicalfixtures.Gas Temperature Profile DuringVacuum Pump Down40- - - Air (55% r h )--Oo~-20i~ ~ , ~E -40p-I~FIG.
5.40TO = 2 5 ~.~/~ ~l "-60-80P--'~= 650 Torr- ^ 1'0II2V = 47.3 liters= 4.0 seciI4It/zI6II8I10M e a s u r e d gas temperature versus reduced time for a t = 40 sec p u m p d o w n of a 47 literv o l u m e from 560 Torr to ~ 30 mTorr (Source: B. Y.-H. Liu, Semiconductor Int'l, p. 75, M a r c h 1994).166R.
POWELL AND S. h.1. ROSSKAGEIClamp RingMechanical clamp rings get coated along with the wafer during PVD andare also subject to periodic heating (e.g., by the substrate heater table) andcooling when lifted to release the wafer. These rapid thermal cycles cancause particle shedding in poorly adhering materials. It is possible todampen such fluctuations by using a higher-mass clamp (e.g., stainlesssteel) in which temperature swings will be minor. The surface finish of thewafer edge, if sufficiently rough, can abrade during clamping and generateSi dust.
To the extent that this can be controlled by the user, wafers withsmooth edge finish are preferred. These specific issues can be addressed byusing an electrostatic chuck; however, ESCs can also generate particles bythe microscopic-scale physical contact between the rough wafer backsideand the ceramic dielectric. Also.
the presence of backside particles can impede this physical contact and reduce heat transfer efficiency.Shir IdsShields are generally regarded as passive elements that protect the chamber walls from deposition; however, they can be an active source of particles. A shield i n general will consist of several separate pieces ( a shieldset) held together with threaded fasteners or simply dropped into placeand held in position by gravity. By design. threaded fasteners require surfaces that rub against each other and can therefore generate particles consisting of the fastener and/or the lubricant (e.g., TiS, or MoS2).This is anongoing process. as shields are periodically disassembled for cleaning orto access the chamber for maintenance. Therefore, shield sets with a minimum number of parts are desired.
I n general. all screws generate particulates no matter what dry lubrication is used, and threaded hardware ofany kind should be minimized i n PVD systems. Since it is not uncommonfor the majority of the material sputtered t'rom the target to wind up depositing onto the shieIds. these deposits can quickly build up to the pointwhere particle generation and flaking are of concern - a particular problem with TIN deposits, which typically have high film stress and poor adhesion. Flaking is also enhanced by thermal stress of the shields, whichare heated and cooled as the sputtering plasma goes on and off duringPVD. Therefore, some designs incorporate internal lamp heating of theshields to maintain constant shield temperature during the off cycles ofthe sputtering plasma. as well as for more effective shield degassing during chamber bakeout.
Proprietary coatings and surface texturing havealso been used on shields to improve the adhesion of sputtered materialSPUTTERING TOOLS167and increase the time between shield cleaning to as much as 5 0 0 / z m fordifficult films such as TiN.Tooling is generally reused, and this requires removal of the depositedmaterial and cleaning of the parts back to their initial clean-room compatible condition. Many semiconductor fabs contract outside vendors for thistask that could be done by sandblasting or wet chemical means such as dipping in an acid bath, although the latter creates environmental concernswith hazardous waste disposal.
In a typical case, a stainless steel shield setmight be sandblasted, ultrasonically cleaned, and then repackaged underclean-room conditions. The cost of the cleaning procedure plus the lostprocess time needed to change the tooling and recondition the processchamber for deposition is significant and might add as much as 5 - 1 0 % tothe overall cost of ownership of the PVD tool. In general, it is desirablethat a shield set and/or collimator last about the same number of processhours as the target on the magnetron so that both can be changed at thesame time. This time varies from material to material, but might typicallybe 3000 to 6000 wafers worth of deposition.Gas Delivery SystemIt has become a standard practice for process gas lines to use electropolished tubing with orbital butt-welded joints, which prevents rough surfacesand internal crevices that might trap contaminants.
Also, right-angle buttwelded elbows are used to prevent sharp bends in the gas line that can concentrate stress and thereby generate particles. The particle levels in theprocess gas as supplied are generally several orders of magnitude cleanerthan the best clean rooms, or the gas can be filtered at the point of use toremove all but the finest particles ( < 100 ,&).
As a result of practices suchas these on passive components, the majority of particles produced in thegas delivery system are from active components such as valves, mass flowcontrollers, and pressure regulators.In general, it is desired to close-couple the gas flow system (e.g., flowcontroller, shut-off valve, particle filter) as close as possible to the sputtering chamber. This reduces the amount of tubing held at high vacuum aswell as shortens the response time. Needless to say, each modular processchamber typically has its own dedicated gas flow system; however, thesesystems may share a c o m m o n gas bottle or tank. Each process chamber isalso outfitted with a capacitance manometer (usually 0.1 or 1.0 Torr fullrange) as well as an ion gauge. The gas control system on the processchamber can then be feedback-controlled from the capacitance manometer.Depending on the tool manufacturer, the gas operating system will specify168R.
POWELL AND S. M. ROSSNAGELeither an absolute pressure (e.g., 1.0 mTorr) or a fixed flow (e.g., 10 sccm)and then use feedback control to maintain that value during processing. Inaddition, if a gas mixture is used (e.g., Ar + N2), control can be based onmaintaining either a fixed total pressure and relative gas concentrations(e.g., 1 mTorr, 80% Ar, 20% N2) or fixed flows of each species (e.g.,8 sccm of Ar, 2 sccm of N2).
Total pressure control is somewhat complicated by system-related changes in the net pressure, which may be causedby the initiation of a plasma, the breakdown of a gas in that plasma, heating and degassing of chamber and/or fixtures, or pump loading.PVD Target and SourceThere are number of particle generation mechanisms associated with targetquality and related PVD source performance. For example, microbursts ofgases that were trapped within the microvoids of a low-density target canbe released as the target erodes, and the electrical arcing of the sourcecaused by these high-pressure gas bursts can lead to particle generation.This is being addressed by target suppliers through improved manufacturing and by equipment suppliers through PVD sources with electronic arcsuppression.
These and target/source-related issues of particle generationare discussed in more detail in Chapter 11. Particle contamination duringDC magnetron sputtering has been relatively unexplored compared towork on particle formation during reactive plasma etching or plasmaassisted CVD. Recent work using laser light scattering [5.42] suggests thatthe mechanisms of particle generation, transport, and trapping in PVD aredifferent from those of plasma etching and CVD and that this is probablycaused by the inherent spatial nonuniformity of the magnetically enhancedplasma of a DC magnetron.5.3.7ROBOTIC HANDLINGRobotic wafer motion in a PVD cluster tool is a special challenge associated with simultaneous requirements of pressure, temperature, and contamination [5.43]. In particular, wafer handling within the tool must often becarried out under high or even ultrahigh vacuum ( ~ 10 -8 to 10 -9 Torr).Unlike motion under atmospheric pressure, this means that simple vacuumsuction cannot be used to securely hold the wafer in place during rapidchanges in position.
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