Roland A. - PVD for microelectronics (779636), страница 24
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Figure 5.14 shows the general types of vacuum pumps available for semiconductor processing under atmospheric toUHV conditions. Fortunately for PVD, the pumping of hazardous and toxicgases is not an issue (as it can be with CVD and plasma etching) since theprocess gas is typically inert Ar or a mixture of Ar/N 2 for reactive deposition of TiN. This greatly simplifies pump selection and also allows certainpumps to serve dual purposes. For example, the same dry pump could beused as the primary roughing pump of a transfer module and the backingpump for a turbomolecular roughing pump of a UHV PVD module.Thus, pump selection for a PVD cluster tool is determined primarily bythe pressure and gas throughput requirements of the process; the time topump out the chamber volume; and the usual considerations of cost, reliability, and cleanliness common to all semiconductor process tools.
With regard to cleanliness, the major contaminant of interest to PVD continues tobe water vapor, and the ability of pumps to maintain a low partial pressureof water vapor during deposition ( < 1 x 10 -8 Torr) is required to producefilm quality suitable for ULSI devices.R. POWELL AND S. M. ROSSNAGEL122FIG.
5.14General types of vacuum pumps available for semiconductor processing.Figure 5.15 presents one possible pumping scheme for a PVD clustertool, and Figs. 5.16-5.18 illustrate the major pump types used on PVDtools for low, medium, and high vacuum, respectively: dry pump(Fig. 5.16), turbopump or "turbo" (Fig. 5.17), and cryopump or "cryo"(Fig. 5.18). In the system configuration shown in Fig. 5.15, a common, dryroughing pump is used to rough out the process chambers to a pressure lowenough for use of a dedicated cryopump. The turbo/drag pump on thechamber can be used to further rough out the process chamber and/or pumpgases released by the cryopump during its regeneration cycle (discussed inmore detail later in this section). The industry trend is away from oilsealed mechanical pumps and toward the use of oil-free, dry, mechanicalpumps for roughing and backing purposes, using either established multistage pumps or more recent orbital scroll pumps.
Dry pumps virtuallyeliminate the possibility of oil backstreaming and offer reduced maintenance. Pumping is provided by trapping and removing small pockets ofgases in several stages from the inlet to the exhaust, with each stage compressing the gases more.Small turbomolecular or turbopumps are sometimes used to evacuatewafer load stations from atmospheric pressure as well as being used ondegas modules. The turbopump is a clean, compression pump that basi-SPU'Iq'ERING TOOLS123FIG. 5.15 Pumping scheme for a PVD cluster tool, illustrating that a mix of pump types is used depending on the level of vacuum and process conditions required.
For simplicity, the pumping on onlyone of the three process chambers is shown.cally consists of an alternating stack of rotating and fixed disks into whichhave been machined a large number of angled blades (see Fig. 5.17).Pumping action occurs when gas molecules bounce off of the rapidly moving rotors (e.g., 70,000 rpm), which are angled so as to increase the molecules' momentum in the direction of the pump exhaust. Turbopumps offervery high pumping speed and constant throughput at moderate pressures,and are sometimes used to rough out UHV chambers.
On the other hand,124R. POWELL AND S. M. ROSSNAGELRepresentative dry pump used for low-vacuum application in a PVD tool (dry iQ-seriespump shown, courtesy of Edwacds High Vacuum International, Wilmington, MA).FIG. 5.16turbopumps are slow at pumping light gases and water vapor. Also, a backing pump must be used to prevent the turbopump from being overloadedby the gas load it is compressing into its foreline. For example, the turbopump shown in Fig. 5.17 has N 2 pumping speed of 70 liter/sec and mightbe backed with a 4-1iter/min mechanical pump. The trend in turbos is toward compound pumps that combine a regular turbopump with a so-calledmolecular drag pump that is like a turbopump (it uses a rotating drum ordisk) and allows the compound pump to be exhausted at pressures highenough to use simple, low-cost backing pumps.SPU'Iq'ERING TOOLS125FIG.
5.17 Representative turbopump used for medium- to high-vacuum application in a PVD toolwith shaft removed to reveal alternating disks of rotors and stators (turbopump model V70 shown,courtesy of Varian Vacuum Products, Lexington, MA).PVD processing requires a contamination-free, high-vacuum pump withhigh pumping speeds for both process gases and residual gases. As a resuit, PVD chambers are primarily pumped using cryopumps due to theircleanliness (no pump oil means no hydrocarbons) and high pumping speedfor water. In a two-stage cryopump, water vapor and other condensiblegases are pumped in the first stage via physisorption on a cryogenicallycooled surface (T ~ 77 K), while the second stage is used to trap gaseswith high vapor pressuresuch as Ar, He, and H I in the molecularscale pores of a charcoal array (T ~ 15 K).
Given typical PVD chambervolumes (20-50 liters) and pump gate valves (e.g., 20-cm internal diameter with a 10-inch conflat flange), a cryopump equivalent to the one shownin Fig. 5.18 might be used. It should be noted that modern PVD tools(since 1990) are generally operated without throttling of the pumpi.e.,at full pumping speed. This means that the base pressure of the tool (e.g.,1 X 10 -8 Torr) is also effectively the base pressure during operation. Also,during high-temperature deposition, radiant heat from the sample holder( ~ 500~may be sufficient to warm the front of the cryopump and"dump" the pump. In this case, it is necessary to provide some protective126R.
POWELL AND S. M. ROSSNAGELFIG. 5.18 Representative cyropump used for high-vacuum application in a PVD tool (ONBOARD| high-vacuum pump, courtesy of CTI-Cryogenics, Mansfield, MA). For use on an ultrahighvacuum PVD process module, the elastomer-sealed mounting flange would be replaced by a metalsealed conflat flange.radiation shielding for the cryopump. Cryopumps have a high pumpingspeed for argon, but they cannot handle very high Ar flow rates for long,due to their limited absorption capacity.
Under these conditions, thecryopump would be throttled back. Unfortunately, this also reduces thepumping speed for water vapor, which can have an adverse impact on oxidation-sensitive hot PVD processes such as reflow A1 and the two-step"cold-hot" A1 process (see Chapter 7). Hence, some PVD modules utilizean unthrottled turbopump with a relatively small pumping speed for argon(e.g., 200 l/sec) in tandem with a cold trap having high capacity and pumping speed for water (e.g., > 1000 1/sec).There is a slight reliability advantage to cryos over turbos in that cryostend to fail slowly, making loud noises and generally breaking down withina day or two.
Hence, there is usually enough warning to schedule a routinepump replacement without losing any wafers. On the other hand, turbostend to fail quickly and without much external notice. Conversely, there isSPUTTERING TOOLS127a slight maintenance disadvantage with cryos associated with the downtime associated with regeneration of the p u m p - - n a m e l y , the cryogenicsurfaces have a finite gas capacity that requires a periodic bakeout (heatedN 2 gas is flowed through the pump) to remove the adsorbed species. Fastregeneration cryopumps are therefore of great interest. These pumps workby only heating up the colder, second stage.
This can greatly reduce regeneration times (from ~ 2.5 hr at 500~ to 0.5 hr); however, since the adsorbed water is not baked off, such pumps are best used where the absolutevolume of residual water vapor is l o w - e.g., the internal chambers of aloadlocked cluster tool.Finally, we note the recent application of nonevaporable getter (NEG)technology for PVD applications, which has been used to reduce the timefor a PVD chamber to recover to high-vacuum base pressure after chamberventing [5.17, 5.18]. Gettering materials remove residual active gases suchas 02, N 2, CO 2, H20, and H 2 from a vacuum chamber by forming stablechemical bonds or compounds. Semiconductor processing has long takenadvantage of high-vacuum pumping based on the gettering action of a thinfilm of reactive titanium metal ~ either through evaporation as in a titanium sublimation pump (TSP) or by sputter deposition as in a sputter-ionpump, such as the Varian Vac-Ion TM design.
Less familiar to IC manufacturing are bulk getters (or nonevaporable getters) that have been used formany years in other industries ~ e.g., to produce high vacuum in linearaccelerators for high-energy physics research. NEGs are alloys of metalsfrom Group IV-A of the periodic table - - such as Ti, Th, and Zr ~ that arecapable of dissolving their own oxides in the solid state at elevated butmoderate temperature (e.g., 350-500~ for certain Zr-alloys). Therefore,even though the NEG surface eventually saturates during use, it can be renewed by a vacuum anneal that produces a clean and highly reactive metalsurface. As a practical matter, NEG activation annealing might be done aspart of a preventive maintenance cycle during chamber bakeout.Mounted internally within the PVD chamber in conjunction with an external cryopump, the NEG boosts pumping speed in the high-vacuumregime ~ particularly for H 2, which is difficult to cryopump (see Fig.
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