4.0. Chemical Vapor Deposition

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4.0. Chemical Vapor Deposition pdf




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4.0. Chemical Vapor Deposition - page 1
Chapter 4 - Chemical Vapor Deposition 4.0. Chemical Vapor Deposition 4.1. Introduction Chemical Vapor Deposition (CVD) is the process of depositing films by reacting chemical vapors to produce a film on a substrate. CVD reactions may be activated by heat (CVD), RF energy (plasma enhanced - PECVD) or by light (photon induced - PHCVD). CVD processes may be used to deposit a wide variety of insulating - dielectric films, poly and single crystal silicon and metal films. 4.2. Basic theory The CVD process involves one or more gases being introduced into a reactor, most commonly at low pressure. The gases react at the surface of a substrate to form a film on the substrate surface. Figure 4.1 illustrates the steps in a CVD system reaction. Feed gases in Reactants diffuse to the surface adsorption Silicon Reaction products and unreacted gas out Film forming reaction Surface reaction Products diffuse away from the surface Silicon atom desorption Oxygen atom Hydrogen atom Figure 4.1. CVD deposition of second dioxide from silane. In figure 4.1, silane (SiH 4 ) and oxygen (O 2 ) are introduced into a CVD reactor. The flowing gases form a stag- nant boundary layer over the substrate surface that the gases must diffuse through to reach the surface. The gases are adsorbed onto the surface where they react to form silicon dioxide (SiO 2 ) giving off hydrogen (H 2 ) that then desorbs from the surface and diffuse away. The reaction is: SiH 4 ( gas ) + O 2 ( gas ) → SiO 2 ( solid ) + 2H 2 ( gas ) (4.1) There are many other possible reactions, this reaction is presented as an example. As mentioned in the intro- duction the process may be activated by heat, RF or photo energy. CVD reactions may also take place at atmospheric pressure (APCVD) or low pressure (LPCVD). 4.3. Equipment 4.3.1. Horizontal furnace CVD systems Early CVD systems were horizontal tube furnaces modified to operate under vacuum and with expanded gas control capabilities. Figure 4.2 presents a schematic diagram of such a system. The key elements of the system are: • A water-cooled door seals the quartz process tube so that a vacuum can be created in the tube. The water cool- ing keeps the door o-ring seals from breaking down at the relatively high process temperatures required. • The quartz process tube is surrounded by a three zone heating element. • 3 zone temperature control with a center master zone to set the furnace temperature and slave end zones. The end zones compensate for heat lost out the ends of the furnace tube and allow for better temperature uniformity up and down the furnace tube. Horizontal furnaces can achieve plus or minus ½ degree centigrade temperature Copyright © 2000 - 2004 IC Knowledge LLC, all rights reserved. 11
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4.0. Chemical Vapor Deposition - page 2
Chapter 4 - Chemical Vapor Deposition uniformity at temperatures up to 1,200 degrees centigrade although CVD processes are typically lower temper- ature processes. Modern horizontal furnaces suspend the wafers on a cantilever that does not touch the furnace sidewalls. Older style furnaces utilized quartz boats that slid along the bottom of the quartz furnace tube. The friction of quartz on quartz generated large numbers of particles. The cantilever systems have 100 to 1,000 times lower particle levels than older sled style systems. A roots blower and roughing pump is used to pump down the system and maintain vacuum during processing (see chapter 16 for a discussion of vacuum pumps). A pressure controller and throttle valve are used to control the system pressure during processing. The pressure control system allows pressure to be set independently of gas flow. Without a throttle valve the gas flow and pumping package pumping speed would result in a fixed system pressure. The throttle valve can also be used to compensate for changes in the pumping speed of the vacuum pumps as the vacuum pumps age. A gas control system with valves and mass flow controllers. The gas control system allows the gases flowing and flow rates to be controlled during processing. A central computer is used to control all of the elements of the system - temperature, pressure, gas flow, etc. throughout the process sequence. Water Cooled Door Three Zone Heating Element Wafers Gas Inlet Quartz Tube Mass Flow Controllers Valves Gas Controller Pressure Sensor Throttle Valve Thermo- couple Slave Temp Control Thermo- couple Master Temp Control Thermo- couple Slave Temp Control Pres- sure Control 1 2 3 Gases Roots Blower Host Computer SCR Pack SCR Pack SCR Pack 480 volts Input Roughing Pump Figure 4.2. Horizontal CVD furnace system. There are several issues with batch systems. Perhaps the most serious is that the gases are reacting and becom- ing depleted as the gases flow down the process tube. Without some sort of compensation scheme a thicker film results nearer to the gas inlets than further downstream. Sloping the temperature profile so that the temperature is higher as the gas flows downstream can compensate for gas depletion. Increasing the rate of gas flow can also help to compensate for gas depletion. Sloped temperature profiles can compensate to some extent for film thickness variation, but temperature differences can result in different film properties down the process tube. The hot wall nature of quartz tube batch systems results in significant film deposition on the process tube sidewalls. Over time film buildup on the process sidewalls can begin to flake off resulting in particles problems. A variety of in-situ and ex-situ cleans are used to try to control particle problems. 4.3.2. Barrel type epitaxial reactors Early epitaxial silicon (single crystal silicon deposited on a substrate that follows the underlying crystal orien- tation of the substrate) reactors were barrel or pancake type reactors that processed multiple wafers at a time, see figure 4.3 for an example of a barrel reactor. Key feature of the epitaxial reactor illustrated in figure 4.3 includes: Copyright © 2000 - 2004 IC Knowledge LLC, all rights reserved. 12
4.0. Chemical Vapor Deposition - page 3
Chapter 4 - Chemical Vapor Deposition A graphite susceptor is suspended inside a quartz process chamber and holds the wafers on a sloped surface by gravity. A set of heat lamps - reflector assemblies surrounds the quartz process chamber and provides rapid heat-up of the wafers. Barrel type systems may be used for atmospheric pressure epitaxial deposition or for low-pressure epitaxial deposition when outfitted with a vacuum pump. Gas inlets are designed to swirl gas down around the susceptor depositing the films. The gases exit the chamber through an exhaust port at the bottom of the camber. Rotating hanger Cover Seal Gas injectors Heat lamp blocks (front units not shown) Wafers Susceptor Quartz bell jar Exhaust Figure 4.3. Barrel type epitaxial reactor. 4.3.3. Cluster tools As linewidths shrunk below one micron, requirements for greater uniformity and particle control led to the switch over from batch or multiple wafer at a time CVD systems to single wafer at a time CVD systems and the advent of the cluster tool. In a cluster tool a central robot services load-unload ports and multiple process chambers. Cluster tools allow multiple chambers of the same or different process type to be combined on one common plat- form and enable complex multiple step processes to be performed in a computer controlled sequence continuously under vacuum. Figure 4.4 illustrates a cluster tool. Key features of the cluster tool illustrated in figure 4.4 include: • A central robot that transfers wafers from the input and output cassettes to the individual process chambers. The central robot operates under vacuum so that a wafer can be transferred from one process chamber to the next while still under vacuum. The input and output cassettes are also typically under vacuum to minimize transfer times. In the ideal configuration the central cluster tool robot and input and output cassettes are all standards compatible so that process chambers from different vendors can be mixed and matched on the cluster tool for a best of breed approach. In practice only a few process chamber vendors make standards compatible chambers and the largest process chamber manufacture - Applied Materials only supports their own proprietary cluster tools. • The cluster tool has multiple process positions where a variety of different process modules can be attached. The process modules may be different CVD modules or even different process types. Example of cluster tool approaches might include CVD deposition chambers mixed with etch-back chambers for planarization so that a deposition - etch-back - deposition - etch-back process may be performed. Alternately a deposition module might be mixed with a rapid thermal processing module to allow the deposited film to be annealed. Cluster Copyright © 2000 - 2004 IC Knowledge LLC, all rights reserved. 13
4.0. Chemical Vapor Deposition - page 4
Chapter 4 - Chemical Vapor Deposition tools are very widely used in state-of-the-art wafer fabs today providing CVD, RTP (see chapter 13), dry etch (see chapter 5) and dry strip (see chapter 6) functions). RTP chamber Deposition chamber 1 Deposition chamber 2 Open position Cassette load lock with door removed Cassettes of wafers Cassette load lock with cover removed Transfer area with cover removed Wafer transfer arm Figure 4.4. Cluster tool. 4.3.4. Single wafer CVD chamber Figure 4.5 illustrates a single wafer CVD chamber used for submicron processing. Gas inlet Gas distribution “showerhead” Wafer Wafer transfer door Suseptor Vacuum pumping out Heat lamps under a quartz window Vacuum pumping out Figure 4.5. Single wafer CVD chamber. Adapted from [1]. Operation of the system illustrated in figure 4.5 is as follows: A wafer is inserted onto the susceptor through the wafer transfer door. The wafer transfer door closes sealing the vacuum chamber and the chamber is pumped down through the opening arrayed symmetrically around the wafer chuck. 14 Copyright © 2000 - 2004 IC Knowledge LLC, all rights reserved.
4.0. Chemical Vapor Deposition - page 5
Chapter 4 - Chemical Vapor Deposition The heat lamps under the susceptor turn-on heating up the wafer. Gases are introduced through the top of the system and flow down through the “showerhead”. The holes in the showerhead distribute the gas out over the surface of the wafer. • The gases interact on the heated wafer system depositing the desired film. The cold chamber walls help to prevent deposition on the walls minimizing particles. The chamber may also undergo an in-situ clean in between deposition steps. CVD systems such as this one are relatively simple, however pure CVD processes require temperatures >450 o C Generally limiting CVD to usage prior to metallization. 4.3.5. Single wafer PECVD system When linewidths were greater than approximately one micron, batch system such as the one illustrated in fig- ure 4.2 were modified with the addition of radio frequency generators (RF) to create PECVD systems (see chapter 5 for a discussion of RF plasmas). PECVD system add plasma activation to CVD system enabling deposition pro- cesses with deposition temperatures of 250 o C to 400 o C making these deposition processes capable of depositing films over metals without exceeding the maximum stable temperatures for the metals. As linewidths shrunk below one micron the same requirements for film uniformity and particle control that drove CVD system to single wafer processing drove PECVD systems to the same requirements. Figure 4.6 illus- trates a CVD system outfitted for PECVD deposition. 350KHz 13.56MHz Gas inlet L.P.F. Gas distribution “showerhead” Insulator Wafer Wafer transfer door Suseptor Vacuum pumping out Heat lamps under a quartz window Vacuum pumping out Figure 4.6. Single wafer PECVD system. Adapted from [1]. The major difference between the CVD system illustrated in figure 4.5 and the PECVD system illustrated in figure 4.6 is the addition of the R.F. generators for plasma generation. The use of two generators allows the plasma density and ion energy to be independently adjusted, the high frequency 13.56MHz generator primarily determines plasma density and the low frequency 350KHz generator primarily determines the ion energy. The disadvantage of PECVD systems is low film density and a tendency of PECVD films to “cusp” at sharp corners pinching off deposition into high aspect ratio features leaving gaps in the film. 4.3.6. High density plasma (HDP) systems HDP systems utilize high density plasma sources such as Inductively Coupled Plasmas to increase plasma den- sity and also provide tunable ion bombardment. The addition of wafer biasing with ion bombardment cause the Copyright © 2000 - 2004 IC Knowledge LLC, all rights reserved. 15
4.0. Chemical Vapor Deposition - page 6
Chapter 4 - Chemical Vapor Deposition “cusps’ normally seen during PECVD to be etched off by the ions and results in better gap fill, more planarization and a denser film. Remote plasma clean chamber Temperature controlled ceramic dome Dual R.F. coils Gas inlet Throttle valve out to vacuum pump Wafer chuck Wafer Figure 4.7. HDP deposition system. Adapted from [2]. The HDP system illustrated in figure 4.7 includes the following features: A remote plasma clean chamber used for chamber cleans between deposition runs. Dual inductively coupled R.F. coils that achieve high plasma densities with tunable bias. Distributed gas inlets and high flow symmetric pumping to insure uniform gas distributions. 4.4. Recipes Table 4.1 summarizes some CVD applications, the deposited film and the typical system used for the deposi- tion step. Table 4.2 summarizes some CVD recipes. Table 4.1. Dielectric deposition applications summary. Application Shallow Trench Isolation polish stop layer. Shallow Trench Isolation trench fill. Gate polysilicon anti-reflective coating. Sidewall spacers. Intermetal Dielectric layer 0 etch stop. Intermetal Dielectric layer 0. Intermetal Dielectric layer 1+ with aluminum lines. Intermetal Dielectric film 1+ with damascene copper. Film Si 3 N 4 SiO 2 Si 3 N 4 Si 3 N 4 Si 3 N 4 Doped SiO 2 SiO 2 FSG or SiOC Requirements High density film to resist oxidation during trench corner rounding oxidation step. High density film with good gap filling properties. Optical properties. Etch rate difference relative to SiO 2 High density film with good barrier properties and etch rate difference relative to SiO 2 . Thick film without cracking. Typically phosphorus doped or borophosphorous doped SiO 2 . Good gap fill. Low-k films such as fluorine doped oxide or car- bon doped oxide. Can be deposited with HDP or PECVD. PECVD is preferred due to lower com- plexity although many FSG films were deposited in HDP system already installed in fabs from pre- vious generation products. System CVD HDP CVD CVD HDP PECVD HDP HDP or PECVD. Copyright © 2000 - 2004 IC Knowledge LLC, all rights reserved. 16
4.0. Chemical Vapor Deposition - page 7
Chapter 4 - Chemical Vapor Deposition Table 4.1. Dielectric deposition applications summary. Application Intermetal Dielectric film 1+ etch stop layer. Passivation Film Si 3 N 4 or SiC Si 3 N 4 Requirements Etch rate difference relative to FSG or SiOC. Low- est k value possible also desired. Good barrier properties System PECVD PECVD Table 4.2. Common CVD deposition reactants. Film Al Cu Si Reactants TIBA, DIBAH, DMAH Cu(hfac) 2 + H 2 or Cu 1 (hfac)L SiH 4 SiCl 2 H 2 SiHCl 3 SiCl 4 SiH 4 SiH 4 --- SiH 4 + NH 3 SiH 4 + NH 3 + N 2 O SiCl 2 H 2 + NH 3 SiH 4 +N 2 O SiH 4 + O 2 or SiH 4 + N 2 O TEOS + O 2 TEOS + O 2 SiH 4 + O 2 TEOS + O 2 SiCl 2 H 2 + N 2 O O 2 or H 2 O TiCl 2 + NH 3 or TiCl 3 + H 2 /N 2 or TDMAT + NH 3 WF 6 + SiH 4 or WF 6 + H 2 System LPCVD LPCVD APCVD APCVD APCVD APCVD LPCVD LPCVD PHCVD PECVD PECVD LPCVD PHCVD PECVD PECVD APCVD LPCVD LPCVD LPCVD Thermal LPCVD LPCVD Composition --- --- Crystalline Crystalline Crystalline Crystalline Crystalline Polycrystalline --- Si x N y H z Si x O y N z Si 3 N 4 (H) SiO 2 SiO 1.9 (H) SiO x SiO 2 (-OH) SiO 2 (H) SiO 2 (-OH) SiO 2 (Cl) SiO 2 --- --- Step coverage Conformal Conformal --- --- --- --- --- Conformal --- Non Conformal?- Non Conformal? Conformal --- Non Conformal Conformal Isotropic flow Non conformal Conformal Conformal Conformal Conformal Conformal Temperature ( o C) <250 350-450 950-1,050 1,050-1,150 1,100-1,200 1,150-1,250 550-700 580-650 50-250 250-350 250-350 700-800 50-200 250 400 400 450 700 900 700-1,200 400-700, or >700 400-500 Si 3 N 4 SiO 2 TiN W 4.5. References [1] Peter W. Lee, Shinsuke Mizuno, Amrita Verna, Huyen Tran and NBang Nguyen, “Dielectric Constant Sta- bility of Fluorine-Doped Plasma Enhanced Chemical Vapor Deposited SiO 2 Thin Films,” J. Electrochem. Soc., p. 2015, Vol. 143, No. 6 (1996). “Update,” Applied Materials, Vol. 1, Issue 1 (2002) [2] Copyright © 2000 - 2004 IC Knowledge LLC, all rights reserved. 17
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