Low-pressure CVD and Plasma-Enhanced CVD Ronald Curley, Thomas McCormack, and Matthew PhippsCVD overview • “Chemical Vapor Deposition” • Thin films on substrate • Chemical oven + insert gas = deposited filmCVD overview • Four steps[1]: 1. Transport gas species to surface 2. Gas species absorption into surface 3. Reaction deposits products 4. Remove unwanted products and leftover reactantCVD overview • Velocity ratio (molecules/s, not meters/s!): ▫ Mass transport velocity Depends on pressure ▫ Surface reaction velocity Does not depend on pressure • Low ratio -> pure; well-controlled thickness • High ratio -> contaminants; poorly-controlled thicknessCVD overview • Atmospheric-pressure CVD (APCVD) velocity ratio too high: ~1:1 • Mass transport velocity proportional to 1/pressure[2] • 1 atm ~= 100 kPaLPCVD • LPCVD typical pressure: 10-1000 Pa • Ratio 1:100–1:10,000! • Reduced film variation • Increased purityLPCVD • Substrate inserted • Tube evacuated to 0.1 Pa • Process gas (“working gas”) added at 10-1000 Pa • Reaction performed • Substrate removed Source: [3]LPCVD • Best for polysilicon, using SiH4 • Oxides, PSG as well • Nitride encapsulation Source: [4]LPCVD Advantages: Disadvantages: • Excellent uniformity of thickness & purity • Simple • Reliable/reproducible • Homogenous layer • Slows down deposition rate • Requires high temperatures, <600°CPECVD • Plasma added with reactive gases • RF voltage excites plasma • Only electrons are hot, not ions: low temperatures possiblePECVD Film Reactive Gas Thermal Deposition CVD (Celsius) Plasma Enhanced CVD (Celsius) Silicon Nitride SIH4 or S1H2C12 & NH3 750 200-500 Silicon dioxide SiH4 & O2 350-550 200-400 Amorphous silicon SiH441 550-650 200-400 Picture: http://timedomaincvd.com/CVD_Fundamentals/plasmas/capacitive_plasma.html Table: http://www.eng.auburn.edu/~tzengy/ELEC7730/ELEC%207730%20Fall%202003/Fall%202003%20Presentation%201/Park%20-%20PECVD.pptPECVD http://www.hitech-projects.com/dts/docs/pecvd.htm Conformal step coverage of PECVD SixNyPECVD Advantages Disadvantages • Low temperature • Higher film density • Higher dielectric constant • Good step coverage • Chamber easy to clean • Equipment is expensive • Plasma bombardment is stressful • Small batch sizes: 1-4 wafers, one side • Compare to LPCVD: at least 25 wafers, both sides[5]Questions?References: [1] A Stoffel, A Kovács, W Kronast and B Müller, “LPCVD against PECVD for micromechanical applications” J. Micromech. Microeng., Vol. 6 No. 1 pp. 20-33, Mar. 1996 [2] Ivanda, Mile, “Implementation and Development of the LPCVD Process”, [Online], Available: http://www.irb.hr/en/str/zfm/labs/lmf/Previous_projects/LPCVD/ [Accessed: 24 Nov. 2011] [3] Dow Corning, “Chemical Vapor Deposition”, [Online], Available: http://www.dowcorning.com/content/etronics/etronicschem/etronics_newcvd_tutorial3.asp?DCWS=Electronics&DCWSS=Chemical%20Vapor%20Deposition [Accessed: 25 Nov. 2011] [4] Doolittle, Alan, “Thin Film Deposition and Epitaxy”, [Online], Available FTP: http://users.ece.gatech.edu/~alan/ECE6450/Lectures/ECE6450L13and14-CVD%20and%20Epitaxy.pdf [Accessed: 23 Nov. 2011] [5] MEMSnet, “MEMS Thin Film Deposition Processes”, [Online], Available: http://www.memsnet.org/mems/processes/deposition.html [Accessed: 23 Nov. 2011] [6] “Plasma-Enhanced CVD.” Hitech-Projects. 2011. 28 Nov. 2011 <http://www.hitech-projects.com/dts/docs/pecvd.htm>. [7] Mahalik, Nitaigour. Introduction to Microelectromechanical Systems (MEMS). New Delhi, India. Tata McGraw-Hill, 2007. [8] “Plasma (Physics).” Wikipedia. 29 Nov. 2011. 29 Nov 2011. <http://en.wikipedia.org/wiki/Plasma_%28physics%29>. [9] “Fundamentals of Chemical Vapor Deposition – Plasmas for CVD.” TimeDomain CVD, Inc. 2002. 29 Nov.
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