Physical Vapor Deposition (PVD) Zheng Yang

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1 Physical Vapor Deposition (PVD) Zheng Yang ERF 3017, Page 1

2 Major Fabrication Steps in MOS Process Flow UV light Mask oxygen Silicon dioxide photoresist exposed photoresist oxide Silicon substrate Oxidation (Field oxide) Photoresist Coating Mask-Wafer Alignment and Exposure Exposed Photoresist Photoresist Develop Ionized CF 4 gas photoresist oxide Ionized oxygen gas oxide oxygen gate oxide Dopant gas Silane gas polysilicon Ionized CCl 4 gas oxide Oxide Etch CF 4 or C 3 F 8 or CHF 3 O 3 CF4 +O 2 or CL 2 Photoresist Strip Oxidation (Gate oxide) Polysilicon Deposition Polysilicon Mask and Etch Scanning ion beam silicon nitride Contact holes Metal contacts ox S G D Ion Implantation top nitride G G S D S D S G D Active Regions Nitride Deposition Used with permission from Advanced Micro Devices Contact Etch drain G S D Metal Deposition and Etch Page 2

3 Thin Film Deposition Spin-on Films Polyimide (PI), photoresist (PR) Spin-on glass (SOG) Physical Vapor Deposition (PVD) Evaporation Sputtering Chemical Vapor Deposition (CVD) Oxidation LPCVD PECVD Page 3

4 Collision rates with surface. Semiconductor fabrication is based on planar processes, so surface-gas interactions are critically important. Mass=m Number of collisions per unit time per unit area (sec -1 cm -2 ): = (N/V) v/4 Area=A ( p / 4kT) 8kT m For m=50 (amu), T=300 o K, and p given in Torr, ~ p collisions / (sec cm 2). Page 4

5 Numerical examples of surface flux Pressure (Torr) Pressure (Pa) Surface flux (300 o K, atomic weight= 50): molecules/(sec cm 2 ) 1 atmosphere E E+23 medium vacuum E E+21 high vacuum 1.00E E+17 very high vacuum 1.00E E E+14 ultra high vacuum 1.00E E E+11 Note: for Si crystal, surface atomic concentration is approximately 6.3x10 14 atoms/cm 2. Thus under ultra high vacuum (UHV) conditions, the surface could be contaminated in approximately 2000 seconds and in only 2 seconds in very high vacuum. Page 5

6 Thermal Evaporator Crucible containing the material to be deposited is heated. Good for materials with low melting point/high vapor pressure. Problems: Heating of crucible material provides contamination. Radiation from the thermal source heats the substrate. Limited source size (volume) for refractive materials. Page 6

7 PVD: Thermal evaporation Simple schematic: Substrate To vacuum Aperture Boat or filament +V Page 7

8 Step coverage in thermal deposition (PVD). Planetary or perpendicular geometry. Rotating planetary geometry. Additional heating Page 8

9 Rotating planetary system. Photograph of an e-beam evaporation system with planetary substrate holder which rotates simultaneously around two axes. Page 9

10 Vapor pressures of some metals as function of temperature Vapor Pressue (Torr) Ag Al Au Cr Cu Ni W Temperature ( o C) file: metal vap press.sgr revised 01/31/2000 jty Page 10

11 Electron-beam heating. Thermal heating is inconvenient for evaporative deposition for several reasons: Radiation from the thermal source heats the substrate. Limited source size (volume) for refractive materials. Thus electron beam heating is commonly used. Small well-defined region heated Material serves as reservoir. Minimal contaminant due to heading of crucible etc. Deposition rate controlled by ebeam intensity. Minimal heating to substrate. Page 11

12 E-beam source Page 12

13 Electron beam evaporator. Page 13

14 Research Scale Page 14

15 Physical vapor deposition (PVD). Negatives of PVD through evaporation: Difficult to create mixed compositions. Uniformity can be less than desired. Evaporation rates are low, throughput low. Dimensions required are large. Source is localized, can result in poor step coverage. Refractory materials (such as W) are difficult to evaporate. Non-metals are difficult. Page 15

16 Physical vapor deposition (PVD): Sputtering. Improvements: RF plasma to maintain discharge stability, especially where insulating materials are involved. Magnetron for increased electron motion. Geometry: Large targets in proximity to substrates. Typical schematic for RF sputtering system. Page 16

17 Physical vapor deposition (PVD): Sputtering. Mechanism: Positive ions are accelerated toward target (cathode). Ions collide with surface at high energy, drive atoms and/or molecules off. Ions can also exchange charge with neutral atoms which can also be accelerated toward target. Observed angular distribution of sputtered atoms follows cosine law. Deposition rate: micron/minute. Choice of gas: Non-reactive -- inert gases are desirable. Readily ionized with stable positive ions. Reasonable cost. Mass which matches sputtered atoms for most effective energy transfer. Generally Argon is chosen. Distribution of Fe atoms sputtered by normal incidence Ar + ions. Fit is to cosine law. Page 17

18 Step coverage in sputtering. Page 18

19 Typical layout for sputtering equipment. Layout: Applied Materials Endura torr 10-8 torr 10-7 torr 10-6 torr 10-5 torr in out Page 19

20 Physical Vapor Deposition No chemical reactions Deposition flux moves along line of sight from source to wafer. Cosine law applies. Performed in vacuum. Page 20

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24 Thermal Evaporator Page 24

25 Thermal Evaporator Page 25

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27 E-Beam Evaporator Page 27

28 E-Beam Evaporator Page 28

29 E-Beam Evaporator Page 29

30 Sputter Page 30

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