CVD: General considerations.

Transcription

1 CVD: General considerations. PVD: Move material from bulk to thin film form. Limited primarily to metals or simple materials. Limited by thermal stability/vapor pressure considerations. Typically requires relatively high temperature and surface experiences high temperature molecules. Today used primarily for deposition of Al, Al:Cu, Au. Natural coverage: line of sight, with cos distribution. CVD: Provides opportunity to deposit thin films of complex materials and in principle can be accomplished at low or modest substrate temperatures. Natural coverage: conformal. Used today primarily for dielectrics and refractory conductors.

2 CVD Process Continuous gas flow Diffusion of reactants Boundary layer Deposited film Silicon substrate

3 Chemical reaction and typical energetics for CVD. AB + C + (inert carrier) A + BC + (inert carrier)

4 Reactions and reaction control in CVD. Generally energy is needed to stimulate the reaction (overcome activation energy E A ) and to control film growth. Thermal energy. CVD normal chemical vapor deposition. LPCVD low pressure CVD. APCVD. Plasma. PECVD HDPCVD RPECVD Etc Atmospheric pressure CVD. plasma enhanced CVD. High density plasma enhanced CVD. remote plasma enhanced CVD.

5 Polysilicon CVD SiH 4 (gas) + H 2 (gas) 2H 2 (gas) + PolySilicon (solid) Gas delivery 1) Mass transport of reactants CVD Reactor 2) Film precursor reactions By-products 7) Desorption of byproducts 8) By-product removal Exhaust 3) Diffusion of gas molecules 4) Adsorption of precursors 5) Precursor diffusion into substrate 6) Surface reactions Continuous film Si 2 H 6 Substrate Si

6 Film Formation during Plasma-Based CVD RF generator Gas delivery 1) Reactants enter chamber RF field Electrode PECVD reactor 2) Dissociation of reactants by electric fields By-products 7) Desorption of by-products 8) By-product removal Exhaust 3) Film precursors are formed 4) Adsorption of precursors 5) Precursor diffusion into substrate 6) Surface reactions Continuous film Substrate Electrode Same as CVD, but plasma accelerates reactions; charged products deposit anisotropically; and extra energy inproducts increases the final film quality.

7 Some exemplary data for deposition of polysilicon: High temperature: deposition limited by mass transport Low temperature: deposition limited by chemical reaction SiH 4 + SiH 2 Cl 2 SiHCl 3 SiCl 4

8 A different way to plot the growth rate:

9 CVD Reactor designs examples. Horizontal flow reactors, (a) and (b). Pancake reactor (c), and barrel reactor (d). Single wafer reactors, (e) and (f).

10 Plasma Enhanced CVD Processing System Gas inlet Process chamber Capacitivecoupled RF input Chemical vapor deposition Wafer Susceptor Exhaust Heat lamps CVD cluster tool

11 High Density Plasma Deposition Chamber Popular since ~ 1995 High density plasma Highly directional due to wafer bias Fills high aspect ratio gaps (deposition mode) Backside He cooling to relieve high thermal load Simultaneously deposits and etches film to prevent breadloaf and key-hole effects Photograph courtesy of Applied Materials, Ultima HDPCVD Centura

12 High Density Plasma CVD Typical deposition conditions and properties of silicon oxide films deposited using HDP CVD. Process parameter Value Source rf power W. Gases SiH 4 /Ar/O 2 = 32-45/0-40/43-60 sccm. Pressure <5 mtorr. Deposition/sputtered ratio 3.2:1 (filled 0.25 µm, 2.5:1 aspect ratio). Deposition temperature C. Deposition rate nm/min. Refractive index 1.46 ± Film stress (0.7 µm, 25 C) (-) x 10 9 dynes/cm 2. Wet-etching (6:1 buffered HF) x that of thermal oxide.

13 Step coverage variation with process. Films of silicate glass deposited on 0.3 micron features. Conformal coverage of LPCVD process based on TEOS. Bread-loaf profile of PECVD process based on TEOS. Unique profile of HDP CVD process based on silane. IBM J. Res. Dev. 43 (1999). D. R. Cote et. al.

14 Polysilicon or polycrystalline silicon deposition. Uses: primarily as conductive material. Conductor in CMOS, bipolar, and related structures. Resistors. Electrodes for internal capacitors (DRAM for example). Advantages: Compatible with silicon. Withstands subsequent high temperature processing. Excellent interface with SiO 2 (low defect density etc.). Conformal coverage. A layer of polysilicon is then deposited onto the silicon dioxide surface using chemical vapor deposition. This material will serve as the transistor's gate.

15 Polysilicon silicon deposition Primary basic chemistry: SiH 4 Si + 2H 2 Very simple chemistry. Single starting reagent. Starting material readily available in high purity. No bothersome products. No significant side reactions. Reaction readily takes place at modest temperature. Pressure ~ torr. T reaction ~ 580 o 650 o C. Lower temperatures give too slow reaction. Higher temperatures give rise to gas phase reaction (particulates). Deposition rate in realm of 0.01 micron/min. Deposition time approx. 2.5 hours for 0.3 micron film.

16 Silicon dioxide and related dielectric material deposition Uses: Gate dielectric (MOS, CMOS etc.) Isolation of internal transistor from metal conductor. Outer metallization insulation. Storage of charge capacitance (EPROM). Passivation layers. Dopant Diffusion sources. Diffusion and implantation masks. Structure generally amorphous with SiO 2 in local tetrahedral configuration. Sometimes referred to as USG for undoped silicate glass.

17 Silicon dioxide deposition. Basic chemistry: low temperature silane process. SiH 4 + O 2 (nitrogen carrier) SiO 2 + 2H 2 Pressure ~ <1 atmosphere; silane partial pressure in realm of 1 torr; oxygen in excess. T reaction ~ 310 o 450 o C. Activation energy approx. 0.4 ev. Films slightly porous, densification at 700 o o C necessary for high quality films. Film quality measurements: Property Low T silane Thermal oxide Dielectric constant Refractive index Etch rate* 3 - >10 (1.0) Slightly Porous

18 Silicon dioxide deposition Basic chemistry: low temperature plasma enhanced process. SiH 4 + 2N 2 O (argon carrier) SiO 2 + 2H 2 +2N 2 Pressure ~. T reaction ~ 200 o 400 o C. Activation energy approx. 0.4 ev. Films slightly porous, densification at 700 o o C necessary for high quality films. Property Low T PECVD Thermal oxide Slightly Silicon Rich Dielectric constant Refractive index ~ Etch rate* ~2-10 (1.0) Dielectric strength 4-8 MV/cm MV/cm

19 Silicon dioxide deposition Basic chemistry: TEOS (tetraethyl orthosilicate) process. Si(OCH 2 CH 3 ) 4 (nitrogen carrier) SiO 2 + 4C 2 H 4 + 2H 2 O TEOS is stored as liquid (but used as gas phase reactant). Conventional CVD (medium temperature) process. Impurities present such as C add O 2 to minimize. T reaction o C (cannot be used over Al) micron/minute deposition rate. Improved step coverage relative to silane process. PE CVD (low temperature) process. T reaction o C. Total pressure 2-10 torr. 0.1 micron/minute deposition rate.

20 CVD Silicon Nitride Deposition. Uses: Final passivation and mechanical protection. Mask for selective oxidation of Si. Charge storage dielectric in MOS capacitors. Sidewall structures in MOSFETs. CMP stop-layer. Basic chemistry: low pressure CVD process. 3SiCl 2 H 2 + 4NH 3 (carrier) Si 3 N 4 + 6H 2 +6HCl T reaction ~ 700 o 800 o C. Deposition rate 0.01 micron/min. Basic chemistry: Plasma enhanced CVD process. SiH 4 + NH 3 Si x N y H z + H 2 T reaction ~ 200 o 400 o C. P= torr Deposition rate 0.05 micron/min.

21 Other useful CVD deposition processes: WSi 2 : Tungsten Silicide. Uses: Local interconnect (bit lines in memory devices). Adhesion layers (for W for example). WF SiH 4 WSi HF + H 2 ( o C, low pressure) 2WF 6 +7SiH 2 Cl 2 2WSi 2 +3SiCl 4 +12HF +2HCl (600 o C) TiN: Titanium nitride. Refractory (2950 o C) with relatively low resistivity (50 -cm) Uses: Diffusion barriers (for Cu for example). Adhesion layers. 6 TiCl NH 3 6 TiN + 24 HCl + N 2 (600 o C, low pressure)

22 CVD W (tungsten) Deposition Uses: Metal vias ( plug ). Local metallization. Basic chemistries: 2 WF 6 +3 Si 2 W +3 SiF 4 (300 o C, micron) WF 6 +3 H 2 W +6 HF (low pressure, 450 o C) 2 WF 6 +3 SiH 4 2 W +3 SiF 4 +6 H 2 (low pressure, 300 o C)

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