4. Thermal Oxidation. a) Equipment Atmospheric Furnace

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1 4. Thermal Oxidation a) Equipment Atmospheric Furnace Oxidation requires precise control of: temperature, T ambient gas, G time spent at any given T & G, t Vito Logiudice 34

2 4. Thermal Oxidation b) Mechanism Reaction of silicon and oxygen to form silicon dioxide: Dry oxidation: Wet oxidation: For every angstrom of oxide grown, 0.45 angstrom of silicon is consumed. This reaction occurs even at room temperature, yielding approximately 20 to 30Ang of dirty oxide. Elevated growth temperatures are needed to create higher quality, thicker oxides. Vito Logiudice 35

3 4. Thermal Oxidation When oxygen atoms initially come into contact with a bare Si surface, the Si & O 2 atoms can react readily. Thus initially, oxide growth occurs in a linear fashion and the oxidation mechanism is said to be reaction rate limited. Linear oxidation occurs to an SiO 2 thickness of approximately 500 Ang. Oxide growth occurs at the silicon to oxide interface, thus oxygen atoms must diffuse through the growing oxide down to the Si/SiO 2 interface before being able to react with the underlying silicon. Past ~500Ang of oxide, the reaction rate becomes limited by the diffusion rate of the oxygen atoms through the growing oxide layer and at this point, the growth mechanism is said to be diffusion rate limited. Vito Logiudice 36

4 4. Thermal Oxidation Oxide growth therefore goes through an initial linear oxidation to about 500Ang, followed by a parabolic growth stage: The differing thick vs. thin oxide growth rates should be taken into consideration when oxidizing, etching, and reoxidizing silicon substrates successively. Vito Logiudice 37

5 4. Thermal Oxidation Due to this parabolic growth, thick oxides take very long to grow. In order to reduce these times, a combination of higher temperatures and wet ambients (ie.,steam) rather than dry (ie., straight oxygen) can be used during oxidation. Dry oxidation An additional consideration when designing process flows: Silicon wafer orientation affects the oxidation rate. Wet oxidation Vito Logiudice 38

6 4. Thermal Oxidation c) Wet Oxidation Recipe example (~1000Ang SiO 2 ) STEP DESCRIPTION DUR. TEMP. N2 O2 H2 hh:mm:ss Celsius sccm sccm sccm 00 STANDBY n/a PUSH-IN (2inch/min) STABILIZATION 00:10: TEMPERATURE RAMP UP (10C/min) 00:30: > STABILIZATION 00 :10 : WET OXIDATION 00:29: TEMPERATURE RAMP DOWN (10C/min) 00:30: > PULL OUT (2inch/min) COOL DOWN 00:10: Vito Logiudice 39

7 5. Photolithography a) Photoresist Fundamental Properties Negative Resist Chemistry Positive Resist Chemistry PR s are designed to be sensitive to Vito Logiudice different exposure wavelengths 40

8 5. Photolithography a) Positive & Negative Photoresists Negative Resists: Very sensitive, therefore short exposure times High etch resistance Adhesion to substrate may be an issue Swelling during development (low resolution) Not expensive Positive Resists: Less sensistive, therefore longer exposure times High etch resistance Excellent adhesion possible via adhesion promoters No swelling during development = better resolution Expensive Vito Logiudice 41

9 5. Photolithography b) Photoresist Application Static Dispense Process Critical process parameters (repeatability): dispense quantity spin acceleration spin speed (target) spin duration pre-exposure bake post-exposure hard bake (PEB) Critical environmental parameters: temperature humidity stability of these over time Potential Issues Vito Logiudice 42

10 5. Photolithography Newer coat techniques aimed at improving uniformity Apply PR while chuck being spun at low RPM s (Dynamic Dispense) OR Apply PR across wafer prior to spin, rather than applying in center only (Moving Arm Dispense) Vito Logiudice 43

11 5. Photolithography c) Exposure Contact / Proximity Alignment NOTE: Some aligners allow for soft (ie., limited) physical contact between mask and wafer in the case of Proximity aligners. Alignment (theta, x, y) Contact prior to exposure Contact aligners; issues with: Defect densities Mask damage Mask cost Resolution accuracy across wafer Proximity alignment addresses defect and damage issues, but resolution problems remain and may in fact be greater due to light scattering between mask and wafer during exposure. Vito Logiudice 44

12 5. Photolithography d) EVG 620 Front to Back Aligner (McGill Fab) Vito Logiudice 45

13 5. Photolithography e) Exposure Projection Alignment / Steppers Key features: Cheaper Mask/Reticle cost since can have 5 to 10 times reduction of minimum feature size via use of reduction lenses. Excellent resolution across wafer Production systems developed with automatic alignment capabilities Excellent (ie., low) defect densites Vito Logiudice 46

14 5. Photolithography f) Canon Mark III G-Line Stepper (McGill Fab) Vito Logiudice 47

15 6. Wet Etching a) Chemistry Etch processes rely on: The transport of reactants to the surface The surface reaction itself The transport of reaction by-products from the surface Key ingredients: Oxidizer to oxidize the surface being etched (eg.: H 2 O 2, HNO 3 ) Acid or base to dissolve the resulting oxidized surface (eg.: H 2 SO 4, NH 4 OH) Transport media for reactants & by-products (eg.: H 2 O, CH 3 COOH) Process parameters affecting etch results: Solution: temperature filtering agitation/stirring strength Time to rinse & rinse efficiency Surface wetting & bubbles Vito Logiudice 48

16 6. Wet Etching b) Isotropic Etching Vertical and lateral etch rates are identical. Undercutting should be taken into consideration. Orientation of the mask edge to the crystal planes does not affect etch rates Example: HF-based etching of silicon dioxide Wet Etching - Isotropic Vito Logiudice 49

17 6. Wet Etching c) Anisotropic Etching Etch rates depend on crystal orientation. The orientation of a given mask edge to the crystal axes will determine whether the lateral or vertical etch rate is dominant. The orientation of a mask edge and the mask pattern itself will ultimately determine the shape of the etched structure. Example: silicon etching with KOH or EDP solutions. The creation of very complex shapes is possible refer to Madou Chp.4 for more insights. Recall - different crystal planes etch at different rates: (100) fastest (110) (111) slowest Top view Side view along indicated section Vito Logiudice 50

18 6. Wet Etching c) Anisotropic Etching (KOH based) Si (100) etch rate (line) & nonuniformity (column) 1hr etch time (thin line) & for 60um etch depth (thick line) M.J.Madou Vito Logiudice 51

19 6. Wet Etching c) Anisotropic Etching (solutions, selectivity) M.J.Madou Vito Logiudice 52