1Etching D. W. Parent2Etching Goals• Primary goal is precisely transfer the features on the mask into the underlying material.– Hily selective against mask layer material– Highly selective again material under layer to be etched.– Should be uniform.– Etch rate should be as high as possible.– Process should be safe.– Should not cause damage to the substrate or devices– Etch residues should be easily removed.– Should be clean.– Conductive to full automation.3Topics•Goals• Definitions• Wet Techniques• Dry Techniques• Our Process4Definitions• Bias: Error in the lateral dimension of image transfer.• Tolerance: Statistical variation of the bias• Etch Rate: Rate of film removal– Vertical and Horizontal • Anisotropy: How selective to direction the etch is.– Anisotropic: Etches is only on direction.– Isotropic: Etches in all directions equally.• Selectivity: Ratio of etch rates to underlying material (Sfs) or ratio of etch rates to mask (Sfm)• Over Etch: How much extra etching you have to perform to clear all etched areas on a wafer to take into account variences in the thickness and etch rates across the wafer, wafer to wafer or run to run.5Numerical Examples:Etch_Rate_Max .2 104−⋅cms:=Etch_Rate_Min .1 104−⋅cms:=Etch_Rate_UniformityEtch_Rate_Max Etch_Rate_Min−Etch_Rate_Max Etch_Rate_Min+100⋅:=Etch_Rate_Uniformity 33.333=Horizontal_Etch_Rate .2 104−⋅cms:=Vertical_Etch_Rate .2 104−⋅cms:=LRHorizontal_Etch_RateVertical_Etch_Rate:= LR1=A1LR−:= A0=νmv.005 104−⋅cms:=νml.005 104−⋅cms:=The mask has a horizontal and vertical etch ratetc29.333s=tchf1 δ+()⋅ 1 Δ+()⋅νf1 φf−()⋅:=Δ .2:=Fraction of ever etch timeMaximum line width loss occurs where material is thicknest, etch rate is slowest, and thus the time is longest.φf.1:=Varia t io nνf.02 104−⋅cms:=Mean Etch Rate10%δ .1:=Varat io nhf.4 104−cm⋅:=Mean ThincknessAssume a 60 degree sidwall Total line width lossθ60180π⋅:= θ 1.047=W2νmv⋅ hf⋅ 1 δ+()⋅1 Δ+()νf1 φf−()⋅⋅ cot θ()νmlνmv+⎛⎜⎜⎝⎞⎟⎟⎠⋅:= W 4.627 105−× cm=6Wet Techniques• HF for silicon dioxide or “Al” Etch for Al• Selectivities over the mask and substrate are quite high.• Cheap to buy and run• Completely isotropic (Etches is all directions at the same time.)7Dry Techniques• Reactive Ion Etch– Low pressure– Physical and chemical– Directional– More selective than sputtering– Radiation damage8Dry Techniques•Plasma– higher pressure– chemical–Isotropic– Selective – No Radiation Damage9Our Process• 45mTorr• -600V• 45ccm Triflouromethane (CHF3)• 8ccm oxygen• Antisymetrical (RIE)• Anisotropic• Selective over Si10Some Reported ParametersMaterial Gas Etch Rate Etch Rate Ratio to other materials To Resist SiO2 CHF3 220 11(Si) 5.5 SiO2 CHF3 450 15 (poly-Si) 20 SiO2 CHF3 300 20 (Si) 13 H. Toyoda, Journal of Electronic Materials, no. 9, pp 569, (1980)11Etching EE/MatE12912Etching Goals• Primary goal is precisely transfer the features on the mask into the underlying material.– Hily selective against mask layer material– Highly selective again material under layer to be etched.– Should be uniform.– Etch rate should be as high as possible.– Process should be safe.– Should not cause damage to the substrate or devices– Etch residues should be easily removed.– Should be clean.– Conductive to full automation.13Topics•Goals• Definitions• Wet Techniques• Dry Techniques• Our Process14Definitions• Bias: Error in the lateral dimension of image transfer.• Tolerance: Statistical variation of the bias• Etch Rate: Rate of film removal– Vertical and Horizontal • Anisotropy: How selective to direction the etch is.– Anisotropic: Etches is only on direction.– Isotropic: Etches in all directions equally.• Selectivity: Ratio of etch rates to underlying material (Sfs) or ratio of etch rates to mask (Sfm)• Over Etch: How much extra etching you have to perform to clear all etched areas on a wafer to take into account variences in the thickness and etch rates across the wafer, wafer to wafer or run to run.15Numerical Examples:Etch_Rate_Max .2 104−⋅cms:=Etch_Rate_Min .1 104−⋅cms:=Etch_Rate_UniformityEtch_Rate_Max Etch_Rate_Min−Etch_Rate_Max Etch_Rate_Min+100⋅:=Etch_Rate_Uniformity 33.333=Horizontal_Etch_Rate .2 104−⋅cms:=Vertical_Etch_Rate .2 104−⋅cms:=LRHorizontal_Etch_RateVertical_Etch_Rate:= LR1=A1LR−:= A0=νmv.005 104−⋅cms:=νml.005 104−⋅cms:=The mask has a horizontal and vertical etch ratetc29.333s=tchf1 δ+()⋅ 1 Δ+()⋅νf1 φf−()⋅:=Δ .2:=Fraction of ever etch timeMaximum line width loss occurs where material is thicknest, etch rate is slowest, and thus the time is longest.φf.1:=Varia t io nνf.02 104−⋅cms:=Mean Etch Rate10%δ .1:=Vara tio nhf.4 104−cm⋅:=Mean ThincknessAssume a 60 degree sidwall Total line width lossθ60180π⋅:= θ 1.047=W2νmv⋅ hf⋅ 1 δ+()⋅1 Δ+()νf1 φf−()⋅⋅ cot θ()νmlνmv+⎛⎜⎜⎝⎞⎟⎟⎠⋅:= W 4.627 105−× cm=16Wet Techniques• HF for silicon dioxide or “Al” Etch for Al• Selectivities over the mask and substrate are quite high.• Cheap to buy and run• Completely isotropic (Etches is all directions at the same time.)17Dry Techniques• Reactive Ion Etch– Low pressure– Physical and chemical– Directional– More selective than sputtering– Radiation damage18Dry Techniques•Plasma– higher pressure– chemical–Isotropic– Selective – No Radiation Damage19Our Process• 45mTorr• -600V• 45ccm Triflouromethane (CHF3)• 8ccm oxygen• Antisymetrical (RIE)• Anisotropic• Selective over Si20Some Reported ParametersMaterial Gas Etch Rate Etch Rate Ratio to other materials To Resist SiO2 CHF3 220 11(Si) 5.5 SiO2 CHF3 450 15 (poly-Si) 20 SiO2 CHF3 300 20 (Si) 13 H. Toyoda, Journal of Electronic Materials, no. 9, pp 569,