EE143 F05 Lecture 15 Etching Etching Terminology Etching Considerations for ICs Wet Etching Reactive Ion Etching plasma etching Professor N Cheung U C Berkeley 1 EE143 F05 Lecture 15 Etch Process Figures of Merit Etch rate Etch rate uniformity Selectivity Anisotropy Professor N Cheung U C Berkeley 2 EE143 F05 Lecture 15 1 Bias dm hf etching mask film substrate df Bias B d f d m B can be 0 or 0 dm substrate df Professor N Cheung U C Berkeley 3 EE143 F05 Lecture 15 Complete Isotropic Etching Vertical Etching Lateral Etching Rate B 2 hf Complete Anisotropic Etching Lateral Etching rate 0 B 0 Professor N Cheung U C Berkeley 4 EE143 F05 Lecture 15 2 Degree of Anisotropy Af B 1 2h f 0 Af 1 isotropic B 2hf Professor N Cheung U C Berkeley anisotropic B 0 5 EE143 F05 Lecture 15 Etching of Steps with a Slope vv t vv t start final film x1 vv t cot x2 vl t x1 x2 substrate Let etching time t v v vertical etch rate x x1 x 2 vv cot vl t vl lateral etch rate To minimize x make large cot 1 tan b a Professor N Cheung U C Berkeley 6 EE143 F05 Lecture 15 Example start 90 vvt o vlt final film sub 0 o film sub Professor N Cheung U C Berkeley 7 EE143 F05 Lecture 15 Worst Case Design Considerations for Etching step height variation variation of film thickness across wafer etching mask can be eroded during film etching Mask film step Substrate a Film thickness variation hf max hf 1 target thickness value variation factor Professor N Cheung U C Berkeley 8 EE143 F05 Lecture 15 b Film etching rate variation v f min v f 1 f variation Worst case etching time required to etch the film h f max h f 1 film v f min v f 1 f c Overetching around step Overetch time fraction h2 step h1 Professor N Cheung U C Berkeley h2 h1 h2 h2 9 EE143 F05 Lecture 15 total tT h f 1 v f 1 f 1 d With Mask Erosion before hf W 2 film h1 h2 Let v m v m be the vertical and lateral etching rates of the mask Let v f be the vertical etching rate of the film ignoring lateral film rate for simplicity after film Professor N Cheung U C Berkeley 10 EE143 F05 Lecture 15 W v m cot v m t T 2 v m vm 1 1 cot hf v v 1 f f m Goal Minimize W Small W 90 o v f m hf small Professor N Cheung U C Berkeley 11 EE143 F05 Lecture 15 Etching Selectivity Sfm between film material and mask material S fm Vf Vm vertical components only Wet Etching Sfm is controlled by chemicals concentration temp RIE Sfm is controlled by plasma parameters plasma chemistry gas pressure flow rate temperature Professor N Cheung U C Berkeley 12 EE143 F05 Lecture 15 Examples HF solution mask SiO2 Si etched by HF solution SiO2 Si SSiO2 Si Selectivity is very large infinity SiO2 Si etched by RIE e g CF4 SSiO2 Si Selectivity is finite 10 Professor N Cheung U C Berkeley 13 EE143 F05 Lecture 15 For a given allowable W 2 what is the minimum selectivity required hf 1 1 vm S fm min cot W vm 1 f 2 Note If v m varies from run to run v m max v m 1 m S fm min h f 1 1 1 m v m cot W 1 vm f 2 Ufm uniformity factor Professor N Cheung U C Berkeley 14 EE143 F05 Lecture 15 Wet Etching 1 3 2 1 Reactant transport to surface 2 Selective and controlled reaction of etchant with the film to be etched 3 Transport of by products away from surface Professor N Cheung U C Berkeley 15 EE143 F05 Lecture 15 Wet Etching cont Wet etch processes are generally isotropic Wet etch processes can be highly selective Acids are commonly used for etching HNO3 H NO3HF H FH is a strong oxidizing agent high reactivity of acids Professor N Cheung U C Berkeley 16 EE143 F05 Lecture 15 Si wet etching mechanisms Professor N Cheung U C Berkeley 17 EE143 F05 Lecture 15 Wet Etch Processes 1 Silicon Dioxide Etch rate A min 6 1 BOE To etch SiO2 film on Si use 1200 650 HF H2O 18 26 T oC SiO2 6HF H2 SiF6 2H2O Note HF is usually buffered with NH4F to maintain H at a constant level for constant etch rate NH4F NH3 HF Professor N Cheung U C Berkeley 18 EE143 F05 Lecture 15 Wet Etch Processes cont 2 Silicon Nitride To etch Si3N4 film on SiO2 use H3PO4 phosphoric acid 180oC 100 A min etch rate Typical selectivities 10 1 for nitride over oxide 30 1 for nitride over Si Professor N Cheung U C Berkeley 19 EE143 F05 Lecture 15 Wet Etch Processes cont 3 Aluminum To etch Al film on Si or SiO2 use H3PO4 CH3COOH HNO3 H2O phosphoric acid acetic acid nitric acid 30oC 6H 2Al 3H2 2Al3 Al3 is water soluble Professor N Cheung U C Berkeley 20 EE143 F05 Lecture 15 Wet Etch Processes cont 4 Silicon i Isotropic etching Use HF HNO3 H2O 3Si 4HNO3 3SiO2 4NO 2H2O 3SiO2 18HF 3H2SiF6 6H2O ii Anisotropic etching e g KOH EDP Professor N Cheung U C Berkeley 21 EE143 F05 Lecture 15 Effect of Slow 111 Etching with KOH or EDP Mask opening aligned in 110 direction 111 sidewalls Professor N Cheung U C Berkeley 22 EE143 F05 Lecture 15 110 Oriented Silicon 111 planes oriented perpendicular to the 110 surface possible to etch pits with vertical sidewalls Bottom of pits are flat 110 plane if KOH is used 100 etches slower than 110 V shaped 100 planes if EDP is used 110 etches slower than 100 Professor N Cheung U C Berkeley 23 EE143 F05 Lecture 15 3 Field Emission Tips Professor N Cheung U C Berkeley 24 EE143 F05 Lecture 15 Drawbacks of Wet Etching Lack of anisotropy Poor process control Excessive particulate contamination Wet etching used for noncritical feature sizes Professor N Cheung U C Berkeley 25
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