N CHEUNG EE143 F2005 Homework Assignment 7 Due Oct 27 Thursday 9 30am Reading Assignment 1 EE143 reader Week 8 Chapter on Etching by G Anner Planar Processing Primer 2 EE143 Lecture Notes Problem 1 Etching Profile a Poly Si of the following structure is to be etched using a completely anisotropic dry etch process to remove poly Si at a rate of 0 1 m min However this etch process has poor selectivities selectivity to SiO2 is 5 selectivity to photoresist is 2 1 um 0 7um photoresist 0 1um 0 5 um oxide Poly Si a Sketch the cross section after 5 minutes of etching b Calculate the angle of the SiO2 sidewalls after 5 minutes of etching Problem 2 Poly Si cross section if mask is also etched The structure shown below is subjected to an etching process to form a poly Si line with the following etching characteristics Vertical mask etching rate 0 01 m min Vertical poly Si etching rate 0 1 m min Degree of anisotropy for mask etching Am 1 Degree of anisotropy for poly Si etching Af 0 60 5 um mask 0 5 um 1 um poly Si Substrate X Y The mask and poly Si have absolute uniform thicknesses The mask and poly Si have absolute uniform etching rates i After the poly Si has just been cleared at points X and Y sketch the cross sections of the poly Si and mask Include the original two vertical dash lines at X and Y for reference ii Find the maximum width of the poly Si line cross section iii Find the minimum width of the poly Si line cross section iv Find the maximum linewidth of the mask cross section v Find the maximum thickness of the mask cross section Problem 3 Worst case design considerations A 0 5 m SiO2 on a Si substrate is to be etched away It is known there is a 5 variation of the oxide thickness and a 5 variation in the oxide etch rate a How much overetch in etch time is required to ensure all oxide on Si is removed b What minimum selectivity of the oxide etch rate to the Si etch rate is required so that a maximum of 5nm 0 005 m of Si etched with the overetch calculated in part a Problem 4 Deep submicron patterning by overetching We would like to fabricate devices with deep submicron dimensions but our lithography equipment can only pattern resist linewidths down to 0 5 m at best For resists with vertical sidewalls propose a simple process flow which can give 0 2 m poly Si linewidth with vertical sidewalls You cannot use undercut with wet etching because the sidewall will be curved 0 5 um Resist SiO2 Your Process 0 2um poly Si Substrate Substrate Problem 5 Sidewall slope of contact holes Taking advantage of photoresist erosion during reactive ion etching of SiO2 we can vary the slope angle of SiO2 contact holes The following schematic shows the resist and SiO2 cross sections before solid lines and after dash lines reactive ion etching Given 80 VRv vertical etching rate of resist 1000 min VRl lateral etching rate of resist 500 min VOv vertical etching rate of oxide 1000 min VOl lateral etching rate of oxide 0 min 2 Before RIE Resist After RIE when SiO2 is just cleared SiO2 Substrate i Use algebra to prove that the slope of SiO2 after etching is a straight line with constant ii If VRv VRl and VOv all have a variation of 10 from run to run Find the maximum and minimum values of Problem 6 RIE Questions a How can we increase the degree of anisotropy in reactive ion etching b Selectivity in reactive ion etching can be increased by using gas mixtures Quote one example to illustrate the principle c In class and in the reader we discuss the formation of an inhibitor during RIE can enhance etching anisotropy However with excessive inhibitor deposition rate the sidewall of the etch film can be more tapered than less inhibitor deposition illustrated in the attached figures Fill in the intermediate crosssections to show progression of the etching process Inhibitor 3
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