EE 143 MICROFABRICATION TECHNOLOGY SPRING 2010 C Nguyen PROBLEM SET 3 Issued Thursday Feb 11 2010 Due Thursday Feb 18 2010 7 00 p m in the EE 143 homework box in 240 Cory I Lithography 1 Consider the overhead view NMOS device pattern below obtained after S D and gate lithography Circle the misaligned areas For each misalignment state whether or not the error is catastrophic i e it will lead to a non functional or dead device and explain why or why not Gate poly Active area S D contact 2 The linear coefficient of thermal expansion of the glass used for a photolithography mask is given by the expression TCF 1 L L 1 L T L T where L is a length on the mask and T is temperature Let TCF 3 ppm oC for a given mask Suppose that an alignment accuracy of 0 5 m across a 6 inch silicon substrate is required from one layer to the EE 143 MICROFABRICATION TECHNOLOGY SPRING 2010 C Nguyen next Assume the thermal expansion of silicon is negligible in comparison and that all previous masking steps were done with masks at the same temperature as the silicon wafer a Assuming that a scanning 1 1 projection printer with global alignment is used how close should the temperature of the mask be kept relative to the silicon wafer during alignment in order to achieve this accuracy i e what is the maximum allowable temperature deviation b Repeat a for the case of a 5 1 projection stepper with die to die alignment Assume the die size is 1 cm2 Is this better than for the 1 1 projection printer II Process Flow Layout to Cross Section 3 Suppose you applied the following fabrication process flow to the layout of a cantilever shown below 1 Wet oxidation 0 5 m on silicon substrate 2 Do lithography of anchor mask spin and expose resist with the Anchor mask layer shown below 3 Wet etch the SiO2 using buffered hydrofluoric acid BHF for 6 minutes where the etch rate of thermal oxide in BHF is 100 nm min 4 Remove photoresist 5 Deposit 0 3 m of p type doped polysilicon 6 Do lithography of anchor mask spin and expose resist with the Poly mask layer shown below 7 Wet etch the polysilicon using a premixed silicon etchant for 2 minutes where the etch rate of polysilicon in this etchant is 200 nm min 8 Remove photoresist EE 143 MICROFABRICATION TECHNOLOGY SPRING 2010 C Nguyen 9 Release the structure by dipping the wafer in 10 1 buffered hydrofluoric acid BHF for 5 minutes where the etch rate of wet oxide in 10 1 BHF is 500 nm min 6 1 5 A 6 1 5 3 5 m A 7 1 5 3 1 5 Anchor Poly Si Draw the final cross section of the structure after release along A A with dimensions labeled Note For this problem assume that wet etching only etches vertically and laterally with 100 isotropy for simplicity i e draw the etch front is as a straight sidewall such as Resist
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