3) a) Describe, with the aid of suitable diagrams where necessary, the principal steps in a typical lithography process flow. Your answer should include a discussion of i) Wafer surface preparation

ii) Resist application iii) Soft bake iv) Alignment and Exposure v) Development [15 1/3 marks] b) A proximity X ray mask is made up of tungsten absorbing patterns on a silicon nitride membrane. The minimum feature size is 0.1 microns. i) If the membrane heats up by 9°C during the exposure and the field is square with 3 cm sides, how much will the field be distorted due to thermal expansion of the membrane? Express your answer as a % increase in the length of a side. ii) If the maximum allowable distortion is 1/3 the minimum feature size, what is the maximum allowable temperature rise on the mask. Take the Thermal Coefficient of expansion for silicon nitride to be 2.7 X 10-6 (°C-¹). [10 marks] c) An optical lithographic system has an exposure power of 0.3 mW/cm². The required exposure energy for a positive resist is 150 mJ/cm² while the exposure energy needed for a negative resist is 12 mJ/cm². i) Neglecting load and unload times, calculate the wafer throughput for both the positive and negative resists. ii) Comment on your results, explaining why positive resist is preferred in general. [8 marks]