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]