3)

a) Discuss, in detail, the following light sources used in lithography.

i) Mercury lamps (Ultra Violet UV)

ii) Excimer lasers (Deep Ultra Violet DUV)

iii) Laser pulsed plasma (Extreme Ultra Violet EUV)

Make sure you consider the mechanism by which the radiation is produced, the

technology used to construct the light source and the advantages and disadvantages

each technology. Include labelled diagrams where appropriate.

[15 1/3 marks]

b) An X ray mask is made up of tungsten absorbing patterns on a silicon nitride

membrane. The minimum feature size is 0.13 microns.

i) If the membrane heats up by 10° C during the exposure and the field is square w

4 cm sides, how much will the field be distorted due to thermal expansion?

Express your answer as an increase in side length in microns. Take the Therma

Coefficient of expansion for silicon nitride to be 2.7 X 10-6 (°C-¹).

ii) If the maximum allowable distortion is 1/4 the minimum feature size, what is t

maximum allowable temperature rise on the mask. Comment on your result.

[10 m

c) A lithographic scanner has an exposure power of 0.25 mW/cm². The required

exposure energy for a positive resist is 160 mJ/cm² while the exposure energy ne

for a negative resist is 11 mJ/cm².

i) Neglecting load and unload times, calculate the wafer throughput, in wafers

hour, for both the positive and negative resists.

ii) Comment on your results, explaining why positive resist is preferred in gene

[8 m