MBE 6005/8125 Micro System Technology

Assignment 2

Softcopy submitted to Canvas.
Due date: Apr 23, 2021

Reminder: Plagiarism is strictly and seriously monitored. Two or more answer sheets
with significant similarity will result in zero mark for ALL answer sheets, and can even
cause penalty at University level for possible termination of degree work.

1. Advanced Lithography. In a phase shift mask, a phase shift of π (1/2 cycle) is achieved by
making the optical lengths different for the neighboring beams. In hard shift mask this is achieved
by etching a precise depth in quartz. Derive the equation for this depth (refractory index of quartz
is n).

Hint: λ = λ0/n
where λ0 is the wavelength in
vacuum (air)
2. Microfluidics. Using the laminar flow at microscale, it is possible to generate concentration
gradient inside a microchannel. Based on the microfluidic circuitry below, please answer the
following questions.
Assumption: the flowrate of each channel of the same stage is identical.

(a) Please derive the concentration of each channel at each stage based on fraction of flow. It is
required to show the calculation procedure, not just the final answer.
(b) Draw the concentration distribution according at stage 4. Is it a straight line?

3. Integrated Process Flow. A sharp tip on a cantilever beam is essential for the function of atomic
force microscope. How do you fabricate it via MEMS technology?
(a) When making the sharp tip, you may use EDP etching on single crystal silicon. Considering
the corner compensation, how would you design the mask for making the sharp tip? You may
draw schematic diagram for description.
(b) For the aluminum layer, you may choose lift-off process or wet etching to pattern it. Explain
what lift-off process is. What kind of technique can be used to assist the lift-off process?
(c) Please draw the process flow for the cross –section of the cantilever beam with the sharp tip.
Note the fabrication starts from a SOI wafer, and the film is deposited with good conformity
at all time. Please clearly, step-by-step, list the planned etching/deposition method and
explain why this method is chosen, e.g. based on what advantage.
(d) After the etching, the stiction problem may bend the cantilever beam and result in the failure
of structure. One of the solutions to resolve the stiction problem is to make the surface of
cantilever beams hydrophobic. Explain why it can solve the problem (hint: use Young-
Laplace equation).