Tutorial 1 (answer)
1. Prove that the (111) surface intercepts the (100) surface at an inclination angle of approximately 54.75o.
Two vectors, <111> and <100>, are perpendicular to (111) and (100) planes. The inner product of two vectors a and b is given by
/a/ ⋅/b/ = /a// b/ cosθ
If a=<111> and b=<100>, the magnitude of the inner product is
√1+ 0 + 0 = √1 =√ 3 ⋅1⋅ cosθ
Hence
θ =cos-1(√1/√ 3)= 54.7356
2. A plane intersects the crystallographic axes at (2,0,0), (0,4,0), (0,0,4). Find the Miller Index for this plane.
a. Step 1 : Identify the intercepts on the x- , y and z- axes.
- Intercepts (2,4,4)
b. Specify the intercepts in fractional co-ordinates
- Fractional Intercepts (2/2,4,2,4/2) = (1,2,2)
c. Miller index (122)
3. Define bulk micromachining and surface micromachining process. Give one (1) simple example for each of the micromachining processes.
Bulk micromachining - processes that involve partial removal of bulk material in order to create three dimensional structures or free devices.
Surface micromachining- processes that builds microstructure by adding materials layer by layer on top of the substrate.
Example of Bulk micromachining; Membrane micromachining processes
4. Name three (3) main components for surface micromachining process and list three (3) examples for each component.
- A sacrificial component (also called a spacer layer)- E.g. phosphosilicate glass (PSG), SiO2, photoresist, polyimide
- A microstructural component. Polysilicon, Gold, Copper and NiFe
- An insulator component
• The etching rates for the sacrificial components must be much higher than those for the two components. E.g.
5. List and briefly explain the criteria to select materials and etching solutions.
- Selectivity – etch rate on structural layer/etch rate on sacrificial layer must be high.
- Etch rate– rapid etching rate on sacrificial layer to reduce etching time
- Deposition temperature– in certain applications, it is required that the overall processing temperature be low (e.g. integration with CMOS, integration with biological materials)
- Intrinsic stress of structural layer– to remain flat after release, the structural layer must have low stress
- Surface smoothness– important for optical applications
- Long term stability
- Explain how to solve the stiction (sticking and friction) problems which commonly happen during the sacrificial layer removal process.
- Use magnetic actuation to pull structures away form the surface reduced surface tension length of arm. Limitation only works for structures with magnetic material.
- Use organic pillar to support the structure during the liquid removal.The organic pillar is removed by oxygen plasma etching.
- Avoid surface tension by relaying on phase change with less surface tension than water vapor. Supercritical state: temp > 31.1 oC and pressure > 72.8 atm.
a. Step 1: Change water with methanol
b. Step 2: Change methanol with liquid carbon dioxide (room temperature and 1200 psi)
c. Step 3: Content heated to 35 oC and the carbon dioxide is vented.
Free-standing cantilever beams up to 850 µm can stay released.
- Forming low stiction, chemically stable surface coating using self-assembly monolayer (SAM). SAM file is comprised of close packed array of alkyl chains which spontaneously form on oxidized silicon surface, and can remain stable after 18 months in air. OTS: octadecyltrichlorosilane (forming C18H37SiCl3)