ELEC9703
Microsystems Design &Technology
Term 1, 2025
Assignment 1
Due on 11:59pm 28th of March 2025
Question 1 (50 marks)
The fabrication of a pressure sensor requires the fabrication of a 20µm membrane. In using a 400mm diameter <100> orientation silicon wafers, the average thickness of a batch of wafers is 600µm with a variation of ±20µm.
a) Suppose you decided to use TMAH (25%) as an anisotropic wet etchant at 70°C to form. the pressure sensor with a 400µm x 400µm square diaphragm of 20µm thickness. Anisotropic etching characteristics of TMAH can be obtained from the Prof. K Sato’s paper: “Anisotropic etching rates of single-crystal silicon for TMAH water solution as a function of crystallographic orientation”, Sensors and Actuators A: Physical; Vol.73, Issue 1-2, 1999, pp131-137.
(i) Assume you decided to achieve a 20µm thick diaphragm by timed etch. For a plain double side polished 600µm thick 400mm <100> orientation silicon wafer, what is the required mask size opening to be patterned on the backside of the wafer, to achieve the desired pressure sensor diaphragm size, and what is the required etching time? (20 marks)
(ii) With this etch time, what is the range of the final diaphragm sizes you will obtain over a batch of such wafers? (5 marks)
(iii) Suppose that during the photolithography step, the mask was misaligned to the major flat by 1.5°. What will be the final diaphragm thickness and size if the same etch time obtained in part (i) was used. State assumptions made. Provide a sketch of the relation between the mask and the final opening on the backside of the wafer. (15 marks)
(b) Suggest what approach you would like to adopt so that repeatable diaphragm thickness from one process run to another can be achieved. Give a brief description of the process steps. (10 marks)
Question 2 (50 marks)
Below in Figure 1(a) and (b) are illustrations of the top and cross-sectional views of the analogue device accelerometer. The accelerometer has a shuttle mass (1) suspended by two springs (2) that are anchored (3) to the substrate at their other ends, two sets of interdigitated fingers with one end attached to the shuttle mass and another one anchored to the substrate, and electrical connectivity to the shuttle mass (Vx) and the anchored fingers of the two sets of the interdigitated fingers (Vs and -Vs). C’1 and C’2 are the capacitances defined by the overlapping fingers in each of the two sets of the interdigitated fingers and are of the same value when no acceleration is measured. When the device is subjected to an acceleration, the shuttle mass will experience a force and is displaced to increase the distance (decrease capacitance, C’1) between the overlapping fingers of one set of the interdigitated fingers and decreases (increase capacitance, C’2) for the other. The difference in the capacitances, which is proportional to the acceleration, produces an output voltage Vx that can be further processed.
The accelerometer is to be fabricated using POLYMUMPS process. POLYMUMPS is 3 structural polysilicon layers (POLY0, POLY1, and POLY2) with 2 sacrificial SiO2 layers and one metal layer (Au/Cr). The polysilicon layers are deposited by LPCVD process. Note that there is LPCVD Si3N4 isolation layer between the substrate and other deposited layer.
(i) Which layers of the three structural polysilicon layers (POLY0, POLY1 and POLY2) do you use to form. each component of the accelerometer? The shuttle mass, the interdigitated fingers, the anchors, and metal pads (10 marks)
(ii) Describe the possible fabrication process steps to realise the accelerometer. You may use figure 2(b) to illustrate each fabrication step. Your answer for each step should be as detailed as possible. For example, is etching step dry (plasma) or wet? What etching recipe would you possibly use? (30 marks)
(iii) What is the reason for having holes on the shuttle mass? Is that necessary? (5 marks)
(iv) The accelerometer operates in vacuum. Describe the possible additional fabrication processing steps required to achieve this. (5 marks)
(1) Shuttle mass; (2) Spring; (3) Anchor; (4) Metal pads; (5) Interdigitated fingers
(a)
(b)
Figure 1: (a) top; (b) cross-sectional view of the accelerometer.