chapter 6: liga and micromolding

6.1 Describe why a synchrotron is the best tool for producing the X-rays required to make the most accurate LIGA parts and make a table comparing X-ray lithography with UV lithography.

6.2 An electroplating solution containing nickel sulphamate is used to electroplate nickel LIGA structures. The formula incorporates 450 g of nickel sulphamate (Ni(NH₂SO₃)₂) per liter of water (plus some additives). The solution is heated to 50X10^-6 cm²/sec, calculate the limiting current density (mA/cm²) assuming the boundary layer thickness is governed by the height of the PMMA.

(iv) Plot the current density versus potential curve at the cathode using the values provided from part (iii).

(v) If the current density is specified at 75 mA/cm², what is the likely result (qualitatively)?

(vi) If the pH of the solution is 4 and the hydrogen ion diffusivity is 3 ?  10^-5 cm²/sec, calculate the limiting current associated with the reaction H⁺ + e ? 0.5 H₂.

Professor Kevin Kelly, Louisiana State University

6.3 What is the difference between a thermoplastic, a thermoset, and an elastomer.  Give three examples of each? Write a short essay on plastic micromachining—different types, challenges, and future prospects.

6.4 Assume a polymer is under a tensile load at a variety of temperatures from -20 ?( C to its breakdown temperature at 150 ?( C. Outline the potential stress strain diagrams that may be found for a thermoplastic polymer under very short loading times.

6.5 What is viscoelastic polymer?  Give an example of the changing behavior of a viscoelastic material. Plot the elastic portion of stress strain curve for a steel, an aluminum alloy, a ceramic, a viscoelastic, a thermoplastic, and a thermoset.

6.6 Let's consider a laboratory setup (Galvanic cell) with a bar of copper (Cu) and a bar of silver (Ag) immersed in an electrolyte solution.  The following are the REDOX half reactions and electrochemical potentials. The Gibbs free energy = (-) number of electrons exchanged ?  electrochemical potential ?  Faraday constant. For a reaction to spontaneously occur, the Gibbs free energy must be negative.

Ag⁺ + e^- ? Ag                                      E ?( = 0.80 V;

Cu⁺⁺ + 2e^- ? Cu                                   E ?( = 0.34 V

(a)   Will Ag react with Cu⁺⁺ (i.e., will we be able to plate copper on the silver bar)?

                        Yes                  No

(b)   Will Cu react with Ag⁺ (in other words, will we be able to plate silver on the copper bar)?

                        Yes                  No

(c)   If you place a sheet of copper into an AgNO₃ solution, what would you see happen? What is the reaction?

6.7 Explain the three-fold effects of mechanical, electromechanical, and electrical conversions in mechanical sensors. What are the advantages of LIGA mechanical sensors in this regard?

6.8 Electrodeposition:

(a) In an electrochemical cell we deposit Zn from a Zn²⁺ solution. After the deposition we monitor the voltage of the Zn deposit with respect to a reference electrode (in this case we are using a standard hydrogen electrode, i.e., SHE); first we measure the potential in a ZnSO4 solution at 10^-2M and then in a ZnSO₄ solution of 10^-3M. What will be the change in potential of the Zn electrode between those two solutions? What is the potential in the 10-2M solution? (Eo vs. SHE for Zn is -0.763V.)

reaction: Zn²⁺ + 2e- ---> Zn

(b) Explain the difference between electroless deposition and electrodeposition

6.9 Why is downscaling for an amperometric device more advantageous than the miniaturization of a potentiometric device? Use equations to prove your point.

6.10 What issues do traditional plating and polymer molding companies face when trying to plate and mold LIGA type structures?

6.11 Calculate the minimum grating feature size that can be lithographically patterned by using a light source with a wavelength of 0.4 nm and employing proximity printing with a resist thickness of 100 micrometers. The gap between the resist and the absorber equals 100 micrometers.

Professor Kevin Kelly, Louisiana State University

6.12 For the following designs, decide which standard MEMS process (CMOS, LIGA, MUMPs) you would use and give a brief justification.

1. You want to build a very cheap impact sensor.

2. You want to build a rotating disk for a micro-optical stroboscope.

3. You want to build a strong micro gear train.

4. You want to build an array of structures with different resonance frequencies to test new theories on viscous damping.

6.13 Compare the pros and cons for injection molding, reaction injection molding, and hot embossing in LIGA. What are the differences in a micro part made with LIGA and pseudo-LIGA? How would you find out which technology was used, LIGA or pseudo-LIGA?

6.14 Describe two MEMS applications where the use of LIGA may be justified.

6.15 Fabricate an X-ray mask detailing all materials and processes required. The mask should be adequate for the fabrication of Au parts with an aspect ration of 50 (height over width).

6.16 Describe all the techniques you know for the fabrication of stepped or slanted Au structures. The stepped or slanted wall must have a length of 3 mm and be 500 ??m high at the highest point.

6.17 Circle the RIGHT answer. The organic molecular structure on the left belongs to what chemical group?

Problem Figure 6.17

Professor Karl B ??hringer, University of Washington, Seattle