Chapter 7: Processing Technology

Questions:

1. What is meant by the word monolithic in the MMIC acronym?

2. What is important about the structure of the substrate material?

3. How are the wafers produced?

4. Why is it useful to use a low-conductivity, semi-insulating substrate material?

5. How is the conductivity of a semiconductor increased for the active layers in the transistors?

6. What are the different ways to create an active layer?

7. What are the two common photoresist processes?

8. What are two typical ways to isolate active devices from each other?

9. How does mesa isolation impact the orientation of the transistors?

10. Can the ohmic contact metallization be used for lateral current conduction?

11. Which of the source, gate, and drain contacts of a field effect transistor is a nonconducting Schottky contact?

12. What is the high-dielectric layer normally used for?

13. What advantages do thin-film metal-alloy resistors (nichrome) have over mesa resistors?

14. How is the electromagnetic coupling between crossing tracks on separate interconnecting layers minimized?

15. How does the final wafer/chip thickness influence the performance of the MMIC?

16. What parasitic impedance do substrate vias add to microstrip shunt components?

17. What is the primary function of the back-face metallization?

18. How is the quality of a processed wafer assessed?

Answers:

1. The word monolithic in the name MMIC means that the circuit is constructed on one solid piece of semiconductor material.

2. The structure of the substrate material must be manufactured as a single crystal lattice if the transistors fabricated on the substrate surface are going to benefit from the high electron mobility of the substrate semiconductor material.

3. Wafers are produced by sawing the single-crystal boule into slices and then polishing the surfaces.

4. Higher-conductivity (lossy) substrates reduce the Q-factor of components, such as inductors, and limit the sharpness of filter responses.

5. Dopant atoms are introduced into semiconductor lattice that have more or fewer electrons than the atoms they are replacing.

6. Implantation and epitaxy.

7. Etch and lift-off lithography.

8. Meas and implant isolation.

9. The metal tracks to the transistor gate contacts must run off the positive slope of the mesa, so all transistors must be aligned with their gate contact parallel to each other.

10. No, the ohmic contact metallization is normally only used to conduct current vertically into the semiconductor.

11. The gate contact.

12. The high dielectric layer is normally used for the dielectric insulator for MIM capacitors and for passivation of the semiconductor surface.

13. Nichrome thin-film resistors have virtually zero variation of resistance with temperature, which mesa resistors vary considerably.

14. A relatively thick layer of the low-dielectric material helps to minimize the coupling.

15. The chip thickness affects the number of modes that can propagate along a microstrip transmission line and the amount of heat that can be conducted out from the back face of the device.

16. Primarily inductance.

17. The back-face metallization forms the ground plane for microstrip transmission lines.

18. Test devices on the wafer are measured, and their parameters are compared to predetermined limits set by the foundry.