Midterm Examination

This midterm examination comprises of two parts:

Part I is a set of questions that must be completed within a period of 120 minutes. If you go over the allowed period, write down the time that you worked on it.  It is open book, open notes, but you must not discuss or collaborate on the solution with anybody in the class or outside the class.

In Part II you MUST collaborate in groups of two or three students.

Part I (65 %)

Questions carry equal points.

Question 1: (Oxidation) (20 %)

A 100 nm gate oxide is required in an integrated circuit fabrication process  to form the MOS transistor oxide. Silicon wafers with <100> orientation are employed in CMOS fabrication. As this oxide must be of the best possible quality, It has been decided that the oxidation will be carried at 1000^o C in dry oxygen. Before doing oxidation, the surface of the silicon is cleaned so that there is NO residual oxide on the surface. How long should the oxidation be done? You can look get the result from some oxidation chart, justify your answer with numerical calculations. Is the oxidation in the linear regime, parabolic or in between the two?

Question 2: (Process design) (45 %)

A square silicon membrane , 50 microns thick  must be formed through bulk micromachining in 450 micron thick wafers, to design a pressure sensor (See 1994 class project). To form the membrane a cavity must be wet etched to a depth of 400 microns from the surface of the wafer.  The mask to this etch step is Si02 (silicon dioxide) and we will use KOH as the etchant.

I. Design the process flow for this task.

1. Growth of silicon dioxide-masking material- (how thick SiO2 do we need?, how long do we need to oxidize?, what temperature?,  wet or dry?)
2. Etching of silicon (what concentration of KOH?, what temperature? how long?)

II. What is the minimum mask opening to achieve the 450 micron deep cavity. I.e what is the smallest possible membrane that you can make. Does it depend on wafer orientation?

Part II  (35 %)

In this course we are learning the basis of fabrication technologies within a context of a laboratory that has limited scope and facilities. We have a  contact mask aligner, a photoresist spinner, an oxidation furnace, and a thermal evaporator as well as basic chemical setups for wet etching. The following four papers discuss  both processing improvements in fabrication  and integration (1,2) as well as applications in biology/medicine (3) and communications (4).

1. Fabrication of a Cylindrical Display by Patterned Self-Assembly
2. Microscope Projection Photolithography for Rapid Prototyping of Masters with Micron-Scale Features for Use in Soft Lithography
3. Single Ion Channel Sensitivity in Suspended Bilayers on Micromachined Supports
4. Direct Coupling of VCSELs to Plastic Optical Fibers Using Guide Holes Patterned in a Thick Photoresist.

Read and do a written critical review (1 page)  for ONE of the above papers.

    If you choose to review papers 3 and 4, discuss how you would fabricate structures in our lab, i.e. design the process flow within the constraints of our facilities,  using the equipment available in the lab (1 page).

    If you are reviewing papers 1 and 2 discuss how you would go about augmenting our facilities to perform the required function and elaborate on basic test procedures to evaluate the performance and or efficacy of the new fabrication capability (1 page).