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3.1 What is the mean free path (MFP)? How can you increase the MFP in a vacuum chamber? For metal deposition in an evaporation system, compare the distance between target and evaporation source with working MFP. Which one has the smaller dimension? 1 atmosphere pressure = 760 mm Hg = 760 torr. What are the physical dimensions of impingement rate?
Professor Karl BÖhringer, University of Washington, Seattle
3.2 Analyze the Si oxidation growth curve. During Si oxidation, oxygen moves through the oxide already formed; in the NiO case, Ni cations move through oxide already formed. Use the latter information to make a hollow NiO tube of less than 25 ?(m inner diameter.
3.3 Why is the orientation dependence of oxidation and etching of a single crystal Si opposite on the (111) versus the (100) planes?
3.4 If we want to deposit a metal film on a substrate by resistance heated evaporation (as opposed to E-beam), what kinds of metals are preferred? How is the thickness of a deposited thin film measured during evaporation?
3.5 Demonstrate the equality of Equation 3.26 and Equation 3.39.
3.6 Assume we use a Kundsen cell to deposit a layer of Al on a nonsmooth surface by the thermal evaporation method (as shown below) for 10 minutes. Compare the thickness of the film at the indicated points.
Problem Figure 3.18
Professor Karl B ??hringer, University of Washington, Seattle
3.7 Why is sputter deposition so much slower than evaporation deposition? Make a detailed comparison of the two deposition methods.
3.8 Develop the principal equation for the material flux to a substrate in a CVD process, and indicate how one moves from a mass transport limited to reaction-rate limited regime. Explain why in one case wafers can be stacked close and vertically while in the other a horizontal stacking is preferred.
3.9 Describe step coverage with CVD processes. Explain how gas pressure and surface temperature may influence these different profiles.
3.10 Compare sputter deposition with evaporation for a simple metal such as Ag. Compare the two techniques for as many different parameters as you remember. Give examples where you would use one technique over the other.
3.11 CVD: (a) Show different types of step coverage and explain what are the most important parameters influencing each. (b) Explain the difference between PECVD and LPCVD.
3.12 Boron with an energy of 100kev is implanted into a 0.18 ohm-cm n-type silicon wafer to achieve a peak concentration 1 ? 1018/cm3:
(a)Find the locations of the p-n junction.
(b)What thickness of silicon dioxide is required to mask this ion implantation?
Assume the projected range and straggles in Si and SiO2 are the same and the criteria of minimum silicon dioxide thickness is that the implanted concentration is less than 1/10 the background concentration at the interface between the silicon and silicon dioxide.
3.13 A 10 ?(m wide channel of single crystal silicon is to be diffusion doped with boron atoms. The concentration at the surface of the silicon is 1020 atoms/cm3 throughout the doping process. The doping is continued until the concentration at a depth of 1 ?(m is 1017 atoms/cm3. Calculate the time required to dope the silicon.
Professor Kevin Kelly, Louisiana State University
3.14 Compare the relative change of the junction depth in silicon for the following two scenarios. Assume that the junction depth is the depth at which the concentration is 1018/cm3. In case A, boron diffuses into Si until a concentration of 1018/cm3 is achieved at a depth of 0.5 micrometer. The line width is 1 ?(m. The diffusion process takes place at 1100 ?C and the diffusivity of boron is 1 ? 10-13 cm2/sec. The surface concentration of the boron is 1021 /cm3 throughout the process. Case B is identical to case A except the junction depth is 1.5 mm and the line width is 4.0 mm. A thermal anneal of both wafers (case A and case B wafers) for 30 minutes at 1100 ?C is used to anneal polysilicon that has been deposited in a surface micromachining process. Calculate the percentages by which the junction depths in case A and case B increased. Assume that a 1D approximation applies.
Note: The goal of this problem is to show why the strategy of using larger line-widths allows one to avoid aluminum planarization. You will need to write a program to solve this problem.
Professor Kevin Kelly, Louisiana State University
3.15 A <100> silicon wafer has 400 nm of oxide on its surface. How long will it take to grow additional 1 mm of oxide in wet oxygen at 1100 ?C? Compare graphical and mathematical results. What is the color of the final oxide under vertical illumination with white light?
3.16 Describe two methods to make DNA arrays using a virtual mask approach.
3.17 What are the characteristics of electron beam heated evaporation compared to resistance heated evaporation? Mark correct answers with an X.
( ) More complex.
( ) Very versatile.
( ) Only works under lower temperatures.
( ) Everything a resistance heated evaporator can deposit can also be accommodated by electron beam heated evaporation.
( ) A magnetic field can be used to increase the temperature.
( ) It is not possible to provide a large evaporant surface area in electron beam heated evaporation.
( ) The adhesion between an evaporant and a substrate is accomplished by local reactions (sticking). Titanium and chromium are often used as a "glue" to improve the adhesion between an evaporant and a substrate.
( ) When the partial pressure of an evaporant vapor exceeds its equilibrium vapor pressure, it will condense.
( ) When a system is under equilibrium vapor pressure, the net transfer rate of material from one state to the other state is equal to 1.
( ) The cosine law is the underlying principle that a Kundsen cell can deposit a perfectly uniform coating inside a spherical glass jar.
Professor Karl B ??hringer, University of Washington, Seattle
3. 18 Shown is a typical vacuum evaporation system.
Problem Figure 3.18
(a) Where are the liquid nitrogen traps/baffles? (A, B, or C)
(b) What is the purpose of those traps?
(c) Do we need to fill the trap with liquid nitrogen before we warm up the pumps?
(d) Do we need to fill the trap with liquid nitrogen before we open the gate valve?
(e) Does the trap need to be refilled during the operation?
Professor Karl B ??hringer, University of Washington, Seattle