Mid-term Exam Thin Film Engineering 4/28/2009

1. Mid-term Exam Thin Film Engineering 4/28/2009 A mechanical pump has an ultimate pressure of 0.001 torr and is connected to a 30 cm x 30 cm x 30 cm chamber. Assuming the initial chamber pressure is 760 torr, estimate what the intrinsic pumping speeding of the pump will be if the chamber is to be evacuated to a pressure of 0.1 torr in 1 min. (20%) Compare the differences between thermal evaporative deposition and sputter deposition. (20%) When examining the growth rate uniformity for the case shown in the figure below, 2 of the boundary conditions are (a) DC/r = kC at the back side of wafer zi (b) -DC/r = kC at the front side of wafer zi+1 Explain why the signs are different. (10%) Identify the ion-surface interactions shown in the following figure. Also determine which interactions are likely to occur during a regular sputter deposition process. (20%) Compare the differences between thermal evaporation of alloy and compounds. (15%) Explain the following terms. (15%) Thin film and thick film Gas throughput Conductance Plasma Gas breakdown 1

2. Q2: Vacuum/Evaporation vs. Plasma/Sputtering 2

3. Q4: Ion-Surface Interactions Fig. 4.9 (12) (7) (11) (1) (10) (5) (2) (3) (6) (4) (9) (8) 3

4. Q5: Evaporation of Compounds Elements evaporates as atoms and occasionally clusters. Compounds Evaporation without dissociation MX(s or l) → MX(g) Evaporation with dissociation– chalcogenides MX(s) → M(g) + ½X2(g) Evaporation with (partial) dissociation– oxides MO2(s)→MO(g) + ½O2 Decomposition– Type I MX(s) → M(s) + ½X2(g) Decomposition– Type II MX(s) → M(l) + 1/n Xn(g) Evaporationwith partial dissociation 4

5. Evaporation of Alloys Ideal alloy Usually and Therefore from (3-2), (3-9), & (3-10) Note that gA, gB, XA, and XB change with the evaporation time. Therefore, the flux ratio varies with time. • The use of two separate melt sources, or • Continuous adjustment of the melt composition by using a feed-rod. 5

6. Q6 Thin film and thick film Thin: the formation is from atoms, molecules, and/or clusters. Thick: the formation is from macro-particles. Gas throughput Gas throughput, Q, is the quantity of gas that passes a plane in a known time. (Unit: Torr-liter/s or Pa-liter/s) Conductance, C C is known as the conductance, which depends on the geometry, as well as the flow regime. 6

7. Q6 Plasma A plasma may be defined as a quasineutral gas hat exhibits a collective behavior in the presence of applied electromagnetic fields. Therefore plasmas are weakly ionized gases consisting of charged and neutral species, including electrons, ions, atoms, and molecules. The fourth state of matter Gas breakdown Before a plasma or glow discharge can be ignited, very small amounts of current occur. If the current and voltage continue to increase, electron avalanching takes place to cause ignition of a plasma or glow discharge, then we call it gas breakdown. 7

8. Final Exam Thin Film Engineering 6/23/2009 The use of RF power in sputter deposition would result in target self-bias. Please explain such self-bias and why the self-bias is always negative. (20%) Explain how to eliminate the hysteresis that occurs during reactive sputter deposition using a high pumping speed and by varying the target-to-substrate distance? (20%) Describe Walton-Rhodin theory and the existence of a critical temperature associated with the theory. . (20%) What are X-ray reflectivity method and X-ray photoelectron spectroscopy? (20%) Explain the following terms. (20%) Condensation coefficient Chemisorption reaction probability Utilization fraction Thermal accommodation coefficient Trapping probability What is the fundamental difference between a dye-sensitized solar cell and a CIGS solar cell? (5%) In optical spectroscopy analysis, what are the p-polarization and s-polarization? (5%) What is oblique-angle deposition? (5%) What is an organic concentrator for photovoltaics? (5%) 8

9. The use of RF power in sputter deposition would result in target self-bias. Please explain such self-bias and why the self-bias is always negative. (20%) 1 1 V (kV) Vt (kV) t t 0 0 -1 -1 Why not positively biased? Because e has much higher mobility then positive ions 9

10. Explain how to eliminate the hysteresis that occurs during reactive sputter deposition using a high pumping speed and by varying the target-to-substrate distance? (20%) The relation between FRG and pRG is where SRG is the pumping speed of the pumping system for the RG and sFRG is the flow rate of RG gettered by sputtered material. For a stable system, the last inequality becomes: FRG/pRG > 0 Take this into Eq. 6-A: The condition for the deposition system to become stable i.e., to eliminate the hysteresis effect, is then It was found that during the DC reactive magnetron sputtering of Ti in an Ar/O2 atmosphere, when the target-to-substrate distance was small, gas-rich films of TiO2.4 were formed. This is because at such small distances, the flux density of sputtered Ti was high and hence a high O2 partial pressure was needed for the reaction to take place. However, the high PO2 caused the target to switch to the reactive or poisoned mode, which is characterized by a low sputtering rate and gas-rich films. On the other hand, when the target-to-substrate distance was made larger, the stoichiometric TiO2 films were formed at a lower PO2. The lowerPO2 also prevented or reduced the target poisoning. Lower PO2 is used because the flux density of sputtered Ti was then lower due to the cosine distribution law. 10

11. Describe Walton-Rhodin theory and the existence of a critical temperature associated with the theory. . (20%) An important advance was done by Walton et al. Treat clusters as macromolecules and applyi statistical mechanics in describing them. Introduce the critical dissociation energy, Ei*., which is defined as the energy required to break a critical cluster having i atoms into separate i atoms. (Ei* can be viewed as the negative of a cluster formation energy.) Therefore the equilibrium between a monomer density (N1) and the critical concentration of clusters having size i* (Ni*) is Analog to Eq. 7-27, the monomer density is given by the supply rate (R) and the adatom life time (7-25) The monomer supply rate is given by the supply rate and area (X2) over which adatoms can diffuse before desorbing. From Eqs. 7-34 to 36 Eq. 7-37 predicts that, in a thermally activated nucleation, the nucleation energy depends on the critical nucleus. also suggests the existence of critical temperatures where the nucleus size and orientation undergo change. where n0 is the density of adsorption sites. where Therefore and 11

12. What are X-ray reflectivity method and X-ray photoelectron spectroscopy? (20%) X-ray reflectivity method is a non-destructive technique for the characterization of density, thickness, roughness, and layering of coatings. A critical angle exists, below which x-rays are totally reflected by the film. A critical angle is given by The reflected waves from the top surface to the film-substrate interface above the film critical angle. This periodicity gives the film thickness. In general, a thinner film has a longer period. The relation between the periodicity Δqof the fringes and the layer thickness d is d~l/2Δq. XPS is a surface analytical technique that detects electrons from a surface, leading to the determination of surface composition. XPS is capable of readily providing information on the nature of surface chemical bonding and valence states Explain the following terms. (20%) Condensation coefficient: The fraction of the arriving vapor that becomes chemisorbed Chemisorption reaction probability: Condensation coefficient is NOT used in the case of chemisorption on a foreign substrate. For foreign substrate, it is called chemisorption reaction probability. Utilization fraction: The fraction of the vapor entering the reactor that becomes incorporated into the film instead of being carried downstream and pumped away. Thermal accommodation coefficient: It represents the degree to which a molecule accommodates itself to the temperature, Th, of the surface from which it is reflected. Trapping probability: The fraction of approaching molecules that is weakly adsorbed (physical adsorption or physisorption). 12