UNIVERSITY OF CALIFORNIA. College of Engineering. Department of Electrical Engineering and Computer Sciences. Professor Ali Javey.

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1 UNIVERSITY OF CALIFORNIA College of Engineering Department of Electrical Engineering and Computer Sciences EE 143 Professor Ali Javey Spring 2009 Exam 2 Name: SID: Closed book. One sheet of notes is allowed. There are a total of 15 pages on this exam, including the cover page. Problem 1 41 Problem 2 32 Problem 3 19 Problem 4 8 Total 100 1

2 Physical Constants Electronic charge q C Permittivity of vacuum ε F cm 1 Relative permittivity of silicon ε Si /ε Boltzmann s constant k x 10 5 ev/ K or J K 1 Thermal voltage at T = 300K kt/q V Effective density of states N c 2.8 x cm 3 Effective density of states N v 1.04 x cm 3 Silicon Band Gap E G 1.12 ev 2

3 Solid Solubility Limits Information which may be useful: 3

4 Diffusion Information which may be useful: 4

5 Ion Implantation Information which may be useful: 5

6 Problem 1: Doping (41 pts) 1. An ion implantation step (dose= =10 14 cm 2, R p p=0.0226um, ΔR p =0.0102um) implants Arsenic (As) into a p type semiconductor material with uniformly doped boron (B) background concentration of cm 3. The total thickness of this Si wafer is 300 µm. A. Find the peak As concentration, N p ( 5 pts) N P = Δ R P 1 Q 10 = 2π = π points for equation 2.5 points for the result B. Qualitatively draw the As and B concentration profiles. Clearly indicate and label the projected range and the peak concentration. Assume an ideal Gaussian profile. (5 pts) 6

7 C. Find the junction depth(s) right after the implantation (5 pts) N B x = 10 j 16 = = 64nm N p 2.5 points for equation 2.5 points for the result ( x j R p ) 2 exp 2 2ΔR p D. The sample is now thermally annealed at 950 C for a fraction of a second. Qualitatively draw the Arsenic and boron profiles of this sample after high temperature annealing with and without transient enhanced diffusion (TED). Indicate clearly which curve has TED effect. Here, you do not have to consider electric field enhanced diffusion. (5 pts) E. If we increase the implantation energy and dose, will the TED effect be more prominent? Briefly explain (1 sentence). (3pts) Yes. More defects will be induced by using higher energy and/ /or dose. 7

8 F. Redraw the As and B profiles of the sample after thermal annealing, this time ignoring TED but including the electric field enhanced diffusion. Label the direction of the e field as well. (5pts) G. We do high temperature annealing at 1000 C for a long time, say two week. Redraw the As and B profiles. Assume the result in section F to be your starting point. (5 pts) 8

9 H. Besides the various effects covered in the previous parts of this question, channeling could also affect the dopant profile after the ion implantation step. Briefly explain the channeling effect and list/describe two approaches for reducing the channeling effect (4 sentences max). (4 pts) If the ions are implanted through the Si lattice in the <110> direction, then they may miss the Si atoms and channel much more deeply into the material than otherwise predicted Tilt the substrate and amorphization of the surface 2 points for the explanation 2 points for the prevention ways I. Achieving ultrashallow junctions as the source/drain extensions of nanoscale MOSFETs is a challenging field of active research. Speculate whether enabling ultrashallow junctions is more difficult for p+ or n+ doping. Briefly justify your answer (3 sentences max). (4 pts) p+ is harder because B is lighter than As, and hence go deeper upon implanting. Also, B diffuses faster than As (i.e. B has a higher diffusion coefficient) during high temperature post implant steps such as annealing. 9

10 Problem 2: Etching (32 pts) A. List the three main mechanistic steps involved in a wet etching process. (3 pts) 1. transport_(absorption) of reactants_ 2. reaction 3. desorption of by product B. Increasing the temperature typically results in an increased etch rate. Briefly explain why that is the case. Specifically answer this question in reference to the mechanistic steps listed in A. (5 pts) [1] Reaction rate increases with increasing temperature. [2] Transport (absorption) rate increases with increasing temperature. [3] Desorption rate increases with increasing temperature. +0 for none +5 for [1] only +2.5 for [2] only +2.5 for [3] only +5 for both [1] and [2] +5 for both [1] and [3] +2.5 for both [2] and [3] +5 for all three of them, [1] and [2] and [3] C. Increasing the concentration of the reactant typically results in an increased etch rate. Briefly explain why that is the case. Specifically answer this question in reference to the mechanistic steps listed in A. (5 pts) [1] Reaction rate increases with increasing concentration. [2] Transport (absorption) rate increases with increasing concentration. [3] Desorption rate is not affected. +0 for none +2.5 for [1] only +5 for [2] only +0 for [3] only +5 for both [1] and [2] +1.5 for both [1] and [3] +4 for both [2] and [3] +4 for all three of them, [1] and [2] and [3] 10

11 D. Stirring the etching solution typically results in an increased etch rate. Briefly explain why that is the case. Specifically answer this question in reference to the mechanistic steps listed in A. (5 pts) [1] Reaction rate is not affected. [2] Transport (absorption) rate increases with increasing agitation. [3] Desorption rate increases with increasing agitation. +0 for none +0 for [1] only +2.5 for [2] only +2.5 for [3] only +1.5 for both [1] and [2] +1.5 for both [1] and [3] +5 for both [2] and [3] +4 for all three of them, [1] and [2] and [3] E. List the three main disadvantages of wet etching over dry etching. (3 pts) Isotropic, Contamination, Lack of control, Lack of uniformity, Inadequate for defining very fine features, Potential of chemical handling hazards +1 for each disadvantage stated, max 3pts F. Consider a 1um thick photoresist pattern shown below. A dry etch is performed on the sample with the etch rate of oxide being 100 nm/min. 1 um PR PR 0.5 um Oxide Si Substrate If the selectivity of PR versus oxide is 1:10 (i.e., Oxide etch rate is 10X faster than PR), redraw the figure after 1min and 5mins etching. (6 pts) 1.02 um 1.1 um PR 45? 45? PR PR 45? 0.5 um Oxide 0.4 um 0.5 um Oxide Si Substrate Si Substrate 45? PR 11

12 +3 for the correct profile after 1min; +1 for the lateral dimension of the trench, 1.02 μm +1 for the vertical dimension of the trench, 0.1 μm +1 for the sidewall angle, ~ for the right etch profile after 5mins; +1 for the lateral dimension of the trench, 1.1 μm +1 for the vertical dimension of the trench, 0.5 μm +1 for the sidewall angle, ~84.29 G. Starting with sidewall angle of PR, θ P R (sidewall angle is the angle between sidewall and the substrate. For example, θ PR =45 shown above), derive the sidewall angle of the etched oxide, θ OX, as a function of selectivity (i.e. oxide etching rate is s times faster than PR etching rate). (5 pts) 12

13 Problem 3: Deposition (19 pts) a) Assume a starting substrate profile shown below. A conformal deposition of Al2O3 is then performed with a deposition rate of 0.1 micron/min. Sketch the cross sections of the deposited film for a completely conformal deposition after 1 min, 2 min, and 4min of deposition. (6 pts) Total 6 points. (2 pt for each curve and 1pt for conformal coverage each curve) 13

14 b) For chemical vapor deposition of poly Si using SiCl 4 as a gaseous source, the vapor phase masstransfer coefficient hg = 1 cm/sec, the surface reaction rate constant k s = 2x10 6 exp( 1.9eV/kT) cm/sec, and the concentration of Si atoms in the gas stream Cg = 3x10 16 atoms/cm 3. (The atomic concentration of solid Si is 5x10 22 atoms/cm 3.) (6 pts) i) What is the growth rate at 500 C? The growth rate of a silicon layer can be calculated using v J kh s g Cg N k + h N s = = s g 1.5pt Use h = 1 cm/sec, g ks 5 k ev / K v 6 = 2 10 exp( 1.9 / kt), Cg =, T=773K, we can get the growth rate is 13 = cm/ s 1.5pt ii) At what temperature does k s = h g? 16 3 = 3 10 atoms/ cm, N 22 3 = 5 10 cm, If ks = h, then we have g exp( 1.9 / kt ) 1 =. 1.5pt We can get T=1519.7K by solving the equation. 1.5pt c) List the major advantages of using chemical vapor deposition versus physical vapor deposition for thin films. (4 pts) Adv. of CVD over PVD: - Better coverage of the surface. 2pt - Better Uniformity. 2pt d) List the major advantages of using sputtering deposition versus evaporation deposition for thin films. (3 pts) Advantages of sputtering over evaporation: - For multi component films, sputtering gives better composition control using compound targets. Evaporation depends on vapor pressure of various vapor components and is difficult to control. - Better lateral thickness uniformity(lager target for sputtering and superposition of multiple point sources). - Better coverage. Total 3 points. For each advantage, you get 1.5 point. 14

15 Problem 4: Oxidation (8 pts) We have a structure shown below. This structure is placed in a furnace and oxidized at 900ºC in the presence of dry oxygen in order to grow a thin layer of SiO2. Draw the schematic of this structure after the dry oxidation process. In particular, make sure that your drawing emphasizes the relative thickness and the surface profile/ /morphology of SiO2 obtained in different regions. (8 pts) (Newly formed) oxide thickness: region (ii) > (iii) > (i) > (iv) (no oxide) Bird s beak: 2 pts 15

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