Adhesion Copper Interconnects in Microelectronics University of Minnesota – Twin Cities MatS 4221W Lab Group 2 Timothy S. Marass Loc Nguyen Peter Sylvestre Ethan Taylor Date(s) of Laboratory: November 20 & Decmber 4, 2007 Date of Report: December 12, 2007 Adhesion – MatS 4221W – Dec. 12, 2007 I. Executive Summary This experiment investigated… Page 2 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Table of Contents Executive Summary ............................................................................................................... 2 Introduction ............................................................................................................................ 4 Methods .................................................................................................................................. 5 Results .................................................................................................................................... 7 Discussion ............................................................................................................................ 16 Conclusions .......................................................................................................................... 17 References ............................................................................................................................ 18 Appendix A: Data................................................................................................................. 19 Appendix B: Sample Calculations ....................................................................................... 20 Page 3 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 II. Introduction Periods of time have been named for, and often defined by, various technological advances. By the same convention, if 2000 to 700 B.C is known as the Bronze Age1 because of the appearance of bronze tools and weapons, then surely the last 100 years could be characterized as the beginning of the Silicon Age. The microelectronics industry is rapidly enlarging due to the ever-present demand for smaller, more powerful electronic devices with extended battery lives. Most integrated circuits use aluminum for interconnects; however, there are issues with miniaturizing aluminum connections because of electro-migration and stressmigration issues2. Aubel et. al. found that “Copper shows a much higher electromigration resistance compared to aluminum3.” Copper can diffuse through the silicon wafers it is deposited on, resulting in short circuits, and poorly adheres to the silicon wafers. To combat these problems with copper, it has been theorized4 that an intermediate layer of Ti between the Cu and silicon wafer could form a diffusion barrier and increase the overall adhesion of the system. In this lab, three samples will be analyzed to determine if the adhesion issues can be resolved by depositing a layer of Ti. These three samples include: A: 3um, Ti, Cu on a Si wafer; B: bare Si wafer; and C: 200nm, Ti, Cu on a Si wafer. 1 www.le.ac.uk/archaeology/ulas/birstall.html Lab 6 Handout. 3 http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/7298/28198/01261738.pdf 4 Lab 6 Handout. 2 Page 4 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 III. Methods The first experiment was performed using micro-indenters. The three samples were mounted onto a mounting platform shown below. Figure 1. Sample on testing platform. A: 3um, Ti, Cu on a Si wafer; B: bare Si wafer; and C: 200nm, Ti, Cu on a Si wafer. Location of indentation marked with arrows per sample. Tribo-indentor and Vickers indenters were used to produce indentation blisters which will later be analyze to determine the adhesion properties of the film. The initial process was preparing and determining a clean area on the film for indentation via Video Enhanced Microscope (VEM). Areas were then marked with sharpie marker; Figure 1 showed the circled marked area. Sample C was chosen for the first Tribo-indentation test then sample A and B. Two experimental tests were done for sample C and A, and one test for sample B; sample C was performed with 100nm and 20nm indentation depth, sample A with 1000nm and 2000, and sample B with 1000nm depth. VEM images were then taken of the indentation blister. Vicker hardness was then performed on sample A and C using NanoXp indenter. Four experiments were taken for sample A and six for sample C with various load setting. Relative indentation locations are shown in Figure 2 below. Figure 2. Vicker hardness indentation number and location for sample A and C. Page 5 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Indentation areas were then imaged via VEM. Profile scan of the indentation blisters using profilometer were then taken for sample A with 2 blisters in the same scan path. The second profile scan was from a third indentation blister for sample A. Sample B and C were then scanned using the profilometer. Page 6 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 IV. Results Initial VEM images revealed possible film buckling prior to indentation for sample C with the 200nm film on Si wafer shown in Figure 3. (a) (b) (c) (d) Figure 3. VEM images of 200nm copper film. (a) 2.5x Possible Blister: shows location of blister shown in 1.d. in respect to sample; arrow denotes blister location. (b) 50x Prior indentation on film. (c) 50x Possible Blister: shows 5um difference between blister surface and film background. (d) 50x Possible Blister: image showing 3um difference between blister surface and film background. Figure 4 showed indentation marks and indentation blisters for the 6 indentation experiments. Table 1 contains the load for each indentation mark as well as the calculated hardness value; data for indentation 1 and 2 was not recorded. Page 7 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 (a) (b) (c) (d) (e) (f) Figure 4. Indents made on 200nm film sample C. (a) Indent 1. (b) Indent 2. (c) Indent 3. (d) Indent 4. (e) Indent 5. Surface height = 0.004mm, maximum indent depth = 0.003mm, overall change (indentation depth) = 0.001mm. (f) Indent 6. Surface height = 0.004mm, maximum indent depth = 0.003mm, overall change (indentation depth) = 0.001mm. Table 1. Sample C, 0.2um of Cu. Hardness Measurements. Position Load,P[mN] D1[um] D2[um] HV 3 2942 22.1 25.7 973.9 4 490.3 12.8 12 603 5 490.3 11.9 12.7 617.8 6 980.7 15.4 15.9 757.1 Note: No data was taken from positions 1 and 2. Page 8 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Figure 5 showed VEM images of indentation mark and blister for sample A with 3um film and Table 2 contains load value and hardness that correspond to each indentation. (a) (b) (c) (d) Figure 5. Indents made on 3um-film sample A. (a) Indent 1. (b) Indent 2. (c) Indent 3. (d) Indent 4. Table 2. Sample A, 3um of Cu. Hardness Measurements. Position Load,P[mN] D1[um] D2[um] HV 1 490.3 22.4 26.1 157.6 2 980.7 29.3 32.4 192.9 3 2942 39.5 41.4 340 4 9807 60.1 55.9 551.2 (a) (b) (c) Figure 6. NanoXP indentation groups. (a) Overall view of Groups 1 (left) and 2 (right). (b) Group 1 indentations. (c) Group 2 indentations. Page 9 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Figure 7 showed VEM images of spontanious blisters for sample A that was not related to indentation. (a) (b) (c) (d) € (f) Figure 7. Sample A observed blisters. (a) Sample A, Blister 1. (b) Sample A, Blister 1 sample location. (c) Sample A, Blister 2. (d) Sample A, Blister 2 sample location. (e) Sample A, Blister 3. (f) Sample A, Blister 3 sample location. Table 3 and 4 are result form nanoxp indentation data which contains modulus from unload and hardness from unload for sample A. Table 1. Sample A, 3um, Ti, Cu Material Properties. Trial 1. Modulus From Unload Hardness From Unload (GPa) (GPa) Test 1 148 2.59 2 139 2.57 3 154 2.70 4 145 2.71 Mean 147 2.64 St. Dev. 5.9 0.072 Table 2. Sample A, 3um, Ti, Cu Material Properties. Trial 2. Modulus From Unload Hardness From Unload GPa GPa Test 1 158 3.22 2 156 3.23 3 158 3.23 4 165 3.33 Mean 159 3.25 Std. Dev. 4.0 0.053 Page 10 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Table 5 and 6 are result from nanoxp indentation data which contains modulus from unload and hardness from unload for sample C. Table 3. Sample C, 200nm, Ti, Cu Material Properties. Trial 1. Modulus From Unload Hardness From Unload GPa GPa Test 1 92 2.92 2 95 3.41 3 90 3.39 4 73 2.54 Mean 87 3.07 Std. Dev. 10.1 0.419 Table 4. Sample C, 200nm, Ti, Cu Material Properties. Trial 2. Modulus From Unload Hardness From Unload GPa GPa Test 1 23 1.72 2 47 1.78 3 78 1.22 4 0 **** Mean 49 1.57 Std. Dev. 27.9 0.31 Table 7 showed result from nanoxp indentation data which contains modulus from unload and hardness from unload for sample B. Table 5. Silicon (Si) Sample Material Properties. Modulus From Unload Hardness From Unload GPa GPa Test 1 179 13.04 2 197 12.28 3 186 12.63 4 195 12.38 Mean 189 12.58 Std. Dev. 8.3 0.34 Page 11 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Figure 8 and 9 showed graph of depth versus hardness and depth versus modulus for sample A with 3um film thickness from trial 1 and 2 respectively. Figure 8. Hardness and Modulus Load/Unload Curves. Sample A, 3um, Ti, Cu. Trial 1. Figure 9. Hardness and Modulus Load/Unload Curves. Sample A, 3um, Ti, Cu. Trial 2. Page 12 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Figure 10 and 11 showed graph of depth versus hardness and depth versus modulus for sample C with 200nm film thickness from trial 1 and 2. Figure 10. Hardness and Modulus Load/Unload Curves. Sample C, 200nm, Ti, Cu Material Properties. Trial 1. Figure 11. Hardness and Modulus Load/Unload Curves. Sample C, 200nm, Ti, Cu Material Properties. Trial 2. Page 13 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Figure 12 showed graph of depth versus hardness and depth versus modulus for sample B of silicon without and film. Figure 12. Hardness and Modulus Load/Unload Curves. Silicon (Si) Sample. a) b) c) Figure 13. NanoXP indentation using conical tip. a) Sample A with 4um displacement. b) Sample C with a 1um displacement. c) Sample C single indentation. Page 14 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 Table 6 and 7 present more data for sample A using NanoXP indenter. Figure 13a showed VEM image of conical indentation using NanoXP; indentations are 4um apart. Table 6. NanoXP data for sample A (3um film) with a 4um displacement between trial 1. Test Modulus From Unload Hardness From Unload GPa GPa 1 278.438 17.378 Table 7. NanoXP data for sample A(3um film) with a 5um displacement between trial 1 and 2. Test Modulus From Unload Hardness From Unload GPa GPa 1 275.599 15.601 2 291.314 17.979 Mean 283.456 16.79 Std. Dev. 11.112 1.682 % COV 3.92 10.02 Table 8 and 9 showed more data for sample C from NanoXP indenter. Figure 13b and c, showed VEM image of conical indentation using NanoXP; indentations are 1um apart. Table 8. NanoXP data for sample C(200nm film) with a 1um displacement between trial 1 and 2. Test Modulus From Unload Hardness From Unload GPa GPa 1 198.779 36.707 2 200.092 36.962 Mean 199.436 36.835 Std. Dev. 0.929 0.181 % COV 0.47 0.49 Table 9. NanoXP data for sample C(200nm film). Test Modulus From Unload Hardness From Unload GPa GPa 1 190.211 42.756 Page 15 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 V. Discussion A Page 16 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 VI. Conclusions Experimental data showed… Page 17 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 VII. References Page 18 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 VIII. Appendix A: Data Page 19 of 20 Adhesion – MatS 4221W – Dec. 12, 2007 IX. Appendix B: Sample Calculations Page 20 of 20