Applied Mechanics and Materials Vol. 393 (2013) pp 259-265 Online available since 2013/Sep/03 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMM.393.259
Effect of Double Sided Process Parameters in Lapping Silicon Wafer ABDUL RAHIM M Sahab1, a, NOR HAYATI Saad2,b, AMIRUL Abd Rashid2,c , NORIAH Yusoff 2,d , NASSYA Mohd Said 1,e , AHMAD FAIZ Zubair3,f and AHMED Jaffar 1,g 1
AMREC, SIRIM berhad, No. 1, Persiaran Dato’ Menteri, Section 2, P.O. Box 7035, 40700 Shah Alam, Malaysia 2
Faculty of Mechanical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia
3
Faculty of Mechanical Engineering, Universiti Teknologi MARA, 13500 Pulau Pinang, Malaysia a
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected],
[email protected], g
[email protected]
Keywords: Silicon wafer, double sided lapping, total thickness variation, surface roughness, waviness.
Abstract. Silicon wafer is widely used in semiconductor industries for development of sensors and integrated circuit in computer, cell phones and wide variety of other devices. Demand on the device performance requires flatter wafer surface, and less dimensional wafer variation. Prime silicon wafer is hard and brittle material. Due to its properties, double sided lapping machine with ceramic grinding agent were introduced for machining high quality standard silicon wafers. The main focus is the silicon wafer with high accuracy of flatness; to reduce total thickness variation, waviness and roughness. In this paper the lapping experiment and analysis showed that the double sided lapping machine is able to produce total thickness variation less than 10 um at controlled process parameters within short processing time. Machining using low mode method reduced the total thickness variation (TTV) value. The lapping load and speed directly reflected the performance and condition of final silicon wafer quality. Introduction Lapping is an abrasive machining process used to obtain materials with high dimensional accuracy and excellent surface finish. Polishing is a process with smooth and lustrous surface finish. The shiny smooth surface is reflected from the action of fine abrasive powders and smooth plate surface [1, 2]. Lapping and polishing alter surface qualities such as roughness, waviness, and flatness [3]. Modern lapping machines (double sided lapping) can produce exceptionally flat surfaces with a flatness of less than one light band (helium light, wavelength 23.2 millionths of an inch). Single-sided lapping is a time consuming process and tedious if both sides of the wafer surface need to be lapped. It requires a skilled workmanship. The process is highly undesirable for mass production due to the time factor involved [4]. Semiconductor industry is the fastest growing industry in the world. Most of semiconductor products are produced from silicon wafers. Silicon is a crystal structure with covalent atom bonding. High hardness performance and brittle properties of silicon cause difficulties in machining [5] the wafer. Year by year the price of semiconductor products decreases rapidly. To keep this trend the manufacturer is required to significantly reduce the processing cost of silicon wafers. Silicon wafers (material for fabrication of microchips, sensor and other micro devices) must have high accuracy of flatness in order to print device patterns or circuits by a lithographic process. High accuracy of flatness of silicon wafer directly impacts devices line-width capability of process latitude, yield and throughput. The feature sizes of semiconductor devices also strongly demanded flatter wafer surface [6, 7, 8, 9, 10]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 103.6.236.38, Universiti Teknologi Mara (UiTM), Shah Alam, Malaysia-02/10/13,01:54:43)
260
Advances in Manufacturing and Mechanical Engineering
Total Thickness Variation (TTV), is an important measurement done on a silicon wafer at the final lapping process. The TTV represents the difference between the minimum and maximum wafer thickness. In general a typical TTV measurement on a good 8 inch prime wafer would be less than 15µm. Site Total Indicator Reading (TIR) is a site by site measurement of flatness of prime silicon wafer (20mm x 20mm or larger site). TIR measurement must be less than 0.3 µm. Waviness is also an important specification in determining the quality of final wafer. Waviness is how a concave or convex deformation of a median surface of the measurement of unclamped silicon wafers. In general a typical waviness value on a good 8 inch prime wafer would be less than 30 µm [11]. In this study the relationship between double sided lapping parameters setting and different lapping mode methods are evaluated for the final quality of the silicon wafer. The main target objective is to get the TTV less than 10 µm. Experimental Details Materials and Grinding Agent. In this experiment 150 mm silicon wafers diameter with an average thickness of 650-700 micron were selected for lapping and polishing process. Two type of grinding agents and a type of diamond slurry were used for the grinding and polishing process. Table 1 shows the grinding and polishing agent which are used in the experiment. Table 1: Type of grinding and polishing agent used for lapping process Process Type of grinding agent Average size, µm First Lapping
Silicon Oxide
74
Second Lapping
Aluminium Oxide
25
Polishing
Diamond slurry
5
Lapping Machine, Carriers and Process. PR Hoffman 3100 Double Sided Lapping Machine was used for the lapping and polishing process. The machine requires 60 teethes carrier and minimum of 3 carriers attached per lapping process for producing high accuracy and quality surface of silicon wafer. Each carrier is only able to guide a piece of 150mm silicon wafer (1 silicon wafer disk for a carrier). The metal carrier used in the experiment was installed with rubber inserts at the edges of the silicon wafer guide. The insert was used to cover the edges of the silicon wafer from chipping or broken during lapping process. Fig. 1 and 2 show the double sided lapping machine and the insert carrier used for the experiment. Two types of lapping mode or methods were applied in the first analysis. The lapping modes are low and high mode methods. The differences between both modes are rotation carrier direction and lapping speed. In the second lapping experiment the top plate speed, lapping time and lapping load were varied to obtain the relation of the lapping performance of silicon wafer and the processed parameters. In this study the machine was set to low mode method. Table 2 and Table 3 highlight the lapping parameters used in the first experiment. Fig. 3 shows and illustrates the differences between the low and high lapping mode method.
Fig. 1: Pr Hoffmann Double Sided Lapping Machine
Fig. 2: Insert lapping carrier
Applied Mechanics and Materials Vol. 393
261
Table 2: Low mode and high mode method parameters of lapping silicon wafer Parameters
Low Mode Technique High Mode Technique
Top Plate Speed, RPM
20
15
Ring Gear Speed, RPM
9
8
Centre Gear Speed, RPM
15
5
Load, lb
22
22
Lapping Time, minute
10
20
Abrasive Agent, micron
74
74
Table 3: Low mode method parameter of lapping wafers with varied lapping speed and load Parameters Low Mode Technique Low Mode Technique Top Plate Speed, RPM
25
30
Ring Gear Speed, RPM
10.5
12
Centre Gear Speed, RPM
20
25
Load, lb
15
11
Abrasive Agent, micron
25
25
a
b
Fig. 3: (a) High mode and (b) low mode method of lapping process Measurement and Analysis. After lapping process the wafer samples were cleaned with clean water, dried with air pressure and ready for wafer analysis. A Coordinate Measuring Machine (CMM) was used for thickness measurement of the silicon wafer after lapping process. The data obtained are used to calculate the Total Thickness Variation (TTV) according to formula shown in Equation 1. An Alicona Infinite Focus machine was used for waviness and roughness measurement of the silicon wafers. Fig. 4a and Fig. 4b illustrate the CMM equipment and points measured on a silicon wafer.
Fig. 4a: Coordinate Measuring Machine (CMM) TTV = Tmax - Tmin
Fig. 4b: Points measured on a silicon wafer (1)
262
Advances in Manufacturing and Mechanical Engineering
Test Result and Discussion Total Thickness Variation (TTV), Roughness and Waviness according to Low Mode method and High Mode method. Table 4 shows the test results of the total thickness variation (TTV), waviness and roughness of wafer which were lapped using double sided lapping machine at low mode method. The experiments used silicone oxide as grinding materials for the lapping process. Test results show that the average TTV value (using low mode method) is 11 micron. While the average waviness and roughness of the samples (selected sample from low mode method process) is 0.44 and 0.71 respectively. Table 4: Total thickness variation (TTV), waviness and roughness values lapped by low mode method Sample TTV (µm) Measurement Waviness (µm) Roughness (µm) point of Sample 1 1
12
1
0.50
0.62
2
12
2
0.32
0.74
3
8
3
0.49
0.76
Average value
11
0.44
0.71
Table 5 shows the test results of the total thickness variation, waviness and roughness of the wafer which were lapped using double sided lapping machine at high mode method. The average TTV value in this experiment (using high mode method) is 12 micron. The average waviness and roughness of samples (selected sample from high mode method process) is 0.63 and 0.42 respectively. The experiment results demonstrate that the low mode method of double sided lapping produced good lapping performance and consequently good quality of silicon wafer. The produced wafer has low value of TTV and waviness compared to the high mode method machining. Table 5: Total thickness variation (TTV), waviness and roughness values lapped by high mode method Sample TTV (µm) Measurement point Waviness (µm) Roughness (µm) of Sample 1 1
18
1
0.70
0.50
2
12
2
0.59
0.33
3
7
3
0.61
0.43
Average value
12
0.63
0.42
Total Thickness Variation Measurement versus Lapping Speed and Load. Table 6 shows the Total Thickness Variation (TTV) values for six selected samples with different lapping speed and load parameters. In this experiment the lapping uses low mode method process. The low mode method was selected due to its performance in the first experiments which produces stable and accurate measurement result compared to the high mode method. In the experiment, lapping loads of 11 lb and 15 lb were set with different speed of top plate, ring gear and centre gear. Both lapping processes used aluminium oxide grinding material with an average particle is 25 um. Test results for both samples show that after 20 minute lapping time, 1b and 2c samples were broke for both process parameters. It can be explained due to the wafer disc slipped from carrier and broke after long time period. At lapping load of 11 lb, speed of the top plate is 30 rpm, ring gear is 12 rpm and centre gear is 25 rpm, the measured TTV for both samples are 6 and 9 µm after 20 minute lapping.
Applied Mechanics and Materials Vol. 393
263
While, with lapping load of 15 lb, speed of the top plate, ring gear and centre gear of 25, 10.5 and 20 rpm respectively, the TTV for both samples are 2 and 8 µm. Fig. 6 highlights and compares the plotted TTV test result versus lapping parameters. From the Table 6 and Fig. 6, increasing lapping load and reducing the lapping speed parameters resulted small TTV values. Sample
Table 6: Total thickness variation with different load and speed Load, Speed, RPM Time, Ib minute Top Plate Ring Gear Centre Gear
TTV, µm
1a
15
25
10.5
20
20
2
1b
15
25
10.5
20
20
broken
1c
15
25
10.5
20
20
8
2a
11
30
12
25
20
6
2b
11
30
12
25
20
9
2c
11
30
12
25
20
broken
Total Thickness Variation (TTV), um
Relationship: low mode and high mode method versus TTV, waviness and roughness. Fig. 7a shows the TTV comparison between low mode and high mode lapping methods. Three samples (Sample 1, 2, and 3) were machined together in the selected lapping mode process. From Figure 7a, TTV of samples (1, 2 and 3) which used low mode method is more stable compared to the high mode method. The thickness variation of low mode method is only 4 micron compared to high mode method, 11 micron (refer to Table 4 and Table 5). Fig. 7b shows the waviness relation between low and high mode method lapping process. The result illustrates that the low mode method produced low waviness compared to the high mode. The average waviness values of low mode and high mode lapping method are 0.44 µm and 0.63 µm respectively. Fig. 7c shows the roughness relation between low and high mode lapping method. In this experiment the surface roughness of silicon wafer by low mode lapping method is higher compared to high mode method. The average values of low mode and high mode lapping method are 0.71 µm and 0.42 µm respectively. The high surface roughness of low mode method compared to low mode method mostly because of the different selection of lapping speeds (top plate, ring gear and centre gear) and time of the lapping process (refer to Table 2). 10 5
Load 15 speed 25
0
Load 11 speed 30
0
1
2
Sample
Fig. 6: The relation of TTV value for each silicon wafer disc with different setting of load and speed
Fig. 7a: TTV in relation to low mode and high mode method of lapping process
264
Advances in Manufacturing and Mechanical Engineering
Fig. 7b: Waviness in relation to low mode and high mode method of lapping process
Fig. 7c: Roughness in relation to low mode and high mode method of lapping process
Conclusion The experiment results of the double sided lapping process (PR Hoffmann 3100 machine) for lapping 150 mm wafer disc portray that the low mode method process is considered the best method in producing low values of the total thickness variation (TTV) of wafer. Increasing speed of the top plate, ring gear and centre gear and reducing the lapping load, produced low value of TTV of wafer. The 150 mm silicon wafer lapped by using low mode method of double sided lapping machine is able to reduce the total thickness variation (TTV) less than 10 um. The application of double sided lapping machine and appropriate selection of type and sizes of grinding materials for wafer lapping is able to reduce processing time and manufacturing cost for production of semiconductor products with high quality standard. Acknowledgements The authors would like to acknowledge the Universiti Teknologi MARA (UiTM) Shah Alam and the Malaysian Ministry of Higher Education (MOHE) for supporting the research cost under ERGS Grant (600-RMI/ERGS 5/3 (16/2011). References [1] Yuan-Cherng Chiou, Rong-Tsong Lee, Chang-Li Yau, A novel method of composite electroplating on lap in lapping process, International Journal of Machine Tools & Manufacture 47 (2007) 361-367. [2] J.H. Liu, Z.J. Pei, Graham R. Fisher, Grinding Wheels for manufacturing of silicon wafers: A literature review, International Journal of Machine Tools & Manufacture 47 (2007) 1-13. [3] W.J. Liu, Z.J. Pei, X.J. Xin, Finite element analysis for grinding and lapping of wire-sawn silicon wafer, Journal of Materials Processing Technology 129 (2002) 2-9. [4] Z.C Li, Z.J. Pei, Graham R. Fisher, Simultaneous double side grinding of silicon wafers: a literature review, International Journal of Machine Tools & Manufacture 46 (2006) 1449-1458. [5] P.S. Sreejith, G. Udupa, Y.B.M. Noor, B.K.A. Ngoi, Recent Advances in Machining of Silicon Wafers for Semiconductor Applications, International Advanced Manufacturing Technology 17 (2001) 157-162. [6] Z.J. Pei, Graham R. Fisher, Milind Bhagavat, S. Kassir, A grinding-based manufacturing method for silicon wafers: an experimental investigation, International Journal of Machine Tools & Manufacture 45 (2005) 1140-1151.
Applied Mechanics and Materials Vol. 393
265
[7] Z.J. Pei, Alan Strasbaugh, Fine grinding of silicon wafers, International Journal of Machine Tools & Manufacture 41(2001) 659-672. [8] Nor Hayati Saad, Xueyong Wei, Carl Anthony, Hossein Ostadi, Raya Al-Dadah and Michael C.L. Ward, Impact of Manufacturing Variation on the Performance of Coupled Micro resonator Array for Mass Detection Sensor, Procedia Chemistry 1,1 (2009) 831-834. [9] Xueyong Wei, Nor Hayati Saad, and Michael C.L. Ward , Analysis of manufacturing variation in a coupled microresonators array based on its designed values and measured eigenfrequencies, Micro & Nano Letters, 5,5 (2010) 300-303. [10] Nor Hayati Saad, Amirul Abd Rashid, Noriah Yusoff, Ahmad Faiz Zubair, Farrahshaida Salleh, Ahmed Jaffar, Assessing Level of MEMS Process Variation on Fabricated Micro Resonator Sensor Structure, 1st Joint Symposium on System-Integrated Intelligence: New Challenges for Product and Production Engineering, Hannover Germany (2012) 63-65. [11] Information on http://www.processpecialties.com/siliconp.htm
Advances in Manufacturing and Mechanical Engineering 10.4028/www.scientific.net/AMM.393
Effect of Double Sided Process Parameters in Lapping Silicon Wafer 10.4028/www.scientific.net/AMM.393.259
