Process optimization of gold stud bump manufacturing using artificial neural networks

Expert Systems with Applications 29 (2005) 264–271 www.elsevier.com/locate/eswa

Process optimization of gold stud bump manufacturing using artificial neural networks Leo Chau-Kuang Liau*, Burce Shin-Ching Chen Department of Chemical Engineering and Materials Science, Yuan Ze University, 135 Yuan-Tung Road, Chungli 320, Taiwan, ROC

Abstract The optimal operating conditions of a gold stud bumping process were determined using a process optimization scheme for a microelectronic packaging foundry. The schematic procedure of the process optimization is first to evaluate effects of the operating parameters on bump size and height, and shear stress, using a design of experimental method. Several operating parameters, such as compression force (bonding load) and electronic flame off (EFO) current and time, were analyzed to affect the formation of the stud bump significantly in the bumping process. Artificial neural networks (ANN) modeling was adopted to establish the relationship between the operating parameters and the bump properties with the experimental data. Some optimization cases of the bumping process with constraints were evaluated using the optimization scheme. q 2005 Elsevier Ltd. All rights reserved. Keywords: Gold stud bump; Process optimization; Artificial neural networks; Design of experiment; Shear stress.

1. Introduction The packaging of microelectronic devices has been attractively adopted a stud bond bump-flip chip (SBB-FC) technology to the packaging manufacturing. The main advantages of utilizing this technology in the packaging are not only to shorten the interconnect distance of the electrical signals between chips and substrates but to increase the number of input–output (I/O) counts. The electrical performance of the packaging can be greatly enhanced by applying this bonding technology. Besides, this connection structure can effectively reduce the geometrical dimensions of the electronic products. Moreover, the product stability and reliability in the first level IC interconnections can be improved using this bonding method. Furthermore, these advantages can be expanded to the demands of modern electronic products, especially for high frequency applications. For the SBB-FC technology, one of the critical steps is on the processing of the stud bumps which greatly influences * Corresponding author. Tel.: C886 3 4638800x573; fax: C886 3 4559373. E-mail address: [email protected] (L.C.-K. Liau).

0957-4174/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eswa.2005.04.023

the packaging performance. In recent years, gold (Jordan, 2003) and copper (Zama, Baldwin, Hikami, & Murata, 2001) stud bumps have been found as better electrical and mechanical properties in the element interconnections. From the material property consideration, gold stud bumps applied on the flip chip packaging are regarded as high quality and reliability for the packaging components, especially for high frequency modulus (Jordan, 2003). The operating factors of the gold stud bumping process can affect the properties (including the shape) of the produced stud bumps as shown in Fig. 1 (Klein, Oppermann, Kalicki, Aschenbrenner, & Reichl, 1999). Properties, such as electrical and mechanical properties of the packaging, are major requirements to evaluate the quality of the fabricated stud bump discussed by Lui and Ni (2002) and Klein et al. (1999). On the other hand, the size of the bump diameter (BD) can also influence the design of I/O numbers and the adhesion of the bump on the IC substrate. In fact, the adhesion (shear stress) of the stud bump attached on the substrate has to be satisfied with required regulations. In addition, variations of the bump height (BH) can affect the contact performance between the substrate and the chip. Therefore, if the bumping process has to be operated effectively, the effects of the operating factors on the bump properties have to be fully understood from the process operation standpoint.

L.C.-K. Liau, B.S.-C. Chen / Expert Systems with Applications 29 (2005) 264–271

Fig. 1. The typical shape of the stud bump.

2. Gold stud bumping process The gold stud bumping process is originally derived from the modified wire bonding process (Harman, 1997). The unit operations of processing a gold stud bump with double bumping modes include the forming of a free air ball (FAB), the first bumping mode, and the second bumping mode, as illustrated in Fig. 2. The operating units of the gold stud bump are described as following: (1) In the beginning of the process, the gold wire is fed through a capillary and an IC substrate on a bond pad is heated as shown in Fig. 2(a).

(2) For the FAB step, a gold ball forms at the end of the melted wire by sparking the electronic flame off (EFO) modulus as depicted in Fig. 2(b). (3) For the first bumping mode as shown in Fig. 2(c)–(e), the capillary tube is pushed down (bonding load) to the bond pad and the FAB is then compressed as shown in Fig. 2(c). The ultrasonic power is then applied to let the ball adhere to the bond pad as illuminated in Fig. 2(d). After the stud bump is formed, the tube is raised up to a certain position to complete the bonding as shown in Fig. 2(e). (4) The second bumping mode proceeds and repeats the previous procedure of the first bumping mode as illuminated in Fig. 2(f)–(h). (5) The capillary is then raised and the gold wire is cut off to complete the whole gold stud bumping process as demonstrated in Fig. 2(i). From these unit operations, the important factors to influence the formation of the FAB are the EFO current or voltage, the EFO time, and the tail length of the wire in the capillary; while the operating parameters to affect the stud bump bonding condition are analyzed as the ultrasonic power and time, and the compression force both for the first and second bumping modes. However, not much research was studied in the effects of the operating parameters on the properties of the produced gold stud bumps. Furthermore,

Gold wire EFO Module Capillary

I.C. Bond Pad

Heater

(a)

(b)

(f)

(c)

(g)

265

(d)

(e)

(h)

(i)

Fig. 2. The processing steps of a gold stud bump with double bumping modes.

266

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Table 1 The operating factors with three levels for the DOE Operating parameter A B C D E F G H I J

the bump properties using an artificial neural networks (ANN) approach. The built model can then be applied to determine the optimal operating conditions of the gold stud bumping process.

Level

Bond pad temperature (8C) Tail length (mm) EFO time (ms) EFO current (mA) Ultrasonic energy: first mode (W) Ultrasonic time: first mode (ms) Compression force: first mode (gf) Ultrasonic energy: second mode (W) Ultrasonic time: second mode (ms) Compression force: second mode (gf)

1

2

3

150 0.6 2.52 20 0.36 40 30 0.036 5 10

180 0.7 3 25 0.504 50 50 0.108 15 15

210 0.8 3.52 30 0.648 60 70 0.18 25 20

3. Experiment The gold wire used in this study is Nippon B3 series type (1.0 mil.) provided by Nippon Corp., Japan. The bonding machine is adopted by Panasonic (HW27U-Ha) and the capillary is selected as Gaiser (1570-15-437GM). The bond pads (substrates) are silicon wafer coated with gold. The gold stud bump is produced utilizing a double mode process and follows the steps as described in Fig. 2. The bond pads were put in the heating zone for 10 min at a setting temperature in the beginning. The bumping process follows the procedure described in Section 2 with the defined operating parameters. There are 15 samples fabricated for each experimental run for the data collection and analysis. The stud bump properties of these fabricated samples were measured by analytical instruments. The size and height of the stud bumps are measured using a Nikon type VM250 3-D instrument. While, the shear stress of the stud bump is analyzed using an EPACK type Dage

the process optimization cannot be determined if the process model, relationship between the operating parameters and the bump properties, is not available. In this work, the optimization scheme was developed for evaluating the optimal operating condition of a gold stud bumping process for a practical foundry manufacturing. The effects of the operating factors on the bump properties, such as bump diameter, bump height, and shear stress of the stud bump was demonstrated using the design of experiment (DOE). The process model is established to describe the relationship between operating factors and Table 2 A Latin square table (L27) for the DOE No

A

B

C

D

E

F

G

H

I

J

SSa (gf/mil2)

BSb (mil)

BHc (mil)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3

1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3 1 1 1 2 2 2 3 3 3

1 1 1 2 2 2 3 3 3 2 2 2 3 3 3 1 1 1 3 3 3 1 1 1 2 2 2

1 1 1 2 2 2 3 3 3 3 3 3 1 1 1 2 2 2 2 2 2 3 3 3 1 1 1

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

1 2 3 1 2 3 1 2 3 2 3 1 2 3 1 2 3 1 3 1 2 3 1 2 3 1 2

1 2 3 1 2 3 1 2 3 3 1 2 3 1 2 3 1 2 2 3 1 2 3 1 2 3 1

1 2 3 2 3 1 3 1 2 1 2 3 2 3 1 3 1 2 1 2 3 2 3 1 3 1 2

1 2 3 2 3 1 3 1 2 2 3 1 3 1 2 1 2 3 3 1 2 1 2 3 2 3 1

1 2 3 2 3 1 3 1 2 3 1 2 1 2 3 2 3 1 2 3 1 3 1 2 1 2 3

6.76 2.98 1.03 6.54 6.84 4.03 5.35 7.95 5.22 7.15 7.31 6.39 5.33 8.18 5.30 5.42 7.90 4.83 7.65 5.62 8.51 7.68 5.32 8.10 6.08 3.29 5.59

3.14 3.88 4.53 3.10 4.08 4.82 3.12 4.08 5.02 4.19 3.14 4.33 4.12 3.42 4.26 3.99 3.36 4.11 3.81 4.49 3.60 3.88 4.54 3.65 3.86 4.46 3.65

1.14 0.85 1.33 2.31 1.51 1.06 2.99 2.04 1.40 1.58 2.21 1.41 1.36 1.55 1.09 1.17 1.51 1.05 1.70 1.28 1.76 1.51 1.32 1.73 1.19 1.01 0.96

a b c

Shear stress. Ball size. Ball height.

L.C.-K. Liau, B.S.-C. Chen / Expert Systems with Applications 29 (2005) 264–271

9

25

8

23 21

7

19

6

17

5

15

A1A2A3 B1B2B3 C1C2C3 D1D2D3

E1E2E3 F1F2F3 G1G2G3 H1H2H3

I1I2I3

J1J2J3

4

Operating factor

(b) 46

S/N (db)

5 42 40 4

4.2. ANN process modeling The experimental data carried out for the above analysis were applied to construct the process model using an ANN

36

A1A2A3 B1B2B3 C1C2C3 D1D2D3 E1E2E3

F1F2F3 G1G2G3 H1H2H3 I1I2I3

J1J2J3

3

Operating factor

(c) 36

3 2.8

34

2.6

S/N (db)

2.4 32

2.2 2

30

1.8 1.6

28

1.4

Bump Height (BH) (mil)

where y is the average of the experimental output data and s is the standard deviation. From the DOE results, the effects of each parameter on shear stress, bump diameter, and bump height are analyzed and illustrated in Fig. 3. The important operating factors affecting the shear stress are analyzed as compression force (G) in the first bumping mode, the bump pad temperature (A), and EFO time (C) and current (D) in the FAB processing as shown Fig. 3(a); while the bump size is greatly influenced by the first bumping mode (E, F, G) as demonstrated in Fig. 3(b). In addition, the FAB process (C, D) and first bumping mode (G, E) affect the bump height significantly as illustrated in Fig. 3(c). On the contrary, the step of the second bumping mode (H, I and J) reveals little influence on the stud bump properties. Results of the experimental analyses indicates that the stud bump properties were most influenced by the FAB (C, D) and the first bumping mode (E,F,G).

38

Bump diameter (BD) (mil)

6

44

4.1. Design of experiment (DOE) The selected parameters used in the experimental design are listed in Table 1 with three levels in the operating ranges. An orthogonal array of a L27 table as shown in Table 2 was designed to set the operating parameters for the experimental runs in this work. In the data analysis, the signal to noise ratio (S/N) related to the standard deviation of the experimental data of the stud bump samples in these analyses is defined as  2 y S=N Z 10 !log 2 (1) s

Shear stress (gf/ml2)

10

27

4. Results and discussion The scheme of the stud bumping process optimization includes the design of experimental analysis, the ANN process modeling approach, and the optimization approach. The objective of the experimental analysis is to evaluate effects of the operating parameters (input) on the stud bump properties (output) described above. Several optimization cases were discussed and the optimal operating conditions were determined utilizing the built ANN process model and the optimization approach.

11

(a) 29

S\N (db)

Series 4000 based on the EIA/JEDEC22-B116 (Wire Bond Shear Test Method) regulations. In addition, the ANN modeling simulation and the process optimization approach utilize the MATLAB commercial software provided by MATLAB Corp.

267

1.2 26

A1A2A3 B1B2B3 C1C2C3 D1D2D3 E1E2E3 F1F2F3 G1G2G3 H1H2H3

I1I2I3

J1J2J3

1

Operating factor

Fig. 3. Results of the design of experimental analysis for the operating factors vs. (a) shear stress (b) bump diameter and (c) bump height.

approach. The built ANN model can then be expressed the interactions and relationships between the operating parameters and the stud bump properties by the process model. The detailed procedure of carrying out the ANN modeling can be found in Heykin (1999) or our earlier study (Liau, Yang, & Tsai, 2004). In the previous analytical results of DOE, the significant operating factors to affect the shear stress were the compression force in the first bumping mode, EFO current and time, and the pad temperature. Fig. 4 illustrates the shear stress associated with these factors. Fig. 4(a) demonstrates that the shear stress increases if higher EFO current and time, and pad temperature but lower compression force is adopted in the first bumping mode. The shear stress of the stud bump does not increase with the use

268

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Fig. 4. The shear stress of the stud bump associated with the operating parameters.

of larger compression force. This is because that if the compression is too large, the cratering condition can be generated inside the stud bump. Thus, the adhesion of the stud bump with the substrate becomes worse. Besides, the stress can be higher if the substrate temperature increases as shown in Fig. 4(b). This shows that it can improve the better gold-gold bonding condition on the substrate surface coated with gold in higher temperature during the formation of the gold stud bump. It was observed that the bump size is greatly influenced by the compression force and ultrasonic power in the first bumping mode in Fig. 3(b). Fig. 5 depicts that lower ultrasonic powers and time, and compression forces but higher EFO current can produce smaller bump size. Fig. 6 shows that higher EFO current and time but lower

compression force and ultrasonic time can fabricate higher bump height. Therefore, the ANN process model can demonstrate the relationship between the operating parameters and the produced stud bump properties. 4.3. Gold stud bumping process optimization The optimization problem of the stud bumping process can be described as Max

f ðx; m; yÞ

subject to

mlb & m& mub y Z Uðx; mÞ gðx; m; yÞS 0

(2)

L.C.-K. Liau, B.S.-C. Chen / Expert Systems with Applications 29 (2005) 264–271

269

Fig. 5. The bump diameter correlated with the operating parameters.

where f is the objective function, x is the state variable, m is the operating parameter, y is the stud bump property, U is the process model of the operating system which describe the relationship between the input (x,m) and the output (y) variables, and g is the inequality constraint in this problem. The notations of subscript lb and ub represent lower bound and upper bound, respectively. The optimization problem can be solved using a nonlinear constrained optimization approach provided by the MATLAB commercial software tool. The system model U can be obtained and represented by the ANN model. There are three process optimization cases discussed as the following:

Case 1. The objective function is defined to maximize the shear stress of the stud bump under the constraint of fixing the bump size at 3.5 mil. Case 2. The objective function is defined to maximize the shear stress of the stud bump under the constraint of fixing the bump height at 2.0 mil. The optimal operating conditions of both cases were estimated using the optimization approach. The shear stress is calculated as 11 gf/mil 2 for case 1 and 12.5 gf/mil2 for case 2. The results of the determined operating parameters and the maximum shear stresses are listed in Table 3.

270

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Fig. 6. The bump height as a function of the operating parameters.

The variations of the bump height can also affect the interconnection performance of the flip chip to attach with the stud bumps during the packaging. The operating factors of the bumping process can influence the formation of the bump height analyzed previously as depicted in Fig. 3(c). The variation is represented by S/N ratio as described in Eq. (1) in this study. For example, Fig. 7 demonstrates the S/N ratio of the bump height as a function of the compression force and EFO current. It reveals that the variations are significantly if the operating factors are set up at lower compression force and lower EFO current. Therefore, the uniformity of the bump

height is an issue to maintain the product quality in the process. This optimization problem is described in the case 3. Case 3. The objective function is stated to maximize the S/N ratio (or minimize the variation) of the bump height under the constraint of maintaining the shear stress at 11 gf/mil2. The optimal operating condition of the case was evaluated by the optimization scheme as demonstrated in Table 3. Hence, the process optimization problems of the gold stud bump were solved using the optimization scheme developed in this work.

L.C.-K. Liau, B.S.-C. Chen / Expert Systems with Applications 29 (2005) 264–271

271

Table 3 The optimal operating conditions for different cases

A B C D E F G H I J

Operating parameters

Case 1

Case 2

Case 3

Bond pad temperature (8C) Tail length (mm) EFO time (ms) EFO current (mA) Ultrasonic energy: first mode (W) Ultrasonic time: first mode (ms) Compression force: first mode (gf) Ultrasonic energy: second mode (W) Ultrasonic time: second mode (ms) Compression force: second mode (gf)

210 0.65 3.5 25 0.454 60 30 0.036 25 20

180 0.65 2.7 30 0.511 60 30 0.036 30 20

190 0.60 3.5 30 0.612 40 30 0.036 5 20

Fig. 7. The S/N ratio associated with EFO current and compression force.

5. Conclusions The optimal operating parameters of the gold stud bumping process were determined using the optimization scheme developed for the practical packaging manufacturing. The critical operating parameters to affect the stud bump properties were analyzed using the design of experimental analysis. The relationship between the operating parameters and the stud bump properties was established by the ANN modeling approach. The optimization problems of the bumping process were solved to evaluate the optimal operating conditions using the optimization scheme. This optimization scheme can be further applied on the packaging industry to improve the process operating efficiency, and product quality and yield. Acknowledgements This work is partially supported by the National Science Council, Taiwan under grant NSC 92-2212-E-

155-009. The acknowledged.

financial

support

is

gratefully

References Harman, G. G. (1997). Wire bonding in microelectronics. New York: McGraw-Hill. Heykin, S. (1999). Neural networks. Upper Saddle River, NJ: Prentice-Hall. Jordan, J. (2003). Gold stud bumps in flip chip applications. Microwave Journal , 86–103. Klein, M., Oppermann, H., Kalicki, R., Aschenbrenner, R., & Reichl, H. (1999). Single chip bumping and reliability for flip chip processes. Microelectronics Reliability, 39, 1389–1397. Liau, L. C.-K., Yang, T. C.-K., & Tsai, M.-T. (2004). Expert system of a crude oil distillation unit for process optimization using neural networks. Expert System with Applications, 26, 247–255. Lui, D.-S., & Ni, C.-Y. (2002). The optimization design of bump interconnections in flip chip packages from the electrical standpoint. Microelectronics Reliability, 42, 18931–11901. Zama, S., Baldwin, D. F., Hikami, T., & Murata, H. (2001). Flip chip interconnect systems using copper wire stud bump and lead free solder. IEEE Transaction on Electronics Packaging Manufacturing, October, 261–268.