Polymers for Microelectronics - American Chemical Society

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Chapter 26

Recent Progress of the Application of Polyimides to Microelectronics Downloaded by PENNSYLVANIA STATE UNIV on September 10, 2012 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1994-0537.ch026

Daisuke Makino Yamazaki Works, Hitachi Chemical Company, Ltd., Hitachi, Ibaraki 317, Japan

Highly purified and high heat resistant PI for microelectronics was first devel­ oped in the early 1970s (1). Purpose of the development was the application to the interlayer dielectric of dual metal semiconductor device. By coating liquid PI precursor on a stepped first Al wiring layer, flat PI insulating layer is obtained and step-free second Al layer can be formed as shown in Fig. 1. As a result, reliability of the electrode, especially the second level electrode, is remarkably improved. Result of high temperature and high humidity life test of the PI intercon­ nect ICs is shown in Fig. 2 (2). Time to reach to 1% cummulative failure is about 10 years under 65 °C-95%RH condition. This life is comparable with the PSG (phosphorous doped silica glass) passivated single layer device. In this way high reliability of PI interconnect ICs was confirmed in the market, and application of PI to microelectronics rapidly expanded. Fig. 3 summarizes the application of PIs to microelectronics in chronological order (3). They include α-ray shield of memory device, interlayer insulation of thin film facsimile thermal head, thin film magnetic head and so forth. Out of these applications, this paper introduces the recent progress of the application to LSI, Liquid Crystal Display, Multi Chip Module and Optical Wave Guide. PI BUFFER COAT Mounting method of semiconductor package to printed wiring board is shifting from pin inserting to surface mounting. In a surface mount, soldering is carried out by dipping the package in a molten solder or exposing to high temperature gas, vapor phase soldering. During this process mold stress generates in a package because of the difference of thermal expansion of mold resin and semiconductor chip. This mold stress gives fatal damage to the package such as passivation crack, Al electrode slide and package crack. Accordingly, it is a very

0097-6156/94/0537-0380$06.75/0 © 1994 American Chemical Society In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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26.

Application of Polyimides to Microelectronics

MAKINO

381

85/100 144/100

(mmHg)

Figure 2. Failure time dependence of PI interconnect devices on absolute water vapor pressure. TO •

1

75 I

I

I

II

*80 I

i

i

Transistor

i

I

*85 I

I

I

1

I

^0 1

I

1

I

T""

Bubble Memory

o'-Ray Shield for VLSI • ^s.

Linear IC

"=>

GTO Thyristor lyristor 1

>

Facsimile

rprr out ectJon i e u i i u N of U T LSI

r Thin Film Magnc Magnetic Head Thin LC D i s p l a y ^ i |N Multi-chip Module

Figure 3. Application fields of Pis.

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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POLYMERS FOR MICROELECTRONICS

important issue to reduce the mold stress during soldering process. For this purpose, coating the surface of the chip by PI is currently remarked and utilized in the industry.

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Prevention of Passivation Crack Fig. 4 compares the humidity resistance of the package of PSG passivated chip dipped in solder with non dipped sample (4). Failure rate of solder dipped package is higher. However, when 2 fim thick PI ( P I Q : P M D A / B T D A / O D A / Diaminodiphenylether carbonamide) film was coated on the PSG passivation, failure rate was remarkably improved to the equivalent value to non dipped package as shown in Fig. 5 (4). Role of PI chip coat film can be explained as follows. In a case of PSG passivation alone, cracks are generated in the PSG film by the mold stress because of the brittleness of PSG film, and A l electrode corrodes by the moisture penetrating through these cracks. On the other hand, for a package with PI chip coat, mold stress is absorbed by this PI film and the passivation crack is prevented. Fig. 6 compares the passivation crack for various PI chip coat (5). PI-A is low thermal expansion, PI-B is low modulus and PI-C is conventional (Hereafter conventional PI refers to P M D A / O D A PI unless otherwise specified). Among them, low modulus PI-B coating gives the best result. This can be explained by its big stress absorbing ability. On the other hand, relatively good preventing effect is observed for PI-A, which has the highest modulus. In this case stress generation is suppressed by the smaller difference of coefficient of thermal expansion between PI and passivation film. Namely, both PI-A and -B have an ability to prevent passivation crack but their function is different. This can be understood by the fact that stress is proportional to the product of the difference of coefficient of thermal expansion of PI film and substrate and modulus of PI film. In some cases mold stress affects the active area of the device. Table 1 shows the effect of PI chip coat on the threshold voltage shift of Bipolar Interface Device having piezosensitive circuit (6). By applying 12 fim thick PI coat, threshold failure rate decreases from 10 to 0 %. PI plays an important role to stabilize the electrical property of the circuit by relaxing the mold stress. Prevention of Electrode Slide A l electrode in a chip can slide by the mold stress during heat cycle. Effect of PI chip coat on A l slide is shown in Table 2 (5). No A l slide was observed for both low thermal expansion and low stress Pis compared to the large A l slide for non-coated device. Thus, PI coating is also effective to A l slide by absorbing the stress generated by heat cycle. Prevention of Package Crack Recently, so called package crack attracts attention. Package crack occurs as a result of the abrupt evaporation of moisture trapped at the interface of mold

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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26.

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Application of Polyimides to Microelectronics

383

^100 n=30 2 80 - Dip testing condition ; CD 260t/10sec Molding C o m p o u n d - C h i p ( n o coat)

Residual stress

2 6 0 ° C (soldering

temp.)

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Application of Polyimides to Microelectronics

389

Table 7. Ranking of Pis for Chip Coat Application Low thermal Conven- Fluorin- Low modulus expansion tional ated

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Item

PI

PI

PI

PI

Adhesive strength

5

3

5

1

Residual stress

3

3

5

5

Moisture absorption

3

5

2

5

Tg

4

4

1

5

1 : Worst .

-150

0

5 : Best

150

300

450

Temperature CC)

Figure 9. Visco-elastic property of PI. 10

1 r

J13

CC)

Figure 10. Relation of T/3 of Pis with adhesive strength of Pis and molding compounds. In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

390

POLYMERS FOR MICROELECTRONICS

at the cure temperature of mold resin. In other words, PI having dispersion temperature lower than curing temperature shows good adhesion to the mold resin. But some Pis such as low thermal expansion PI has no or very high )3 dispersion temperature and no stress relaxation is expected. In such a case, it is expected that the PI surface treatment is effective from the analogy of the improvement of PI-PI adhesion. By treating the PI surface by N or O ashing, contact angle decreased and good adhesion was obtained as shown in Table 8 ( i i ) . By the plasma treatment PI surface is roughened. Anchoring effect may also be the factor to enhance the adhesive strength. In conclusion, poor adhesion of low thermal expansion PI can be improved by the plasma treatment, and low thermal expansion PI is the most promising material for the chip coat application.

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2

z

APPLICATION TO MULTI CHIP MODULE (MCM)

PI has began to be applied to multi chip module, which can reduce the machine cycle of computer. Cross section of typical M C M is shown in Fig. 11 (12). 5 layers of metallization insulated by 15 to 20 fim thick PI film is formed on ceramic wiring board. Table 9 is the results of the evaluation of various Pis for M C M (13). Among them, low stress PI, namely low thermal expansion PI, seems to be best balanced except for adhesion and planarity. In the following, recent works to improve these properties of low thermal expansion PI will be introduced. In M C M there exists three adhesion interfaces, metal on PI, PI on PI and PI on metal as shown in Fig. 12. Among them adhesion of PI on metal is sufficient. Fig. 13 compares the adhesive strength of conventional PI, PIQ, and low thermal expansion PI, L-100, for various metals (14). Adhesion of low thermal expansion PI is extremely low. But, as shown in Fig. 14, by treating the PI surface by oxygen, CF4 or nitrogen gas plasma, high adhesion can be obtained for Cu and Ti, which shows poor adhesion when untreated (14). Among the gases nitrogen is most effective. Reason of the improvement of adhesion by nitrogen plasma was investigated by the surface analysis using ESCA (14). Imide group decomposes to produce more active group such as amide and amino as shown in Fig. 15. These groups are thought to work to improve adhesion. The second interface, PI-PI adhesion is also able to be improved by either oxygen plasma or Ar sputtering of PI surface as shown in Fig. 16. PI-PI adhesion is influenced by the baking atmosphere. Fig. 17 shows the experimental results of the dependence of the baking atmosphere on the adhesion (15). By baking in nitrogen atmosphere high adhesion is maintained under high humid condition for PI-C, low thermal expansion PI. Planarity of PI film is affected by the solid content of PI precursor. Good planarity can be obtained for high solid content varnish as shown in Fig. 18 (16). However, usually, high solid content means high viscosity and gives the ununiformity of the resultant film. Most effective method to obtain low viscosity varnish with high solid content is to decrease molecular weight by using capped monomer such as half ester of acid dianhydride. Liquid properties of PI precursor obtained by ester oligomer method is shown in Table 10 (16). In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

MAKINO

Application of Polyimides to Microelectronics

Table 8. Effect of PI Surface Treatment on the Adhesion Between PI and Mold Resin

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Test pieces

PI surface treatment

Contact angle (degree)

Adhesion

A

No ashing

68.1

Poor

B

N

2

ashing

48.2

Good

C

0

2

RIE

41.6

Good

D

0

2

ashing

32.1

Good

Polyimide Thin-Film| Signal Layers with Ground Planes

Flip Tab Carrier Top Surface y Metalization ~>>Gnd E32T

Signal Signal Gnd } Power & Ground

I/O Pin

Ceramic B a s e Substrate! with Power & Ground Planes

Figure 11. Cross section of PI thin film multi chip module.

Figure 12. Three adhesion interfaces in PI multi chip module.

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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POLYMERS FOR MICROELECTRONICS

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Table 9. Evaluation of Various Pis for M C M Application

Property

Standard Fluorinated PI

Acetylene- Benzocycloterminated butene

Silicone

Low stress

PI

PI

PI

PI

Solvent compatibility

1

1

1

1-5

1

Thermal properties

2

3

3

1

2

3

Moisture absorption

5

3

3

2

4

1

Stress

5

5

5

1

5

5

Adhesion

1

2

2

5

2

2

Planarization

5

5

5

5

1

1

RIE etchability

1

1

4

1

1

4

Relative ranking 1 - Best: 5 — Worst

• PIQ Buoo

Figure 13. Peel strength of metal films to PIQ and low thermal expansion PI (PIQ-L100).

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

26.

MAKINO

Application of Polyimides to Microelectronics

•

Cu

•

393

Ti

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500 h

As Cured

0

2

CF

4

N

2

Plasma Gas

Figure 14. Improvement of adhesion to low thermal expansion PI by plasma surface treatment.

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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POLYMERS FOR MICROELECTRONICS

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Ar sputtering

Depth of Etching (.Mm)

Figure 16. Improvent of PI-PI adhesion by surface treatment.

O) J=

+•>

D) C CD

s Q.

0

10

100

P C T Time (hr) • : PI-A/PI-A in N A : PI-B/PI-B in Na o: PI-C/PI-C in N

2

2

• : PI-A/PI-A in Air A : PI-A/PI-B in Air • : PI-C/PI-C in Air PI-A

Pis Thermal expansion coefficient (10- /'C) 5

4.5

PI-B Pl-C 1.9

0.3

Figure 17. Dependence of PI-PI adhesive strength on baking atmosphere.

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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26.

MAKINO

Application of Polyimides to Microelectronics

*AI |

395

Polyimide

Al

•127«n

Degree of Planarization-1-t tAI Slope ' $

1.0r

2

0.5

Q.

0

50

100

Resin Content (%)

Figure 18. Relation between resin content and planarity. Table 10. Synthesis of High Solid Content and Low Viscosity Polyamic Acid Polyamic acid Method

Ester Oligomer Method •CO\ 0

Synthesis Method

,C0 R

N

CO'

S

S

2 F R O H

0 C0'

N

N H C

°N ^ n

C 0 0 R

"\

WoOC' CONH-R'4 n

Resin Content Viscosity Molecular Weight

0

0 + H»NR'NHi

R

N

H NR'NH,7 A

HOOC ,COOR" *R R"OOC' C00H

^cor ^co' /NHC0

S

~ \ H O O C

,COOH N

\

CONH-R'^-

40%

14.5%

100cp

1100cp

500-1000

= 30,000

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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POLYMERS FOR MICROELECTRONICS

Cu is widely used as the wiring metal of M C M . When PI precursor is baked in contact with Cu, its properties deteriorates as shown in Table 11. This deterioration is explained by the decomposition of imide ring as a result of the reaction of migrated Cu ion and carboxyl group of polyamic acid (27). So it is necessary to cover Cu wire by another metal such as Cr or to use preimidized PI instead of polyamic acid. Table 12 is a summary of selection of PI and processing in M C M application. Low thermal expansion PI is most promising. Future subject is etching. 0 -RIE is most suitable from the point of fine patterning but the etching time is too long for the production line. Therefore, photosensitive low thermal expansion PI seems to be most promising for M C M application, hopefully preimidized in order to prevent the reaction with Cu, and positive working to get high accurate patterning.

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2

APPLICATION TO LIQUID CRYSTAL DISPLAY (LCD) In L C D picture image is obtained through the change of the transmission of light by orienting the liquid crystal molecule under the electrical field (Fig. 19). The orientation of liquid crystal is controlled by the alignment surface, and PI is dominantly utilized for this purpose. One of the advantages of PI is to be able to control the pretilt angle. As shown in Fig. 20 pretilt angle is defined as an angle between liquid crystal molecule and alignment surface under the absence of electrical field. This pretilt angle is produced by rubbing the surface of PI. If this pretilt angle is not constant throughout the panel, orientation of liquid crystal is dispersed and gives an ununiform image. Fig. 21 shows the relation between surface tension of alignment surface and pretilt angle for two kinds of Pis (18). Pretilt angle increases with the decrease of surface tension. [Usually pretilt angle increases with the increase of surface tension. This result seems to be the mistyping of the original paper]. However, for PI containing long alkyl chain, pretilt angle increases drastically. This indicates that pretilt angle is also affected by the physical structure of the surface. Pis having various methylene chain length were synthesized using alkyl chain containing diamine, and the relation between chain length and pretilt angle was investigated (19). Results are shown in Fig. 22. Low pretilt angle is obtained for odd carbon number PI, and high pretilt angle for even number PI. This phenomenon can be interpreted from the difference of the conformations of Pis as shown in Fig. 23 (19). Assuming that PI is oriented to rubbing direction, PI using odd carbon number diamine has cis conformation. If liquid crystal molecule is adsorbed to alkyl chain no pretilt angle is produced. But for even number PI, since its conformation is trans zigzag, liquid crystal molecule is aligned in one direction with pretilt angle a. Table 13 shows some of the example of the required properties to the alignment surface of TFT LCD and molecular design of PI. Preimidized varnish reacted from alkyl chain containing diamine and cycloaliphatic dianhydride, which is closely related to the voltage retention rate, seems to be the most appropriate PI for the alignment surface of TFT LCD.

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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MAKINO

Application of Polyimides to Microelectronics

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Table 11. Comparison of the Characteristics of Pis Cured on Various Substrates Underlying Film

Decomposition Temperature (°C)

Tensile Strength

Elongation

(kg/mm )

(%)

380

11.0

450

13.5

17.0

Cr

450

13.4

16.8

Al

450

13.5

16.7

Cu Si0

2

2

8.0

Cure: N*. 350°C

Table 12. Summary of the Selection of PI and Processing in M C M Application Required properties

Countermeasure

Low stress

Low thermal expansion

High adhesion

Metal on PI

High planarity

High solid type polyamic acid

No reaction with

Cr over coat

PI on PI

PI

N sputter 2

Ar sputter, 0 plasma, N bake 2

2

Electro/Electroless plating Cu Etchability

0 - RIE but etching time is too long 2

Photosensitive low thermal expansion

PI

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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POLYMERS FOR MICROELECTRONICS

Vso

Vio

Voltage (V)

Figure 19. T-V property of liquid crystal display.

Liquid Crystal Molecule

a : Pretilt Angle

tium Alignment Surface

Rubbing Direction

x x

Substrate

Figure 20. Generation of pretilt angle by rubbing the surface of alignment surface.

In Polymers for Microelectronics; Thompson, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

26.

Application of Polyimides to Microelectronics

MAKINO

399

20 Q) TJ w

15

_© O)

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