Siloxane Polyimides for Interlayer Dielectric Applications - ACS

Chapter 12

Siloxane Polyimides for Interlayer Dielectric Applications P. P. Policastro, John H. Lupinski, and P. K. Hernandez

Downloaded by UNIV OF LEEDS on June 5, 2016 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch012

General Electric Company, Corporate Research and Development Center, Schenectady, NY 12301

The effect of processing conditions on molecular weight and thermal stability of siloxane polyimides prepared from α,ω-3 amino-propylpolydimethylsiloxane was investigated. Adhesion and dielectric properties were also studied. Siloxane polyimide copolymers were also prepared from the aromatic siloxane dianhydride 1,2-bis(4-phthalic anhydride) 1,1,2,2-tetramethyldisiloxane (1) and a variety of aromatic diamines. The copoly mers obtained were analyzed by isothermal gravimetric analysis to determine stability relative to polyimide structures that did not contain subunits derived from monomer 1. Planarized coat ings of the copolymers prepared from monomer 1 were readily obtained employing standard solution spin coating techniques, which were further characterized by peel adhesion testing. A high degree of adhesion of the coatings to silicon substrates was observed in the absence of surface priming agents after exposure to boiling water.

Current interest in siloxane polyimides is triggered by opportunities for such materials in military, aerospace and electronic applications as coatings, films, adhesives, molding compounds and composite matrix materials which are sub ject to demanding operating conditions. These polymers offer advantages such as excellent interlevel adhesion, plasma resistance, low water absorption, and stability at high temperatures. Two synthetic approaches were used to prepare the materials discussed 0097-6156/89/0407-0140$06.00/0 ο 1989 American Chemical Society

Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

12. POLICASTRO ET AL.

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Inier layer Dielectric Applications

in this report: (i) the two stage polyamide acid method (7) and (ii) solution imidization (2). Method (i) involves heating a polyamide acid film, formed by casting from a polar aprotic solvent, under inert atmosphere with a final cure temperature of 300°C. This procedure is applicable to systems in which the final polyimide product is insoluble in organic solvents. Polyimides which are soluble in organic solvents may be prepared as fully imidized polymers in solution at elevated temperature. The diamines and dianhydrides used in this study are illustrated in Figures 1 and 2.


Siloxane Polyimide With Intermediate Thermal Stability SPI-100, a fully imidized aromatic-aliphatic siloxane polyimide, has been prepared in xylene/diglyme mixtures and in p-methylanisole. In the latter solvent, high molecular weight polymers are readily obtained. The extent of polymerization was determined by monitoring the relative intensities of absorption for the imide overtone band (3490 cm" in toluene) and the anhydride band (1860 cm' in chloroform). These polymers with higher molecular weight offer a significant advantage in peak use temperature over the low molecular weight material prepared in xylene/diglyme mixtures as is shown in Table I. The data suggest that high molecular weight SPI-100 can be used at temperatures up to 350°C in N atmosphere. 1

1

2

Table I. Effect of Processing Conditions on Thermal Stability of SPI-100 in N

IV Process Xylene/diglyme II

Methylanisole II

(g/mole) 8000

0.25

II

23000 II

0.44 II

Temp. (°C) 400

2

Percent Weight Loss 1st 2nd 30 min 30 min 6h 19.3 5.9 32.1

350

2.7

3.7

19.3

400

3.4

1.9

16.9

350

0.2

0.2

4.0

Adhesive characteristics of thin SPI-100 films could not be measured directly because adhesive forces are generally larger than cohesive forces. To obtain some information on adhesion values, thin high molecular weight SPI100 films on substrates were overcoated with about .1 mm of a commercially available polyimide (Product A) to provide greater cohesive strength than can be obtained with SPI-100 alone. The combined layers were then pulled in an Instron tester giving the results shown in Table II which also includes the values for commercial products A and B. Adhesive characteristics for lower molecular weight SPI-100 on Si-oxide and nitride substrates are given in Table III.


POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION


142

Figure 1. Diamines Employed for the Preparation of Siloxane Polyimides.


Inier layer Dielectric Applications


POLICASTRO ET AL.

Figure 2. Dianhydrides Employed for the Preparation of Siloxane Polyimides.


144


Table II. Adhesion Values* of Polyimides Adhesion Promoter Not Required

Film Thickness (mm) .127

Product A

Not Required

.025

101

Product Β

Required

.013

18


Polyimide Type SPI-100 Overcoated with Product A

Avg. Peel Strength (g/mm) 368**

* Silicon substrate. **Cohesive failure without peeling.

Table III. Adhesion of Polyimides on Various Substrates

Polyimide Type SPI-100 Standard + Product A

on SiWafer

SPI-100 Standard + Product A

sio on Si Wafer

Substrate

Film Thickness (mm) .76

Avg.* Peel Strength (g/mm) 288.0**

.76

2

298.0

* Average of 3 measurements. **Cohesive failure without peeling. Dielectric constant measurements were performed with an automatic Hewlett Packard (HP-4270A) capacitance bridge on 2.5 micron low molecular weight SPI-100 films on Al-wafers. A second Al-electrode was sputtered on to the polyimide and patterned with Shipley 1470 photoresist to provide a pattern of dots varying in diameter from .050 to .200 inches (1.3-5.1 mm) (see Figure 3). Five measurements were made for each of the dot sizes. The averages for each size are given in Table IV. Table IV. Dielectric Constant Measurements (100 kHz) Diameter of Test Dot (mm)

5M

Σ54

L9Ï

L52

L2T

Dielectric Constant*

2.677

2.692

2.734

2.683

2.721

* Overall Average 2.70. Development of Highly Thermally Stable Siloxane Polyimides A structure/property profile of polydimethylsiloxane imides such as SP-100 indicated that polydimethylsiloxane is the largest contributor to instability; it



POLICASTRO ET AL.

Interiayer Dielectric Applications

OOO OOO

on on on on BHffi on on on OOO O o o OOO

OOO OOO

Figure 3. Pattern Used for Dielectric Constant Measurements. The diameter of the dots varies from .05 to .2 inches (~ 1.3 to 5.1 mm).



146


undergoes retropolymerization with evolution of volatile cyclic siloxanes. This was demonstrated by pyrolysis gas chromatography-mass spectroscopy in which a homologous series of cyclic siloxanes was detected upon heating at 375°C and above (Figure 4). The aliphatic linkages of bis(a-aminopropyl)tetramethyldisiloxane (GAPD, Figure 1) also imparted instability to the system relative to the entirely aromatic control as determined by isothermal T G A studies. A new reaction, referred to as "decarbonylative silylation" discovered by Rich (3), has made readily available the aromatic dianhydride disiloxane, PADS (4) (Figure 2). With the exception of a report by Babu (5), polyimides derived from PADS had not been described. Due to the aromatic nature of PADS, an improvement in thermal stability would be predicted compared to similar materials prepared from aliphatically linked disiloxanes. Three polyimides of comparable molecular weight were prepared from PADS, GAPD and l,2-bis(5-norbornyl-2,3-dicarboxyhcanhydride)-1,1,2,2, tetramethyldisiloxane, (DiSiAn, Figure 2) (6). Examination of the respective polymer structures shows that all three materials contain diaryl ether units either derived from the diamine (4,4'-oxydianiline, ODA) or anhydride (4,4'-oxy(bisphthalicanhydride), ODAN) components. These materials were evaluated for thermal stability by isothermal gravimetric analysis under air and nitrogen at several temperatures (Table V). Table V. Isothermal Gravimetric Analysis of Siloxane Polyimides

Composition PADS/4,4'-ODA

Temperature (°C) 450 II

II

435

Atmosphere

% wt loss Δ 6 hours

air nitrogen air nitrogen

18 12 5 6

II

II

GAPD/ODAN

450 435 435 350 300

air air nitrogen air air

86 85 75 21 3

435 435

air nitrogen

80 47

II II II II

DiSiAn/4,4'-ODA II

A significant improvement in thermostability was observed for the PADS con taining copolymer as compared to the GAPD and DiSiAn containing materials. At 435°C under nitrogen, the PADS/ODA copolymer produced only 6% vola tiles over six hours whereas the GAPD and DiSiAn containing materials lost 75


Interlayer Dielectric Applications


POLICASTRO ET AL.

200 9:00

400 18:00

600 27:00

800 36:00

1000 45:00

SCAN TIME

Figure 4. G C / M S of 450°C Pyrolysis Products of SPI100.

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148


and 47% of their weight, respectively, over the same period. Of additional importance is that the PADS containing material showed less sensitivity to an oxidative environment at 435°C than did the DiSiAn copolymer, which gave a much larger weight loss when the T G A was performed in air. Because of the improved thermal stability for PADS containing polyimides vis a vis other available siloxane polyimides, a screening program to correlate properties such as T , solubility, thermal stability, adhesion properties, and water absorption characteristics to structure was undertaken. Several copolymers were prepared from diamines and co-dianhydrides. An O D A N / O D A copolymer in which 30 mole % PADS was substituted for ODAN, was prepared and T G A analysis at 450°C indicated that the material was the first siloxane containing polyimide identified that exceeded the established thermal stability criteria for interlevel dielectric applications. Stability and solubility of these materials as a function of PADS concentration is illustrated in Table VI.


g

Table VI. Properties of PADS/ODAN/4,4 '-ODA Siloxane Polyimide Copolymers PADS ODAN mole% 100 50 30 10

50 70 90 100

Thermostability* (°C)

Tg (°C) 160 202 216 237 265

435 450 460 460 460

NMP** + +

-

p-methyl anisole +

-

chloroform +

* Temperature at which 1% wt loss/30 min occurs under nitrogen by isothermal gravimetric analysis. * *N-methylpyrrolidone.

Materials containing < 50 mole % PADS were insoluble in all common solvents including the polar aprotic variety, and thus were prepared by the twostage method. A 1:1 ODAN/PADS composition could be prepared directly in NMP in reasonable molecular weight and of acceptable stability at 450°C. The polymer of PADS and ODA was soluble in common organic solvents such as o-dichlorobenzene (ODCB) and chloroform. This material, however, was stable only to 435°C. T ' s for this family of copolymers, as determined by DSC and TMA, ranged from 265°C for the O D A / O D A N material through 160°C for the ODA/PADS copolymer. Both T and thermal stability of PADS/ODAN copolymers were increased when the O D A diamine component was replaced with m- or pphenylenediamine (MPD, PPD) or a mixture of the two diamines (Table VII). A solvent resistant material prepared from PADS/ODAN/PPD/MPD 3:7:5:5 had a Tg of 260°C and exhibited outstanding thermal stability by T G A analysis, volatilizing less than 2 wt % per hour at 480°C under nitrogen. g

g


12. POLICASTROETAI^

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149

Table VII. Thermal Stability of PADS Siloxane Polyimides Max. Use Temp.* (°C)


Composition PADS/4,4 '-ODA PADS/MPD PADS/PPD PADS/BPADA/MPD PADS/BPADA/MPD/PPD PADS/BPADA/PPD PADS/ODAN/MPD/PPD PADS/PMDA/4,4 '-ODA PADS/PMDA/PPD

1:1 1:1 1:1 1:1:2 1:1:1:1 1:1:2 3:7:5:5 3:7:10 3:7:10

T (°C) g

τ

** (°C) -

160 160 180 190 205 210 260 320

435 435 435 435 435 435 480 460 500

350 -

276 -

> 500

-

* Maximum use temperature defined by isothermal gravimetric analysis: ca. 1% wt loss/30 min at stated temperature under nitrogen. ** Τ = peak melting temperature. A family of materials with even higher peak use temperatures ( T ca. 510°C) was prepared through the copolymerization of PADS, pyromellitic dianhydride (PMDA), and PPD. A representative material was prepared with a molar ratio of 3:7:10 (PADS/PMDA/PPD). FTIR curing studies of this material coated to 1 micron thickness on a silicon wafer indicated that the extent of imidization does not increase above 300°C and is complete within lev els of detection. This material did not suffer significant weight loss over short exposure periods (30 minutes) below 500°C. m

Adhesion Studies PADS containing siloxane polyimide compositions were evaluated for adhesion to silicon wafers according to the identical test protocol employed by Davis (7). As shown in Table VIII, these polymers had adhesive properties similar to the GAP derived siloxane polyimides; however, the PADS containing materials are of higher thermostability than the GAP derived materials. Thus, the PADS class of materials offers a balance of adhesive and thermal qualities not hitherto attainable. Table VIII. Adhesion and Water Absorption of Polyimide Materials Composition PADS/4,4 '-ODA PADS/4,4 '-ODA/ODAN PADS/ODAN/MPD/PPD PADS/PMDA/4,4 '-ODA PADS/PMDA/PPD SPI-100

Tg(°C) 160 220 260 320 (T = >500) 120 m

Peel Test Passes

"

Water Absorption 0.25 moles) a cooling bath was employed to control the polymeri zation exotherm.] Upon complete addition of monomer, the contents were stirred for 4 hours until a homogeneous, high viscosity solution was obtained.


12. POLICASTRO ET AL.

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151

The polyamide-acid solution obtained was cast as a film (ca. .25 mm thick) onto a glass plate and dried for 12 hours at 80°C under vacuum (30 min). The resulting film was subjected to the following cure cycle under nitrogen on a hot plate monitored with a surface thermometer to effect imidization: 100°C (2 hours), 150°C (2 hours), 200°C (1.5 hours) and 300°C (1 hour). High T films (T > 300°C) were optionally cured for an additional 0.5 hours at a temperature of 480°C in an inert atmosphere oven to ensure imidization and removal of volatiles. g

g


General Procedure for Solution Imidization To a round bottom flask equipped with a modified Dean Stark trap, condenser, mechanical stirrer and nitrogen inlet, were added equimolar quantities of diamine and dianhydride as well as 0.5 wt % 4-N,N-dimethylaminopyridine and enough ODCB to provide an initial mixture containing 10% solids. The con tents were heated for 6 hours at 180°C with azeotropic removal of water and distillation of ODCB such that the final polymer solution had a concentration of 20 to 25% solids. The polymer solution was cooled and precipitated twice into methanol, dried under vacuum (30 rnin) at 80°C for 12 hours and 170°C for 2 hours. Acknowledgment The authors would like to acknowledge Dr. J.D. Rich for helpful technical dis cussions and for providing the PADS necessary for this study. We are also indebted to Mr. J.H. Mabb for his assistance in providing numerous thermal analyses. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8)

Scroog, C. E ; Endrey, A. L.; Abramo, S. V.; Berr, C. E.; Edwards, W. M.; Oliver, K. L. J: Polym. Sci., A, 1965, 3 (4), 1373. See for example Takekoshi, T. and Kochanowski, J. E . U.S. Patent 3,991,004. Rich, J.D. U.S. Patent 4,709,054. Pratt, J.; Thames, S. JOC 1973, 38, 4271. Babu, G. N. in Polyimides; Mittal, K. L., Ed.; Plenum: New York, 1984; Volume 1, pp. 51-66. Ryan, H.S. U.S. Patent 4,381,396. Davis, G. C.; Heath, Β. Α.; Gildenblat G. in Polyimides; Mittal, K. L., Ed.; Plenum: New York, 1984; Vol. 2, pp. 847-869. Davis, J. H. in Plastics for Electronics, Goosey, M. T., Ed.; Elsevier: New York, 1985; Chapter 3, pp. 67-98.

RECEIVED June 6, 1989


Siloxane Polyimides for Interlayer Dielectric Applications - ACS

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