High-Performance Silicone Gel as Integrated-Circuit-Device Chip

Chapter 19

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 6, 2018 | https://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch019

High-Performance Silicone Gel as Integrated-Circuit-Device Chip Protection Cure Study and Electrical Reliability C. P. Wong AT&T Bell Laboratories, Princeton, NJ 08540 Recent advances in IC device encapsulants and polymeric materials have made high reliability VLSI plastic packaging a reality. High performance silicone gels possess excellent electrical and physical properties for IC protection. With their intrinsic low modulus and soft gel-like nature, silicone gels have become very effective encapsulants for the delicate larger chip size and wire-bonded VLSI chips. Recent studies indicate that adequate IC chip surface protection with high performance silicone gels in plastic packaging could possibly replace conventional ceramic hermetic packaging. This paper reviews some potential IC encapsulants. It focuses on the high performance silicone gels, their cure chemistry, and their application as VLSI device encapsulants. The rapid advances in integrated circuit (IC) technology have had a profound technological and economic impact on the electronics industry. The exponential growth of the number of components per chip, the exponential decrease of device dimensions (1) and the steady increase in IC chip size have imposed stringent requirements not only on IC physical design and fabrication, but also on the IC encapsulants. The increase of integration in Very Large Scale Integration (VLSI) technology has resulted in a large increase in chip size. The effectiveness of high performance encapsulants, such as silicones (elastomers and gels), polyimides, epoxies, silicone-polyimides, and polyxylylene (Parylene) in protecting these large IC devices has been reviewed. (2-3) High performance silicone gels possess excellent electrical, chemical, and physical properties for this type of IC protection. With their high purity, intrinsic low modulus and soft gel-like nature, silicone gels have become very effective encapsulants for the delicate larger chip size and wirebonded VLSI chips. Recent studies indicate that adequate IC chip surface encapsulation with these high performance silicone gels in plastic packaging could possibly replace conventional ceramic hermetic packages. (4-5) This paper reviews some potential encapsulants with special focus on the high performance silicone gel, its cure chemistry and Temperature Humidity Bias (THB) accelerated electrical testing as a VLSI device encapsulant. The purpose of encapsulation is to protect electronic IC devices and prolong their reliability. Moisture, mobile ions, (eg., sodium, potassium, chloride,fluorides),UV-VIS and alpha particle radiation, and hostile environmental conditions are some of the possible causes of degradation or interaction which could negatively affect device performance or lifetime. Silicon dioxide, silicon nitride and silicon-oxy-nitride, commonly used as passivation layers have excellent moisture and mobile ion barrier properties and are, therefore, excellent encapsulants for devices. As for the 0097-6156/89/0407-0220$06.00/0 © 1989 American Chemical Society

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

19. WONG

High-Performance Silicone Gel

221


sodium ion barrier, silicon dioxide is inferior to silicon nitride. However, the use of phosphorous-doped (a few weight percent) silicon dioxide has greatly improved its mobile ion barrier property. A thin layer (~1-2μπι thickness) of one of these dielectric materials is deposited uniformly on thefinisheddevice, except on the bond pad areas. Due to the "edge effect" of pas sivation of IC devices after wire bonding interconnection, protection to the bond pad area becomes necessary (See Figure 1). In addition, these passivation layers are not 100% pinhole or crack-free. To ensure the device reliability, an organic conformai coating is usually used for

Access for

Figure 1 -

Edge Effect of Device Passivation (Due to the exposure of the wire bond pad and the passivation material, this access area for wire bonds must need encapsulation after wire-bonding interconnection).


POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

222


device encapsulation. Epoxies, polyimides, Parylene, silicone-polyimides, and silicones (elasto mers and gels) are typical materials used for this application. Characteristic properties of these potential encapsulants are listed in Table I. General Chemistry of Silicone Gels. The basis of commercial production of the silicones is that chlorosilanes are readily hydrolyzed to give disilanols, which are unstable and condense to form siloxane oligomers and polymers. Depending on the reaction conditions, a mixture of linear poly mers and cyclic oligomers is produced. The cyclic components can be ring-opened to linear poly mers, and it is these linear polymers that are of commercial importance. Chlorosilanes, with reac tive hydro and vinyl reactive groups, are the intermediates for the synthesis of the silicone gels. These hydro, vinyl reactive functional groups, and the platinum catalyst are essential to the sil icone gel chemistry. (6) The reaction mechanism of the hydro, vinyl additional cure system is shown on Figure 2. EXPERIMENTAL A Nicolett 7199 FT-IR spectrometer was used to monitor the cure of silicone gel. The decreasing absorption of the Si-Η absorption at "2129cm" was monitored during the cure of the silicone gel (see Figure 3). Silicone gel (Part A and Part B) wasfreshlymixed at a prescribed ratio prior to the cure study. A NaCl salt plate was used as the IR sample cell. Results of the FT-IR study are shown in Figure 4. In addition to the FT-IR cure study, measurements have been made on the silicone gels with a highly sensitive microdielectric apparatus from 0.05Hz to lOKHz. This microdielectrometry, which utilizes the Micromet Instrument's Eumetric System II microdielectrometer with a minia ture IC sensor and a wide range of frequencies (from 0.05Hz to lOKHz) to monitor the loss factor (E ) of the silicone gels, is a very sensitive technique to detect the final stage of the silicone gel cure. A thin layer of freshly mixed silicone gel ("20 mil thickness) was coated on the miniature IC sensor and placed inside a programmable oven. The temperature of the oven was set to the pre-described temperature (i.e., 120*, 150* or 175*C) and the loss factors (E") at various frequen cies (0.05, 1, 100, 1000, 10000 Hz) was monitored periodically during the curing time. Results of the microdielectric loss factor measurements are shown in Figure 5. 1

M

Temperature Humidity Bias (THB) Electrical Testing of Silicone Gel. The electrical reliability of silicone gels was determined by using a THB test procedure employing an alumina ceramic IC device with a triple track resistor (TTR). The metallization for the TTR is tantalum nitride (Γα Ν)· The feature size of this testing device is 75μπι metal line-width and spacing. Prior to the silicone gel coating, the TTR was cleaned with Fréon TA vapor for 15 min., boiling H 0 for 15 min., deionized waterrinsedfor 30 min., and oven dried at 120*C for 1 hr. Then, the silicone gel was coated on the freshly cleaned TTR device. These test devices were cured at different curing temperatures (120 C, 150 C, and 175*Q for 2 hrs. The cured test devices were subjected to 85*C, 85% Relative Humidity (RH) and 180 volt-dc bias. The leakage current between the biased center track and the two grounded outer tracks was measured at different intervals to determine the electrical performance of the silicone gel. The difference in the triple track resistance with respect to the initial resistance (AR/R) of the biased center track was measured at different intervals during the testing. The electrical performance of different vinyl and hydride mixing ratios (from 1:1 to 10:1 of Part A:Part B) and different curing temperatures (120*, 150*, 175*) of the silicone gels were studied. The results of the THB electrical testing are shown in Figures 6 to 9. 2

2

e

2

e

RESULTS AND DISCUSSIONS Cure Study of Silicone Gel. The chemistry of most silicone gels is based on the reaction of silicon-vinyl (Part A) and silicon-hydride (Part B) addition cure mechanism (see Figure 2). The addition of the hydride functional group from the silicon-hydride (Part B) to the vinyl functional group of the silicon-vinyl (Part A) and the formation of carbon-silicon crosslinking are key steps in the reaction mechanism. A trace amount (a few ppm) of platinum catalyst is needed to


19. WONG

223

High-Performance Silicone Gel

TABLE I


PHYSICAL PROPERTIES OF SOME POTENTIAL ENCAPSULANTS (Unfilled System) Encapsulants

TCE* (ppm/'Q

Modulus (psi)

Epoxy

"40 -80

-1 - 5 χ 10

Polyimide

-3 -80

-1 χ 10

Parylene

"35 -40

-0.4 χ 10

Silicone-Polyimide

~5- 100

"0.4 χ 10

Silicone Gel

-200 - 1000

-0-400

6

6

6

6

*TCE = Thermal Coefficient of Expansion at Room Temperature

CH

I

3

CH

CH

3

I

I

CH

3

I

~ Si - Ο - Si - Ο - Si - O

I

I

CH=CH CH

3

2

CH η

V

Si

I

CH

3

I 3

m m > η

CH

I

3

— O 4- Si — ο - Si — Ο I

I

CH,

3

CH

Ο - Si

3

"PI" & û 2

CH

H

CH,

~ Si + Ο - Si CH, CH

|CH

3

—Ο -

- Si - Ο

I

CH

CH,

3

CH

Si — Ο 2

I

CH

3

- Si - Ο

(CURED GEL) - (1) • • Figure 2 -

3

I

I

CH

3

Excess Hydrides Reactive "Pt" Catalyst Silicone Gel Cure Mechanism.


POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION


224

3.600

r

LEGEND: χ HEIGHT 3.200

•AREA 1

PEAK: Xmax 2129 cm" (1294-2087) 1

REFERENCE: Xmax 1945 enr (2021 1892)

2.800

χ X

High-Performance Silicone Gel as Integrated-Circuit-Device Chip

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