US 5,612,403 AGrant
Low Temperature Flexible Die Attach Adhesive and Articles Using Same
Issue Date:1997-03-18
•3 Claims
•4 Drawing Sheets
Abstract
Described is a flexible adhesive formulation for bonding a semiconductor device to a flexible substrate and a flexible card containing a semiconductor device which can be processed in a computer.
Metadata
Assignee
- Johnson Matthey, Inc.
Inventors
- My N. Nguyen
- Yuan Y. Chien
Application Information
Application Number:US 5433215
Filing Date:1995-10-16
Priority Date:1990-10-24
Art Unit:151
Classifications
IPC:
C08K 308
Field of Search:
524377;440
Patent Drawings (4 sheets)
Description
Brief Description of the Drawings
FIG. 1 is a graph showing the relationship between the percentage of modifier, i.e., flexibilizer and radius of curvature, ("ROC");
FIG. 2 is a graph of die stress against ROC;
FIG. 3 is a schematic view partly in section of a card within which one or more semiconductor devices are enveloped and showing such devices bonded to a substrate; and
FIG. 4 is a graph illustrating % conversion of the polyhydroxyl polymer flexibilizer as a function of time and temperature.
Detailed Description of the Invention
As has been previously explained, a flexible adhesive is needed for a number of applications where semiconductor devices with large surface areas are attached to flexible substrates, particularly where there is a significant difference between the thermal expansion coefficients of the silicon of the semiconductor device and the substrates. In such cases, a low-stress die attach adhesive is required for maximum performance. This is especially important where the semiconductor device is to be attached to lead frames, such as copper lead frames, and polymeric substrates such as polyester, polyamide films, etc.
If a conventional rigid adhesive such as epoxy referred to in the aforementioned Steinegger article is used in the above described applications, the thermally induced tensile stress imparted to the surface of the semiconductor die is so high that it usually results in a silicon fracture.
To assess the phenomenon involved it is necessary to measure the die stress. One such method is by determining the radius of curvature ("ROC"). A stress free die would be flat and would have a very large radius of curvature. The relationship between tensile stress imparted on the die surface and the radius of curvature is illustrated in FIG. 1. Since the tensile stress of silicon is about 100 MPa, an ROC value of about I meter or above is advantageous to eliminate silicon fracture with dies of customary size.
Die attach adhesive compositions including cyanate ester and silver do not give the flexible properties needed for the above applications. However, flexible properties can be achieved by reacting cyanate esters with at least one elastomeric and thermo plastic modifier from the group of hydroxyl (-OH), amine (NH) or epoxide reactive functional groups, or mixtures thereof by, for example, grafting such group to cyanate ester molecular structures.
To illustrate the foregoing, blends of low Tg, (Tg <25.degree. C.), flexible epoxy and cyanate ester resin undergo the following reactions: ##STR1##
The effect of flexible epoxy on ROC is shown in FIG. 2. As can be seen, a mixture of at least 50% epoxy gives acceptable performance and the cyanate ester in this mixture also acts as curing agent for the epoxy. However, at least about 15% cyanate ester is needed in the adhesive formulation to provide satisfactory curing in this system. Other elastomeric modifiers containing hydroxyl reactive groups, including hydroxyl terminated butadiene and butadiene-acrylonitrile copolymers, such as "HYCAR" sold by B. F. Goodrich and "VITEL-3600" which is a saturated copolyester resin with a Tg of -11.degree. C., and polyhydroxyl polymer compounds are useful as well as amine functional groups such as polyurethane, elastomer, urethane acrylate, silicone imides, etc.
All of the foregoing modifiers have proven to be miscible with cyanate ester in the uncured state. During curing, the system is phase-separated, forming a co-continuous structure of hard and soft domains composed of cyanate ester-rich and elastomer-rich agglomerates. This system exhibits many advantageous properties, such as increased flexibility due to the soft modifier, while maximizing adhesive strength at high temperature.
The following are examples of some flexible adhesive formulations; with amounts in wt. %:
Examples 1-4
Example 5
A so-called "smart card" is shown in the schematic illustration depicted in FIG. 3. As can be seen, a card 10, usually made of plastic, includes a metal conductor 12 to which is bonded a semiconductor device 14. Glass fiber reinforced polyester pads 16 may be included and the entire device is encapsulated, such as within an epoxy resin 18. The epoxy resin may be ground flat to produce a thin plastic card.
The adhesive formulation of the present invention is especially suitable for bonding the semiconductor device to a substrate because it is capable of being cured at comparatively low temperatures of 100.degree. to 150.degree. C. and has very good adhesive strength while imparting low stress on the die. It is highly desirable for smart card application to employ a die attach adhesive capable of being cured at temperatures not substantially greater than about 150.degree. C., preferably not greater than about 130.degree. C. The die may be attached to a lead frame, such as a copper lead frame in which case the low temperature curing adhesive minimizes oxidation of the copper and thereby eliminates the need of nitrogen or other inert atmosphere such as is often used in curing ovens.
The following examples are of flexible adhesive compositions used to bond semiconductor devices suitable for smart card manufacture.
Example 6
Flexible adhesive composition:
The cyanate ester Arocy L10 is available from Rhone-Poulenc of Louisville, Ky. The modified bisphenol epoxy resin such as XB 4122 or XUGY 376 is a flexible epoxy supplied by Ciba Geigy. The catalyst system is a solution of copper napthenate or copper acetylacetonate in nonylphenol. The silver flake has the same physical properties described in the previous examples. Adhesive strength results appear in Table I.
The following is another example of a low curing adhesive formulation:
Example 7
The catalysts in the foregoing systems and in the systems hereinafter described may be copper napthenate or copper acetylacetonate, nonylphenol and silver. The formulation in Example 3 was cured at 100.degree. C. for 15 minutes and exhibited an adhesive strength of over 30 Kg for a semiconductor die 150 mil.sup.2 attached on a copper lead frame.
Where flexible adhesive formulations with low viscosity are desired, the cyanate esters are reacted with a polyhydroxyl polymer compound having the formula:
and/or
Preferred polyhydroxyl polymer compounds include polyhydroxyl polyester and polyhydroxyl polyether. Advantageously, the polyhydroxyl polymer compound used has a molecular weight of between about 200 and about 5000 and contains at least two hydroxyl groups. Such preferred low molecular weight polyhydroxyl polymer compounds result in low viscosity adhesive formulations of about 5 to 50 poise, the molecular weight being empirically related to viscosity. A molecular weight range of about 200 to about 5000 is preferred because below about 200 the material tends to be too volatile and above about 5000 the viscosity may be too high for processing conditions. Low viscosity adhesives which have improved workability enabling the formulation to be applied to a wide range of die sizes with varying substrate types, from flexible to rigid, and with reliable performance. Formulations employing polyhydroxyl polymers reacted with cyanate ester can be cured at temperatures between 30.degree. C. to 200.degree. C., as shown in FIG. 4, in the absence of a catalyst to form partially cross length (partially polymerized) B-stage, non-tacking films which can be subsequently hardened by heat to complete the polymerization in the actually applied process, such as during attaching of electronic devices to substrates. These formulations can be more rapidly cured, and at temperatures up to 200.degree. C., in the presence of catalysts by either heat or UV to form adhesive formulations comprising soft rubber-like layers mass or layer and when combined with a rigid, hard, or elastic layer, both layers would be inseparable from one another. Of course, the compositions may be cured at higher temperatures with shorter curing time.
It has been found that adhesive formulations made from the reaction of polyhydroxyl polymers of compounds having the formula described above with cyanate ester possess a high adhesion strength over a wide range of curing temperatures and on different substrates. They also possess high temperature heat resistance of up to 350.degree. C. and considerable flexibility, thereby providing low stress and low modulus characteristics which are useful with a wide range micro-electronic packages and devices. Suitable substrates for die attach adhesives include copper lead frames, printed circuit boards, laminates with "glob"-top coating for substrates varying from rigid to flexible.
The presently preferred low viscosity formulation comprises, in wt. %, 2-22% cyanate ester, 8-32% polyhydroxyl polymer compound, 0.01-0.06 wt. % metal-containing curing catalyst, up to 4.0% of an additive such as amorphous fume silica or clay, e.g., bentonite, for thixotropic behavior control, 0.5-2%, preferably 0.1-0.5%, alkylphenol and up to 90% silver flake and/or powder. The silver flake and/or powder preferably has a particle size within the range of 0.1-50 .mu.m, a surface area of 0.1-2 m/g and a tap density of 2-6 g/cc.
Low viscosity flexible adhesive formulations of the present invention can be applied to a wide range of semiconductor die size and shape. For example, die sizes from 10 mil. to 700 mil. (0.4 mm to 0.28 mm) either square or rectangular may be employed. A substrate, which may be rigid or flexible, from material such as alumina, gold coatings or silver coatings, plain copper lead frames and of varying thicknesses from 4 mil. and above, may be used. The curing temperature can be from 30.degree. C. to 300.degree. C., but can be reduced with curing catalysts to 200.degree. C. and the processing temperature may vary from 30.degree. C. to 350.degree. C., if required.
Other advantages of the low viscosity flexible adhesive formulation as described is that it is able to resist change even during high temperature thermal aging at 150.degree. C.-185.degree. C. and during thermal cycling from -60.degree. C.-150.degree. C. for at least 200 cycles. The formulations may be rapidly cured within 5 minutes at 200.degree. C. or for longer times at lower temperatures.
The following Table II summarizes a number of examples of low viscosity flexible adhesive formulations employing polyhydroxyl polymer compounds described by their trade names, "Multron R-18", "Desmophen 1700, 1915 U, and 1652", respectively. The material identified as "SD-2" is a bentonite clay additive which may be added to assist in improving thixotropic behavior, i.e., thixotropic characteristics. The physical properties of the examples in Table II are summarized in Table III.
To illustrate that the shear strength of the formulations described in Table II is maintained and does not degrade over time the composition of Example 8 was subjected to adhesion stability testing to compare die share against aging time. The average results of ten samples are reported in Table IV. For this test, the substrate used was a copper lead frame of 6 mil. thickness, a curing temperature of 150.degree. C., an aging temperature of 150.degree. C., and a die size of 5.08.times.5.08 mm-200.times.200 mil.
In order to determine whether the die stress remains at acceptable levels for the polyhydroxyl polymer containing adhesive formulations, test of flexibility tests were run to determine the radius of curvature change, if any, over time. The average results of ten samples of the Example 8 composition are reported in Table V which summarizes the tests conducted on the same substrate at the same curing and aging temperature described in connection with the tests reported in Table IV but with different die size. The die size for these tests were 15.24.times.15.24 mm-600.times.600 mil.
It is apparent from the foregoing that various changes and modifications may be made without departing from the spirit of the invention. Accordingly, the scope of the invention should be limited only by the appended claims wherein what is claimed is:
Claims
What is claimed is:
1. A die attach adhesive composition capable of being cured in less than five minutes at 200.degree. C. comprising: about 10 to about 40 weight percent resin vehicle comprising cyanate ester monomers and at least one flexibilizer; about 60 to about 90 weight percent silver; and wherein said composition is free of added metal containing catalysts or added alkylphenol.
2. A die attach adhesive composition capable of being cured in less than five minutes at 200.degree. C. comprising: about 10 to about 40 weight percent resin vehicle comprising cyanate ester monomers and at least one flexibilizer; about 60 to about 90 weight percent silver; about 0.1 to about 0.6 weight percent metal containing curing catalyst; and wherein said composition is free of added alkylphenol or added solvent.
3. A die attach adhesive composition capable of being cured in less than five minutes at 200.degree. C. comprising: about 10 to about 40 weight percent resin vehicle comprising cyanate ester monomers and at least one flexibilizer; about 60 to about 90 weight percent silver; and about 0.5 to about 2 weight percent added alkylphenol; and wherein said composition is free of added metal containing curing catalyst.
Patent Citations (59)
| Patent | Date | Inventor | Cited By |
|---|---|---|---|
| US1743167 | 1930-01-01 | Styer | |
| US2895270 | 1930-01-01 | Blaess | |
| US3586926 | 1971-06-01 | Nakamura et al. | |
| US3595900 | 1971-07-01 | Loudas et al. | |
| US3658623 | 1972-04-01 | Grigat et al. | |
| US4280885 | 1981-07-01 | Savery | |
| US4369302 | 1983-01-01 | Ikeguchi et al. | |
| US4375498 | 1983-03-01 | Le Minez et al. | |
| US4401776 | 1983-08-01 | Munk | |
| US4552690 | 1985-11-01 | Ikeguchi et al. | |
| US4604452 | 1986-08-01 | Shimp | |
| US4608434 | 1986-08-01 | Shimp | |
| US4699888 | 1987-10-01 | Dumesnil et al. | |
| US4709008 | 1987-11-01 | Shimp | |
| US4732702 | 1988-03-01 | Yamazaki et al. | |
| US4740343 | 1988-04-01 | Gaku et al. | |
| US4740584 | 1988-04-01 | Shimp | |
| US4740830 | 1988-04-01 | Ketley | |
| US4754001 | 1988-06-01 | Blahak et al. | |
| US4785075 | 1988-11-01 | Shimp | |
| US4831086 | 1989-05-01 | Das et al. | |
| US4839442 | 1989-06-01 | Craig, Jr. | |
| US4847233 | 1989-07-01 | Shimp | |
| US4861823 | 1989-08-01 | Qureshi | |
| US4902752 | 1990-02-01 | Shimp | |
| US4918157 | 1990-04-01 | Gardner | |
| US4931545 | 1990-06-01 | Shimp et al. | |
| US4940848 | 1990-07-01 | Shimp | |
| US4983683 | 1991-01-01 | Shimp | |
| US4999699 | 1991-03-01 | Christie et al. | |
| US5002818 | 1991-03-01 | Licari et al. | |
| US5036646 | 1991-08-01 | Makihara et al. | |
| US5037692 | 1991-08-01 | Medney et al. | |
| US5056296 | 1991-10-01 | Ross et al. | |
| US5068309 | 1991-11-01 | Shimp | |
| US5114003 | 1992-05-01 | Jackisch et al. | |
| US5143785 | 1992-09-01 | Pujol et al. | |
| US5149863 | 1992-09-01 | Shimp et al. | |
| US5150195 | 1992-09-01 | Nguyen | |
| US5155066 | 1992-10-01 | Nguyen | |
| US5162574 | 1992-11-01 | Craig, Jr. | |
| US5186880 | 1993-02-01 | Gaku et al. | |
| US5195299 | 1993-03-01 | Nguyen | |
| US5215860 | 1993-06-01 | McCormick et al. | |
| US5250600 | 1993-10-01 | Nguyen et al. | |
| US5261157 | 1993-11-01 | Chang | |
| US5325654 | 1994-07-01 | Juntunen et al. | |
| US5330684 | 1994-07-01 | Emori et al. | |
| US5399907 | 1995-03-01 | Nguyen et al. | |
| DEX3423385 | 1985-01-01 | ||
| JPX153338 | 1983-09-01 | ||
| JPX2285968 | 1987-12-01 | ||
| JPX3069883 | 1988-03-01 | ||
| WOX9208247 | 1992-05-01 | ||
| WOX9316921 | 1993-09-01 | ||
| WOX9316987 | 1993-09-01 | ||
| WOX9317059 | 1993-09-01 | ||
| WOX9324564 | 1993-12-01 | ||
| WOX9406844 | 1994-03-01 |
Non-Patent Literature (67)
- D.A. Shimp, et al., "AroCy.RTM. Cyanate Ester Resin, Chemistry, Properties and Applications, " Rh one-Poulenc, Inc.; Jan. 1990, pp. 1-36.
- E. Grigat and R. Putter, "New Methods of Preparative Organic Chemistry VI-Synthesis and Reactions of Cyanic Esters, " Agnew. Chem. Internat. Edit., vol. 6, Nov. 3, 1967, pp. 206-216.
- Hi-Tek Polymers, "AroCy.RTM. L-10 Cyanate Ester Monomer, : " Oct. 1989.
- Hi-Tek Polymers, "AroCy.RTM. Cyanate Ester Resins. "
- Hi-Tek Polymers, "AroCy.RTM. F-40S Cyanate Ester Resin Solution, " Apr. 1989.
- Hi-Tek Polymers, "AroCy.RTM. B-10 Cyanate Ester Monomer, " Oct. 1988.
- Hi-Tek Polymers, "AroCy.RTM. B-40S Cyanate Ester Resin Solution, " Oct. 1988.
- Hi-Tek Polymers, "AroCy.RTM. B-50 Cyanate Ester Hard Resin, " Oct. 1988.
- Hi-Tek Polymers, "AroCy.RTM. M-10 Cyanate Ester Monomer, " Oct. 1988.
- Hi-Tek Polymers, "AroCy.RTM. M-30 Cyanate Ester Semisolid Resin, " Oct. 1988.
- Hi-Tek Polymers, "AroCy.RTM. M-40S Cyanate Ester Resin Solution, " Oct. 1988.
- Hi-Tek Polymers, "AroCy.RTM. M-50 Cyanate Ester Hard Resin, " Oct. 1988.
- Hi-Tek Polymers, "AroCy.RTM. T-30 Cyanate Ester Semisolid Resin, " Oct. 1988.
- Allied Signal Inc., "Primaset--PT Resins, Safety and Handling Bulletin, " Mar. 20, 1992.
- Allied Signal Inc., "Primaset--PT Resins. "
- Allied Signa Inc., "Primaset--PT Resins, Literature List. "
- Tactix Performance Polymers, "XU-71787.02, XU-71787.07, Developmental Polycyanate Resins for Advanced Composites and Adhesives. "
- Sajal Das, "Pheno-Triazine (PT) Resin, A New Family of High Performance Thermosets" Cyanate Ester Symposium, Apr. 9-10, 1992.
- D.A. Shimp and S.J. Ising, "Moisture Effects and their Control in the Curing of Polycyanate Resins, " Cyanate Ester Symposium, Apr. 9-10, 1992.
- S.L. Simon and J.K. Gillham, "Cure of a Dicyanate Ester/Polycyanurates System, " Cyanate Ester Symposium, Apr. 9-10, 1992.
- Fraunhofer Institut fur Angewandte Materialforschung, "Polycyanurates and Their Modifications, State of the Art and New Aspects, " Gordon Research Conference on Thermoset, Jun. 1993.
- Hi-Tek Polymers, "AroCy.RTM. Resins, " Dec. 1988.
- David Shimp and Mark Southcott, "Controlling Moisture Effects During the Curing to High TG Cyanate Ester/Aramid Composites, " 38th International Symposium and Exhibition, May 10-13, 1993.
- Rh one-Poulenc, "Specialty Resin--Reactive Rubber Toughners for AroCy.RTM. Cyanate Ester Resins, " Jan. 1991.
- Rh one-Poulenc, "Specialty Resins--Pyrolysis of AroCy.RTM. Cyanate Esters. "
- Rh one-Poulenc, "Specialty Resins--Quantative Determination of Residual Cyanate in Cured Homopolymers Via FTIR Analysis, " Jan. 2, 1992.
- Rh one-Poulenc, "Specialty Resins--Compatibility of Cyanate Esters with Aramid Reinforcements and Polyamide/Imide Substrates. "
- RH one-Poulenc, "Specialty Resins--AroCy.RTM. L-10 Cyanate Ester Monomer, " Sep. 1990.
- RH one-Poulenc, "Specialty Resins--AroCy.RTM. F-10 Cyanate Ester Monomer, " Sep. 1990.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. M-20 Low Melt Viscosity Prepolymer, " Sep. 1990.
- Rh one-Poulenc, "Specialty Resins--ArcoCy.RTM. B-10 Cyanate Ester Monomer, " Oct. 1990.
- Rh one-Poulenc, "Specialty Resins--Toxicity of AroCy.RTM. Cyanate Esters. "
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. M-10 Cyanate Ester Monomer, " Sep. 1990.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. Cyanate Ester Resins. "
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. Cyanate Ester" List.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. F40-S Cyanate Ester Resin Solution, " Jul. 1990.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. B-30 Cyanate Ester Semisolid Resin. "
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. B-40S Cyanate Ester Resin Solution, " Aug. 1990.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. B-50 Cyanate Ester Hard Resin, " Sep. 1990.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. B-40S Cyanate Ester Resin Solution, " Aug. 1990.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. M-30 Cyanate Ester Semisolid Resin, " Sep. 1990.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. M-50 Cyanate Ester Hard Resin, " Sep. 1990.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. Cyanate Ester Safety and Handling Bulletin, " Sep. 1990.
- Rh one-Poulenc, "Specialty Resins--Formulating ArcOy.RTM. Cyanate Esters for Resin Transfer Molding Applications, " Apr. 1991.
- Rh one-Poulenc, "Specialty Resins--AroCy.RTM. Cyanate Ester Adhesives for Polyimide Flexible Circuitry, " Jan. 1991.
- Rh one-Poulenc, "Galvanic Corrosion of Carbon Fiber Composites, " Jan. 1991.
- Hi-Tek Polymers, "REX-378 Developmental Cyanate Ester Prepolymer. "
- Uri Sela and Hans Steinegger, "Dispensing Technology-The Key to High-Quality, High-Speed Die Bonding, " Microelectronics Manufacturing Technology, Feb. 1991, pp. 47-52.
- Jack R. Christenson and David A. Shimp, "Improvements in Performance and Processing Using Cyanate Ester Blends and Alloys, " Ultralloy '90, pp. 191-208.
- Tony del Rosario, "JM 7000 Low Temperature Die Attach Material Qualification Report, " Olin Interconnect Technologies, Dec. 10, 1992.
- Malcolm L. White, et al., "Attaining Low Moisture Levels in Hermetic Packages, " IEEE/Proc. IRPS, 1982, pp. 253-259.
- David P. Galloway and My N. Nguyen, "A New, Reliable Snap Cure Die Attach Adhesive, " May. Res. Soc. Symp. Proc., vol. 264, 1992, pp. 271-280.
- Sue Oliver, et al., "Silver/Polymer Die Attach for Ceramic Package Assembly, " 1992 IEPS meeting, Sep. 27-30, 1992.
- My N. Nguyen and Michael B. Grosse, "Low Polymer Moisture Adhesive for Hermetic Packages, " IEEE Trans. on Components, Hybrids and Manufacturing Technology, vol. 15, No. 6, Dec. 1992, pp. 964-971.
- Richard H. Estes, "A Practical Approach to Die Attach Adhesive Selection, " Hybrid Circuit Technology, Jun. 1991.
- Tom Ramsey and Gail Heinen, "Controlling Moisture in Ceramic Packages, " Semiconductor International, Aug. 1988.
- Mitsubishi Gas Chemical Company, Inc., "High Heat Resistant BT Resin--Bismaleimide-Triazine Resin, " Fourth Edition, Sep. 1, 1984.
- Sue Oliver, "Qualification Status and Extended Stressing of JM7000 Silver/Polymer Die Attach Adhesive for Ceramic Package Assembly, " Jan. 12, 1993.
- Hi-Tek Polymers, "AroCy.RTM. F-10 Cyanate Ester Monomer, " Apr. 1989.
- Hi-Tek Polymers, "AroCy.RTM. B-30 Cyanate Ester Semisolid Resin, " Oct. 1988.
- M. Bauer, J. Bauer and G. K uhn, "Kinetics and modeling of thermal polycyclotrimerization of aromatic dicyanates, " Acta Polymerica 37 (1986), Nr. Nov. 12, pp. 715-719.
- D.A. Shimp and W.M. Craig, Jr., "New Liquid Dicyanate Monomer for Rapid Impregnation of Reinforcing Fibers, " 34th Int'l SAMPE Symposium, 1989.
- Hi-Tek Polymers, "AroCy.RTM. Cyanate Ester Safety and Handling Bulletin, Arocy Safety and Handling. "
- Hans Steinegger, "Smart Cards: Bonding Technology at its Limits, " Microelectronics Mfg. Tech., Dec. 1992, pp. 13-15.
- Andrew Rosenbaum, "Smarter and Smarter", Electronics, Oct. 1991, 32D & 32E.
- Daniel Webb, "What Future is in the PC Cards?", Electronic Business, Nov. 4, 1991, p. 11.
- "Japan Challenges Intel's Lead in Flash memories", attached chart World EPROM Market, Semiconductors.