Noble metal paste for bonding of semiconductor element

Abstract

A precious metal paste which does not cause contamination of a member, which can be uniformly coated to a member to be bonded, and which is in good condition after bonding is provided. The present invention relates to a precious metal paste for bonding a semiconductor element, of the paste including a precious metal powder and an organic solvent, in which the precious metal powder has a purity of 99.9 mass % or more and an average particle diameter of 0.1 to 0.5 m, the organic solvent has a boiling point of 200 to 350 C., and a thixotropy index (TI) value calculated from a measurement value of a viscosity at a shear rate of 4/s with respect to a viscosity at a shear rate of 40/s at 23 C. by means of a rotational viscometer is 6.0 or more.

Claims

1. A precious metal paste for bonding a semiconductor element, the paste consisting essentially of either a precious metal powder and an organic solvent, or a precious metal powder, an organic solvent and surfactant, and not containing any resin, wherein the precious metal powder has a purity of 99.9 mass % or more and an average particle diameter of 0.1 to 0.5 m, and the organic solvent has a boiling point of 200 to 350 C., wherein the organic solvent is made of one organic solvent only, which one organic solvent is a branched-chain aliphatic dihydroxy alcohol having a carbon number of 5 to 20, and which branched-chain aliphatic dihydroxy alcohol consists of 1,5-pentanediol or derivatives thereof, and wherein the precious metal paste has a thixotropy index (TI) value of 6.1 or more, of a viscosity at a shear rate of 4/s with respect to a viscosity at a shear rate of 40/s at 23 C. by means of a rotational viscometer, and a viscosity at a shear rate of 4/s which is 100 to 1000 Pa.Math.s.

2. The precious metal paste according to claim 1, wherein the precious metal powder comprises any one or more of a gold powder and a silver powder.

3. The precious metal paste according to claim 1, wherein a volume content of the precious metal powder in the precious metal paste is 26 to 66 vol. % (v/v).

4. A method of bonding which comprises die bonding of a semiconductor element to a substrate with the precious metal paste of claim 1.

5. The precious metal paste according to claim 1 wherein, in a bonded part when a semiconductor element is bonded, a bonding rate calculated from a ratio of an area of an adhesion part in the bonded part to a total area of the bonded part in an X-ray fluoroscopic image is 90% or more.

6. The precious metal paste according to claim 2, wherein a volume content of the precious metal powder in the precious metal paste is 26 to 66 vol. % (v/v).

7. A method of bonding which comprises die bonding a semiconductor element to a substrate with the precious metal paste of claim 2.

8. A method of bonding which comprises die bonding a semiconductor element to a substrate with the precious metal paste of claim 3.

9. A method of bonding which comprises die bonding a semiconductor element to a substrate with the precious metal paste of claim 6.

10. The precious metal paste according to claim 2, wherein, in a bonded part when a semiconductor element is bonded, a bonding rate calculated from a ratio of an area of an adhesion part in the bonded part to a total area of the bonded part in an X-ray fluoroscopic image is 90% or more.

11. The precious metal paste according to claim 3, wherein, in a bonded part when a semiconductor element is bonded, a bonding rate calculated from a ratio of an area of an adhesion part in the bonded part to a total area of the bonded part in an X-ray fluoroscopic image is 90% or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an appearance X-ray fluoroscopic image of a bonded part and a result of a cross-sectional observation by an electron microscope (SEM).

(2) FIG. 2 shows a test method of bonding strength.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(3) Hereinafter, suitable embodiments of the present invention will be described.

Example 1

(4) A gold paste was prepared by mixing 95 mass % of a gold powder (average particle diameter: 0.3 m) having a purity of 99.99 mass % manufactured by a wet reduction method, 3.75 mass % of isobornylcyclohexanol (MTPH) as an organic solvent, and 1.25 mass % of -terpineol. A volume content of the gold powder in the gold paste was 49.6 vol. %. With respect to the used organic solvent and the obtained gold paste, the following physical property measurement was performed.

(5) Physical Property Measurement

(6) With a cone rotational viscometer (Rheostress RS75 manufactured by HAAKE, measured with cone plate made of titanium, having a diameter of 35 mm, of 1, a gap of 0.050 mm), a viscosity of each of the organic solvent and the gold paste was continuously measured by being maintained for 30 seconds at respective shear rates of 4/s, 20/s, and 40/s in this order, after being maintained for 30 seconds at a shear rate of 0/s, at a measurement temperature of 23 C. The boiling point of the organic solvent was measured in atmosphere at a temperature increase rate of 10 C./min by TG-DTA (simultaneous thermogravimetric/differential thermal analysis: TG8101D manufactured by Rigaku Corporation). Moreover, the thixotropy index (TI) value was calculated from the viscosity measurement values at the shear rates of 4/s and 40/s in accordance with the following equation. Furthermore, with respect to the gold paste of Example 1, the TG-DTA (simultaneous thermogravimetric/differential thermal analysis) was performed.
TI=(the viscosity at the shear rate of 4/s)/(the viscosity at the shear rate of 40/s)

(7) According to the above result, the viscosity of the gold paste of Example 1 at the shear rate of 4/s was 256 Pa.Math.s. Moreover, according to the TG-DTA, it was confirmed that evaporation of the organic solvent starts at 70 C. and the organic constituent completely disappears at 190 C. in the gold paste of Example 1.

Examples 2 to 5, Comparative Examples 1 to 7

(8) Gold pastes were produced by using the same gold powder as in Example 1 with the use of the organic solvents and gold contents shown in Table 1. In Example 2, a gold powder having the average diameter of 0.1 m was used. In Comparative Example 7, bisalkenylsuccinimide (manufactured by King Industries Inc., product name: KX1223C) was used as the organic solvent. In each of Examples and Comparative Examples, the viscosity and the boiling point of the organic solvent, and the viscosity and the TI value of the gold paste were measured by the same methods as in Example 1. The results are shown in Table 1.

(9) TABLE-US-00001 TABLE 1 paste kind of gold organic solvent *.sup.2 viscosity (Pa .Math. s) organic content viscosity boiling point Shear Rate 4 Shear Rate 40 TI solvent *.sup.1 (vol. %) (Pa .Math. s) ( C.) (/s) (/s) value Example 1 M/T = 3/1 49.6 14.4 313/213 256 27 9.5 Example 2 M/T = 3/1 40.7 14.4 313/213 184 30 6.1 Example 3 MARS 53.5 1.7 264 934 60 15.6 Example 4 MARS 35.8 1.7 264 315 25 12.6 Example 5 MARS 41.4 1.7 264 502 82 6.1 Comparative M/T = 3/1 36.4 14.4 313/213 157 29 5.4 Example 1 Comparative MARS/T = 3/1 29.7 14.4 264/213 52 13 4.0 Example 2 Comparative M/T= 1/1 29.5 1.3 313/213 77 21 3.7 Example 3 Comparative M 44.8 1702 313 not obtain a paste Example 4 Comparative M 35.8 1702 313 144 54 2.7 Example 5 Comparative T 35.8 0.05 213 not obtain a paste Example 6 Comparative KX1223C 43.5 6.5 350-450 328 152 2.2 Example 7 *.sup.1 M represents MTPH, and T represents -terpineol. M/T is a mixing ratio of MTPH to -terpineol. *.sup.2 With respect to M/T, (boiling point of MTPH)/(boiling point of -terpineol) is shown.

(10) According to the above results of the physical property measurement, each of the precious metal pastes of Examples 1 to 5 was shown to have a TI value of 6.0 or more. In contrast, each of the TI values in Comparative Examples 1 to 3, 5, and 7 was less than 6.0. Furthermore, a paste was not able to be obtained in each of Comparative Examples 4 and 6 because, even if the gold powder and the organic solvent were mixed, the precious metal powder settled out immediately and separated from the solvent or handling became difficult.

(11) [Average Particle Diameter of Gold Powder]

(12) In addition to the above Examples and Comparative Examples, gold pastes were produced as in Example 1 by using, as the gold powder, a gold powder having an average particle diameter of 0.7 m and a gold powder having an average particle diameter of 0.05 m. As a result, in the case of the gold powder having an average particle diameter of 0.7 m, dispersion of the gold powder failed to be maintained in the paste and settling occurred, and in the case of the gold powder having an average particle diameter of 0.05 m, partial aggregation was confirmed in the paste.

(13) [Silver Paste]

(14) Furthermore, a silver paste was produced as in Example 1 by using a silver powder (86 wt. %; 37 vol. %) instead of the gold powder, and the physical property measurement was performed. As a result, in the obtained silver paste, the viscosity at the shear rate of 4/s was 176 Pa.Math.s, the viscosity at the shear rate of 40/s was 19 Pa.Math.s, and the TI value was 9.3.

(15) Coating Test for Substrate

(16) Next, the gold paste of each of the above Examples 1 to 5 and Comparative Examples 1 to 3, and 5 was coated to a center of a semiconductor substrate (Si) having an area of 100 mm.sup.2 such that the area of the gold paste is 25 mm.sup.2, and coating performance to the substrate was evaluated. In addition, as the substrate, a substrate having a film of Ti (50 nm) and Au (200 nm) formed on its surface in advance by sputtering was used.

(17) As a result, each of the gold pastes of Examples 1 to 5 had moderate wettability and was easy to be coated to the substrate, and the gold paste after coating also maintained sufficient moldability. On the other hand, in the gold paste of Comparative Example 3, the solvent tended to bleed from the coated gold paste, and the paste after coating was deformed.

(18) Bonding Test

(19) The following bonding test was performed after coating the paste as described above without drying and sintering. The bonding test was performed by placing a Si chip having an area of 4 mm.sup.2 (on which a film of Ti (20 nm) and Au (200 nm) is formed in advance) on the paste after coating, and by heating and pressurizing it. In bonding, application of pressure was 20 N (5 MPa) per one chip and application of heat was set to be 230 C. by heat transferred from a tool, and heating and pressurizing time was set to be 10 minutes.

(20) With respect to a bonded part which has been bonded as described above, a structure observation was performed by an appearance X-ray fluoroscopic image (manufactured by Uni-Hite System Corporation), and a bonding rate was calculated in accordance with the following equation. The results for the bonding rate are shown in Table 2. In addition, the X-ray fluoroscopic image of each of Example 1 and Comparative Example 1, and the result of a cross-sectional SEM observation of Example 1 are shown in FIG. 1. In the X-ray fluoroscopic image, a part in which a void is generated and a space is formed in the bonded part is observed as a white color, and a part in which tight adherence occurs in the bonded part is observed as a gray (black).
bonding rate={the area of the adhesion part (the gray-colored part in the X-ray fluoroscopic image)}/{the total area of the bonded part (the sum of the areas of the gray-colored part and the white-colored part in the X-ray fluoroscopic image)}

(21) TABLE-US-00002 TABLE 2 bonding rate Example 1 98% Example 2 95% Example 3 97% Example 4 91% Example 5 94% Comparative Example 1 77% Comparative Example 2 61% Comparative Example 3 65% Comparative Example 5 66%

(22) According to the results of FIG. 1, in the bonded part by the gold paste of Example 1, a white-colored part generated by a space due to generation of a void was hardly observed in the appearance X-ray fluoroscopic image, and the gold powder particles were close with each other in a nearly point contact state in the cross-sectional SEM image. In addition, the bonding rate shown in Table 2 was 90% or more. Accordingly, it was confirmed that sintering of the gold powder by heating in bonding uniformly proceeded in Example 1.

(23) When the paste of each of Examples 2 to 5 was used, the bonding rate was 90% or more according to Table 2, generation of a void was hardly observed and it was confirmed that sintering uniformly proceeded like in Example 1. Furthermore, when bonding was performed by using the silver paste, according to the appearance X-ray fluoroscopic image, generation of a void was hardly observed and it was confirmed that sintering of the silver powder uniformly proceeded.

(24) On the other hand, when the gold paste of Comparative Example 1 was used, the bonding rate was less than 90% according to Table 2, and many white-colored parts generated by spaces due to generation of voids were observed in the appearance X-ray fluoroscopic image of FIG. 1. Thus, it is thought that when heating in bonding, many voids were generated by outgassing (emitted) derived from the organic constituent and sintering of the gold powder did not uniformly proceed in Comparative Example 1.

(25) Moreover, when the paste of each of Comparative Examples 2, 3, and 5 was used, the bonding rate was less than 90% according to Table 2, and many white-colored parts generated by spaces due to generation of voids were observed. Furthermore, when bonding by using the gold paste of Comparative Example 7, many white-colored parts generated by spaces due to generation of voids were observed in the appearance X-ray fluoroscopic image.

(26) Bonding Strength Test

(27) Next, with respect to the bonding performed above, a bonding strength test was performed in accordance with FIG. 2. For a semiconductor chip (heat-resistant Si chip) 30 bonded to a semiconductor substrate 10 with a sintered compact 20 sandwiched therebetween, a blade was brought into contact with the chip in the lateral direction at a constant rate and an average value of stress (unit: N) when the rupture (separation of chip) occurred was measured so that the bonding strength was obtained. An average value of bonding strength per unit area (unit: MPa) was calculated from the measurement value and the area of the bonded part after rupturing. The results are shown in Table 3.

(28) TABLE-US-00003 TABLE 3 rupture stress per bonded bonding strength part (average value) (average value) Example 1 160 N 40 MPa Example 2 140 N 35 MPa Example 3 160 N 40 MPa Example 4 150 N 38 MPa Example 5 150 N 38 MPa Comparative Example 1 130 N 32 MPa

(29) According to Table 3, when bonding by using the paste of each of Examples 1 to 5, it was confirmed that the bonded part had sufficient bonding strength (20 MPa or more) for bonding or the like of electronic components. Moreover, in bonding by using the paste of Comparative Example 1, the bonding strength similar to that of Examples was obtained. However, as described above, because the bonding rate was low (Table 2) and generation of many voids was observed (FIG. 1), the paste of Comparative Example 1 was not suitable for bonding of electronic components.

INDUSTRIAL APPLICABILITY

(30) The precious metal paste of the present invention is suitable for low-temperature bonding of various members to be bonded, and is useful for bonding a semiconductor element and the like suspected to be influenced by thermal stress, to a substrate.