JPH0455153B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field to Which the Invention Pertains) The present invention relates to a method for forming a metal film that can be soldered and hard brazed on the surface of an aluminum nitride ceramic used as a semiconductor substrate.

[Prior art and its problems]

In recent years, there has been an increasing demand for electronic devices to be smaller, lighter (higher density packaging), faster, higher output, and more reliable. , multi-chip
The trend is toward modularization or higher output. With these trends in semiconductors, the following problems are occurring.

(1) High integration and density increase the amount of heat generated per unit volume of the device, which adversely affects the reliability of semiconductor devices.

(2) The board or package cannot cope with the increase in chip speed (signal slowdown).

(3) As the chip size increases, the difference in thermal expansion between the chip and the substrate increases, making stress more likely to occur and reducing bonding reliability.

(4) As the operating voltage of high-power chips continues to increase, dielectric breakdown of the substrate is likely to occur.

For this reason, various responsibilities have come to be placed on substrates and package materials that play a role in supporting semiconductors. Substrate and package materials that enable high integration and high density of devices are required to maintain the following properties.

(1) High thermal conductivity.

(2) Excellent electrical insulation.

(3) The coefficient of thermal expansion (μ) is comparable to that of Si.

(4) Low dielectric constant and low dielectric loss.

(5) High mechanical strength.

(6) Excellent metallization properties.

(7) Good workability.

(8) Be chemically stable.

(9) Must be heat resistant.

(10) Low cost.

Conventionally, alumina (hereinafter referred to as Al ₂ O ₃ ) has been most widely used as a ceramic substrate for mounting semiconductor devices, and is inexpensive and has a high degree of technological perfection. However, since Al ₂ O ₃ has a low thermal conductivity, it is not suitable as a substrate for high-density packaging where element heat generation is a problem. Yttrium oxide (Y ₂ O ₃ ) or calcium oxide (CaO) has recently been attracting attention as a highly thermally conductive ceramic substrate to replace Al ₂ O ₃ .
Aluminum nitride (hereinafter referred to as
(AlN) and beryum oxide (BeO) 1-2
There are two types of silicon carbide (hereinafter referred to as SiC) containing % by weight. The latter SiC substrate has high thermal conductivity,
Although it has excellent characteristics such as a coefficient of thermal expansion close to that of Si, it also has problems as a substrate for semiconductors, such as a high dielectric constant and low withstand voltage.

On the other hand, AlN substrates have high volume resistivity, high dielectric strength voltage, and low dielectric constant similar to Al ₂ O ₃ , and also have high thermal conductivity (5 to 8 times that of Al ₂ O ₃ ) and low coefficient of thermal expansion (close to that of Si). ) and high strength (approximately twice that of Al ₂ O ₃ ), it is considered particularly promising as a future highly thermally conductive ceramic substrate.

Incidentally, when mounting semiconductor elements on a substrate, methods such as soldering or brazing are employed.
For this reason, it is necessary to form a metal coating layer (metalized layer) for conductor circuits on the surface of the ceramic substrate. The following two methods are known so far for forming a metal film on the surface of an AlN substrate.

(1) Oxidize the surface of AlN at 1000-1400℃
After forming the Al ₂ O ₃ layer, a copper plate is placed on the AlN and bonded in an electric furnace with the partial pressure and temperature precisely controlled to the eutectic temperature of the Cu-O system.

(2) A method in which a paste of Au, Ag-Pb, Cu, etc. is applied to the surface of AlN and then fired.

Method (1) above is DBC (Direct Bond Copper)
Although it is known as a substrate, there is a problem in that slight differences in oxygen partial pressure and temperature tend to cause variations in bonding strength and airtightness. (2) is known as the so-called thick film method and is a simple method, but
Its bonding strength is at the level of 1.2-2.0Kg/ ^mm2 .
Therefore, in order to put AlN into practical use as a semiconductor substrate for high-integration, high-density packaging applications, it is necessary to establish a method for forming a highly reliable metal film.

[Purpose of the invention]

An object of the present invention is to provide a novel method for forming a metal film with excellent airtightness and bonding strength on an AlN substrate.

[Key points of the invention]

The above purpose was to apply a mixed powder of molybdenum powder or tungsten powder to which 15 to 40% by weight of manganese silicate, iron silicate, or silicate powder containing these as main components was applied to the AlN surface, and then apply a weakly oxidizing powder to the AlN surface. This can be achieved by sintering by heating to a temperature range of 1250°C to 1400°C in an atmosphere.

[Embodiments of the invention]

It is known to metallize the joints of ceramics in advance using molybdenum powder, tungsten powder, manganese powder, and titanium powder, which are high-melting point metal powders, and to hermetically bond the ceramic and metal together by brazing, especially for aluminum ceramics. It is widely used as the Mo-Mn or Mo-Mn-Ti method. The present inventors discovered that this Mo-
An attempt was made to apply the Mn method to AlN to obtain a metal coating, but the intended purpose could not be achieved. Therefore, we investigated various additive elements for high-melting-point molybdenum and tungsten powders, and found that it is possible to use a mixed powder in which manganese or iron is added in the form of a silicate compound to high-melting-point metal powders.
We obtained the knowledge that a good metal film can be formed on the AlN surface.

The optimal proportion of silicate powder added to high melting point metal powders such as molybdenum and tungsten is in the range of 15 to 40% by weight. This means that below 15% by weight AlN
The bonding properties of high melting point metal powder to
This is because if it exceeds 40% by weight, the adhesion of plating applied to the high-melting point metal after the formation of the metal film and the bonding strength of brazing or soldering deteriorate, making it impossible to guarantee reliability as a semiconductor substrate. Also,
According to the present invention, the temperature when forming the metal film on the AlN surface must be in the range of 1250° to 1450°C, and the atmosphere must be weakly oxidizing. At temperatures below 1250°C, AlN and high melting point metals -
This is because the silicate mixed powder does not react well, and furthermore, the silicate powder does not wet the high melting point metal powder particles well, causing problems in bonding strength. Furthermore, if the temperature exceeds 1450°C, some of the silicate becomes vitrified and diffuses into the grain boundaries of AlN, degrading its strength and dielectric properties, so this is not possible. If the temperature is in the range of 1250 to 1450°C, the silicate will wet both the AlN and the high melting point metal powder well, and a finely dense metal coating with a strong bond to the AlN can be obtained.

On the other hand, if the atmosphere during heating is reducing, manganese or Fe present in the form of oxides in the silicate will be reduced, making it impossible to form a bonding layer with AlN. On the other hand, if the atmosphere is strongly oxidizing, AlN and the high melting point metal will be oxidized, making it impossible to form a metal film. Therefore, in the present invention, a weakly oxidizing atmosphere is used, and in this case, the lower limit of oxidizing property can be tolerated up to just before reducing property. Water vapor is particularly effective as a weak oxidizing agent, and it is best to bubble elementary gas into heated water and adjust the oxygen concentration in the atmosphere depending on the temperature of the water.

It should be noted that the metal film of the high melting point metal obtained by the method of the present invention has poor wettability with solder and hard solder and is easily oxidized, so after forming the metal film, it is necessary to perform plating with nickel or the like. The present invention will be explained below based on examples.

Example 1 Molybdenum powder 80% by weight, manganese silicate powder 20%
% by weight of the mixed powder is well suspended in an organic solvent to form a paste, which is then printed with AlN using a screen printing method.
It was applied to a thickness of 18 μm on the surface of This was inserted into an electric furnace, and hydrogen gas bubbled at a water temperature of 50°C was flowed through the furnace at a rate of 300°C per hour.
Raise the temperature to 1400℃ and hold at 1400℃ for 1 hour.
A 10 μm thick metal film was formed on the AlN surface. After this, the metal coating is coated with an electroless nickel plating method.
A nickel plating of 4 μm was applied. This was heat-treated at 820°C for 15 minutes in a mixed atmosphere of hydrogen and nitrogen gas, and the plating was inspected for peeling, blistering, etc., but no defects were observed. Next, the plated AlN substrate and the iron-nickel alloy were brazed using silver solder (BAg-8) in a hydrogen atmosphere at 820°C, and the airtightness of the joint was examined. The amount of helium saw at the joint is 1×10 ^-5
Atmml/sec or less, and it was confirmed that the airtightness was good. In addition, it was treated with plating
A copper wire with a diameter of 0.5 mm was soldered onto an AlN substrate using eutectic solder (40% PD - 60% Sn), and a tensile test was performed on the wire. In this test, all of the wires broke at the copper wire part, and no damage was observed at the interface between the metal coating and the plating, or the interface between the AlN and metal coating. The tensile strength of the copper wire at breakage was 25 Kg/mm ² , indicating that the metal coating on the AlN substrate produced by the method of the present invention can withstand even higher strength.

Example 2 A mixed powder of 80% by weight of molybdenum powder and 20% by weight of iron silicate was applied onto the AlN surface in the same manner as in Example 1,
A metal film was formed by holding at 1350°C for 1 hour. This was also evaluated under the same conditions as Example 1, but
The airtightness and bonding strength were the same as in Example 1.

In addition, in Examples 1 and 2, it was experimentally confirmed that even if molybdenum powder was used as the high melting point metal powder, the same results as in the case of the molybdenum powder described above were obtained.

〔Effect of the invention〕

As explained above, according to the present invention, a mixed powder in which 15 to 40% by weight of manganese silicate, iron silicate, or silicate powder containing these as main components is added to molybdenum powder or tungsten powder is AlN.
After being applied to the surface, it is sintered at a temperature of 1250°C to 1450°C in a mildly residual atmosphere, resulting in a dense metal coating with a stronger bond with AlN than conventional methods. Therefore, it is possible to provide a substrate for mounting a semiconductor element, which has excellent airtightness and bonding strength, and which enables electronic devices to be made smaller, lighter, faster, higher output, and more reliable.

Claims (1)

[Claims] 1 After applying a mixed powder of molybdenum powder or tungsten powder to 15 to 40% by weight of manganese silicate, iron silicate, or silicate powder containing these as main components on the surface of aluminum nitride ceramic, weak oxidation is applied. 1250℃ to 1450℃ in a sexual atmosphere
A method for forming a metal film, characterized by heating and sintering to a temperature of °C.