TWI841253B - Method for manufacturing highly reliable metal-to-ceramic active metal brazed substrate - Google Patents

Abstract

The present invention relates to a method for manufacturing a metal to ceramic active metal brazed substrate. The method comprises coating a precisely controlled thickness with extremely low defect ceramic (LDC) layer on both top and bottom surfaces of a ceramic substrate by atomic layer deposition (or ALD), depositing the first active metal brazing material on both top and bottom surfaces of LDC coated ceramic substrate, putting down the third brazing material on both top and bottom surfaces of an active metal brazing material covered and LDC coated substrate, and bonding a brazing metal concealed ceramic substrate with both top and bottom Cu foils in a high temperature vacuum based furnace.

Description

Preparation method of active metal welded ceramic substrate

The present invention relates to a preparation method for an active metal welded ceramic substrate, in particular, a preparation method for reducing the post-welding void rate of an active metal brazing process.

Active Metal Brazing (AMB) is a method of bonding ceramics to metals by using a small amount of active elements in the solder to react with ceramics to generate a reaction layer that can be wetted by liquid solder.

Usually, the ceramic surface is first printed with active metal solder and then clamped with oxygen-free copper, and then high-temperature soldered in a vacuum soldering furnace. After the substrate is laminated, a wet etching process similar to that of a PCB is used to make circuits on the surface, and finally the surface is plated to produce a product with reliable performance.

The AMB substrate is achieved by chemically reacting ceramics and active metal solder paste at high temperatures, so its bonding strength is higher and its reliability is better. However, due to the high cost of this method, the small amount of suitable solder, and the large impact of solder on the reliability of welding, currently only a few Japanese companies have mastered the high-reliability active metal welding technology.

At present, the void ratio of the ceramic interface is one of the important factors in the quality of the substrate. Therefore, the control of the void ratio of the ceramic interface is crucial. If the interface void ratio is good, the service reliability of the substrate under high temperature and high current can be guaranteed.

Taking Ag-Cu-Ti active solder as an example, the causes of void formation are as follows:

(1) Surface quality of raw materials: Scratches, pits, oxidation, organic contamination and other problems on the surface of ceramics, oxygen-free copper and solder pads before welding will have a negative impact on the wetting and spreading of solder, bringing potential void risks to the brazing interface. Regarding this part, the surface defects of Si ₃ N ₄ ceramic materials generally include the following surface contamination and defects: particle contamination, surface roughness, surface cracks, surface contamination, point defects, dislocations, microcracks, and impurities. Point defects, dislocations, microcracks, impurities, and surface contamination are the most common. Contamination) will have the greatest impact. If it is not handled properly, when titanium is wetted on the surface, it will easily penetrate into the ceramic substrate due to surface defects, resulting in the risk of voids and weakening the strength of the substrate copper film bonding interface, further leading to the risk of voids.

(2) Deactivation of active elements: The active element (Ti) of Ag-Cu-Ti is very sensitive to oxygen. Therefore, during high-temperature brazing, the vacuum degree is often required to be better than 10-3Pa. If the vacuum degree cannot meet the welding requirements, Ti will be oxidized and deactivated, and the solder will not be able to wet the surface of Si3N4 ceramics, resulting in large-area voids and leaks.

(3) Brazing process parameters: Ag-Cu-Ti active brazing filler metal can only wet the _Si3N4 surface at a temperature above ₈₀₀ °C. If the brazing temperature is too low or the holding time is too short, the reaction between Ti and the ceramic surface will not be sufficient, resulting in the inability of the brazing filler metal to completely wet the ceramic surface.

(4) Solder paste printing quality: During large-area solder paste printing, it is easy to have problems with solder paste missing and uneven printing. Once the solder melts and does not spread to cover these missed areas, it will directly lead to the formation of voids.

(5) Solder paste degassing: During the brazing process, the gas volatilized in the solder paste will be wrapped by the flux to form bubbles. In addition, the reaction between the organic acid and metal oxide in the flux will also produce bubbles. As the reaction proceeds, the bubbles gradually grow larger. The discharged bubbles will leave dense pores on the surface of the solder paste. The bubbles that are not discharged will also be retained at the brazing interface as the solder melts and solidifies, thereby forming voids and causing the risk of fracture of the substrate copper film joint interface during long-term operation.

The general way to reduce the void rate is to remove oil and oxidation from the ceramic and copper sheets and provide a high vacuum brazing environment during the preparation of silicon nitride copper-clad substrates through the AMB process. This is a well-known method for reducing interface void rate, but its effectiveness is still very limited and an effective improvement method is urgently needed.

Therefore, in this case, a surface optimization layer (silicon nitride, aluminum oxide or aluminum nitride) is first formed on the upper and lower surfaces of the ceramic substrate by ALD to cover the surface defects and flatten the upper and lower surfaces of the ceramic substrate to reduce the probability of voids. In addition, ALD is used to form a first solder layer (titanium or titanium nitride) on the surface optimization layer, and a third solder layer (silver) is formed on the surface of the copper plate. In this way, when the ceramic substrate and two copper plates are stacked up and down and then high-temperature hot-pressing brazing is performed to form an active metal welding ceramic substrate, the probability of voids will be significantly reduced. Therefore, the present invention should be an optimal solution.

The present invention discloses a method for preparing an active metal-welded ceramic substrate, which comprises: (1) performing an atomic layer deposition process on the upper surface and the lower surface of a ceramic substrate to form a surface optimization layer on the upper surface and the lower surface of the ceramic substrate, wherein the composition of the surface optimization layer is silicon nitride, aluminum oxide or aluminum nitride; (2) performing an atomic layer deposition process on the surface optimization layer to form a first solder layer on the surface optimization layer; (3) forming a third solder layer on the copper plate surface facing the upper surface and the lower surface of the ceramic substrate, wherein the third solder layer comprises at least copper and silver; (4) stacking the ceramic substrate and the two copper plates up and down to form a layered structure, and then performing high temperature hot pressing welding on the layered structure to form an active metal welded ceramic substrate.

More specifically, the surface optimization layer is used to cover the surface defects on the upper and lower surfaces of the ceramic substrate.

More specifically, the upper and lower surfaces of the ceramic substrate can be first subjected to an atomic layer etching process and then subjected to an atomic layer deposition process to form the surface optimization layer, wherein the atomic layer etching process is used to remove surface defects on the upper and lower surfaces of the ceramic substrate.

More specifically, the thickness of the surface optimization layer is 1nm~1μm.

More specifically, a second solder layer can be formed on the first solder layer, and the second solder layer is composed of titanium nitride.

More specifically, the thickness of the second solder layer is 0.1~100nm.

More specifically, the thickness of the first solder layer is 0.1~100nm.

More specifically, the thickness of the third solder layer is 0.1 μm ~100 μm .

More specifically, the composition of the third solder layer further includes titanium.

More specifically, the temperature of the high temperature hot pressure brazing is 750~950℃.

1: Active metal welding ceramic substrate

11: Ceramic substrate

12: Copper plate

13: Copper plate

14: Surface optimization layer

15: First solder layer

16: Second solder layer

17: Third solder layer

[Figure 1] is a schematic diagram of the preparation process of the active metal welded ceramic substrate preparation method of the present invention.

[Figure 2A] is a schematic diagram of the structural separation of the active metal welded ceramic substrate in the preparation method of the active metal welded ceramic substrate of the present invention.

[Figure 2B] is a schematic diagram of the structure of the active metal welded ceramic substrate after hot pressing in the preparation method of the active metal welded ceramic substrate of the present invention.

[Figure 3] is another schematic diagram of the structure of the ceramic substrate and the first solder layer in the preparation method of the active metal-welded ceramic substrate of the present invention.

Other technical contents, features and effects of the present invention will be clearly presented in the following detailed description of the preferred embodiment with reference to the drawings.

Referring to FIG. 1, the preparation method of the active metal welding ceramic substrate is as follows: (1) performing an atomic layer deposition process on the upper surface and the lower surface of a ceramic substrate to form a surface optimization layer on the upper surface and the lower surface of the ceramic substrate, wherein the composition of the surface optimization layer is silicon nitride, aluminum oxide or aluminum nitride 101; (2) performing an atomic layer deposition process on the surface optimization layer to form a first solder layer on the surface optimization layer , wherein the first solder layer is composed of titanium 102; (3) forming a third solder layer on the surface of the copper plate facing the upper surface and the lower surface of the ceramic substrate, wherein the third solder layer is composed of at least copper and silver 103; (4) stacking the ceramic substrate and the two copper plates up and down to form a layered structure, and then performing high temperature hot pressing brazing on the layered structure to form an active metal welded ceramic substrate 104.

The temperature of the high temperature hot pressure brazing is 750~950℃.

As shown in FIG. 2A , the active metal welded ceramic substrate includes a ceramic substrate 11 and two copper plates 12 and 13 , wherein the surface optimization layer 14 is attached to the upper and lower surfaces of the ceramic substrate 11 , and the material of the surface optimization layer 14 is Si ₃ N ₄ , AlN or Al ₂ O ₃ .

The thickness of the surface optimization layer 14 is 1nm~1μm.

The surface optimization layer 14 is used to cover the surface defects on the upper and lower surfaces of the ceramic substrate 11.

A first solder layer 15 (titanium) can be attached to the surface optimization layer 14.

The two copper plates 12 and 13 are respectively attached with a third solder layer 17 (copper, silver, titanium) on the surface facing the ceramic substrate 11, wherein the third solder layer 17 comprises silver, copper, and titanium (wherein the silver content accounts for 66-72%, the copper content accounts for 28-32%, and the titanium content accounts for 0-6%).

The third solder layer 17 is formed on the surfaces of the two copper plates 12, 13 by sputtering or atomic layer deposition.

The thickness of the first solder layer 15 is 0.1~100nm.

The thickness of the third solder layer 17 is 0.1 μm -100 μm .

The thickness of the copper plates 12, 13 is 100-1000 μm .

As shown in Figure 2B, after being stacked up and down to form a layered structure, the layered structure is then subjected to high-temperature hot-pressing brazing to form an active metal-welded ceramic substrate 1.

The Atomic Layer Deposition (ALD) process is a common technique that mainly involves reacting a chemical gas containing the components to be deposited with a ceramic substrate, then removing the chemical gas with a large amount of inert gas (such as nitrogen or argon), and then repeating step a. This allows all reactions to occur only on the surface of the ceramic substrate, and each cycle only forms a thin film with a thickness of one atom. This allows the accuracy of each film deposition thickness to reach the atomic level (about 0.1nm) and has excellent uniformity. Because the growth process is confined to the surface of the ceramic substrate, good coverage and uniformity can be achieved on surfaces with structures.

The atomic layer deposition (ALD) process is used to cover the defects on the upper and lower surfaces of the ceramic substrate, such as point defects, dislocations, microcracks, impurities, and surface contamination, to make the surface flat.

Although the ALD process can cover the surface defects in this case, better results can be achieved if the ALE process can be used to remove the surface micro defects first and then the ALD process can be used to cover the surface defects.

Therefore, the upper and lower surfaces of the ceramic substrate 11 can be first subjected to atomic layer etching, and then the upper and lower surfaces of the ceramic substrate 11 can be subjected to atomic layer deposition to form the surface optimization layer. The atomic layer etching is used to remove surface defects on the upper and lower surfaces of the ceramic substrate 11.

The Atomic Layer Etching (ALE) process is a common technology. It mainly supplies an etching gas and an inert gas to a reaction space through a mass flow controller, and then uses plasma for etching. Inert gas or nitrogen is supplied to the reaction space as the basic reaction gas to generate plasma. In addition, oxidizing gas or reducing gas such as hydrogen can be added to control the etching rate. The process temperature is mostly controlled at 0 to 250 degrees.

The atomic layer etching (ALE) process is used to remove defects on the upper and lower surfaces of the ceramic substrate, such as point defects, dislocations, microcracks, and surface contamination.

The surface optimization layer 14, the first solder layer 15 and the second solder layer 16 on the upper and lower surfaces of the ceramic substrate 11 are in contact with the third solder layer 17 of the copper plates 12, 13, and then vacuum-packaged using a heat-conductive material to obtain the layered structure. The vacuum degree of the vacuum packaging is less than ^10-1 Pa, and the heat-conductive material is a flexible metal foil bag.

As shown in FIG. 3, a second solder layer 16 can be formed on the first solder layer 15. The second solder layer 16 is composed of titanium nitride. The second solder layer 16 has an anti-oxidation function to prevent or reduce the possibility of oxidation of the first solder layer 15. The present invention can be implemented by forming only the first solder layer 15, or by forming the first solder layer 15 and the second solder layer 16, wherein the outermost layer of the multiple layers is the first solder layer 15 or the second solder layer 16.

The thickness of the second solder layer 16 is 0.1~100nm.

The second solder layer 16 mainly prevents the first solder layer 15 from oxidizing titanium during subsequent high temperature treatment and effectively controls the formation of intermetallic.

The ceramic substrate 11 is further degreased so that the first solder layer 15 and the second solder layer 16 are heated and degreased in an inert gas or reducing gas atmosphere and attached to the upper and lower surfaces of the ceramic substrate 11. The inert gas includes nitrogen or argon, and the reducing gas includes hydrogen, nitrogen-hydrogen mixed gas or acidic gas.

The copper plates 12, 13 are further degreased so that the third solder layer 17 is heated and degreased in an inert gas or reducing gas atmosphere to adhere to the surface of the copper plates 12, 13.

Since this case uses atomic layer etching (ALE) and atomic layer deposition (ALD) to process the upper and lower surfaces of the ceramic substrate, the probability of voids occurring can be reduced.

Since the first solder layer is formed using Atomic Layer Deposition (ALD), its thickness can be precisely controlled to ensure a uniform and flat surface. In addition to achieving good connectivity with the ceramic substrate, the first solder layer can also be effectively controlled to form an intermetallic layer between the ceramic substrate and the metal solder because the first solder layer is uniformly thin.

The preparation method of active metal welded ceramic substrate provided by the present invention has the following advantages when compared with other conventional technologies:

(1) In this case, the upper and lower surfaces of the ceramic substrate are treated (removing surface micro-defects) through atomic layer etching (ALE). This will reduce the probability of voids and improve the yield of preparing active metal welding ceramic substrates.

(2) In this case, the surface optimization layer (silicon nitride, aluminum oxide or aluminum nitride) is formed on the upper and lower surfaces of the ceramic substrate through atomic layer deposition (ALD) to better treat the surface (cover surface defects), while also reducing the probability of voids and improving the yield of preparing active metal welding ceramic substrates.

(3) This case uses atomic layer etching (ALE) and atomic layer deposition (ALD) to improve the wetting ability of the first solder layer (titanium) on the ceramic substrate and enhance the bonding strength between copper and the substrate.

The present invention has been disclosed through the above-mentioned embodiments, but they are not used to limit the present invention. Anyone familiar with this technical field and having common knowledge can make some changes and modifications without departing from the spirit and scope of the present invention after understanding the above-mentioned technical features and embodiments of the present invention. Therefore, the scope of patent protection of the present invention shall be subject to the definition of the claim items attached to this specification.

Claims (10)

A method for preparing an active metal-welded ceramic substrate, the method comprising: Performing an atomic layer deposition process on the upper and lower surfaces of a ceramic substrate to form a surface optimization layer on the upper and lower surfaces of the ceramic substrate, wherein the composition of the surface optimization layer is silicon nitride, aluminum oxide or aluminum nitride; Performing an atomic layer deposition process on the surface optimization layer to form a first solder layer on the surface optimization layer, wherein the composition of the first solder layer is titanium; Forming a third solder layer on the surfaces of two copper plates facing the upper and lower surfaces of the ceramic substrate, wherein the composition of the third solder layer is at least copper and silver; The ceramic substrate and two copper plates are stacked up and down to form a layered structure, and then the layered structure is subjected to high temperature hot pressing brazing to form an active metal welded ceramic substrate. A method for preparing an active metal-welded ceramic substrate as described in claim 1, wherein the surface optimization layer is used to cover surface defects on the upper and lower surfaces of the ceramic substrate. A method for preparing an active metal welded ceramic substrate as described in claim 1, wherein the upper and lower surfaces of the ceramic substrate can first be subjected to an atomic layer etching treatment and then to an atomic layer deposition treatment to form the surface optimization layer, wherein the atomic layer etching treatment is used to remove surface defects on the upper and lower surfaces of the ceramic substrate. A method for preparing an active metal welded ceramic substrate as described in claim 1, wherein the thickness of the surface optimization layer is 1nm~1µm. A method for preparing an active metal welded ceramic substrate as described in claim 1, wherein a second solder layer can be formed on the first solder layer, and the second solder layer is composed of titanium nitride. A method for preparing an active metal welded ceramic substrate as described in claim 5, wherein the thickness of the second solder layer is 0.1~100nm. A method for preparing an active metal welded ceramic substrate as described in claim 1, wherein the thickness of the first solder layer is 0.1~100nm. A method for preparing an active metal welded ceramic substrate as described in claim 1, wherein the thickness of the third solder layer is 0.1 μm to 100 μm. A method for preparing an active metal welded ceramic substrate as described in claim 1, wherein the composition of the third solder layer further includes titanium. A method for preparing an active metal welded ceramic substrate as described in claim 1, wherein the temperature of the high temperature hot pressure brazing is 750~950℃.

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