Method for manufacturing a submount and method for manufacturing a semiconductor device using the same

Abstract

To provide a manufacturing method of a submount, having an excellent adhesion of a metallic thin film.SOLUTION: A manufacturing method of a submount having a metallic thin film on a front surface of a ceramic substrate, includes: a plasma ashing step of performing a plasma ashing of the front surface of the ceramic substrate; and a thin film formation step of forming the metallic thin film onto the front surface to be executed the plasma ashing. In the plasma ashing step, it is preferable that RF power is 200 W or more and 900 W or less, a plasma irradiation time is 30 seconds or more and 120 minutes or less, and an oxidation gas flow amount is 30 ccm or more and 1000 ccm or less. Also, the ceramic substrate may be an aluminum nitride substrate.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The embodiments described below relate to a method for manufacturing a submount and a method for manufacturing a semiconductor device using the same.

Background Art

[0002] Examples of the ceramic circuit board include those obtained by bonding a metal plate and those formed with a metal thin film. The one using a metal thin film is called a submount. The submount is used for mounting an optical semiconductor element. Examples of the optical semiconductor element include a light-emitting diode, a laser diode, and a photodiode. For example, Japanese Patent No. 5166017 (Patent Document 1) discloses a submount in which a metal thin film is provided on the surface of an aluminum nitride substrate. In Patent Document 1, a three-layer film of a titanium (Ti) film, a platinum (Pt) film, and a gold (Au) film is provided on the surface of the aluminum nitride substrate. Further, a solder layer is provided on the three-layer film. This solder layer uses AuSn solder or the like. As described above, the submount has a multilayer film of a metal thin film. For forming the metal thin film, a film-forming method such as vapor deposition or sputtering is used.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] When a metal thin film was provided using a film formation method, there were problems such as film peeling. In the manufacture of submounts, multiple units are taken. Taking multiple units means a method in which a plurality of metal thin films are provided on a ceramic substrate in a design that is divided in advance and then divided into arbitrary sizes. If there are problems such as film peeling, film peeling and other problems occur on the ceramic substrate provided with the metal thin film before division, so the impact on the yield was significant. When investigating the cause of film peeling, it was found that there was a problem with the ashing conditions. Ashing is a process of cleaning the surface on which film formation treatment is to be performed. For example, in Japanese Patent Application Laid-Open No. 2006-286944 (Patent Document 2), an oxygen plasma asher method is used. The ashing process using oxygen plasma is a method of reacting oxygen plasma with organic substances to remove them. Organic substances present at the location where film formation is desired can be removed, and a cleaning effect can be obtained. However, even when the ashing process using oxygen plasma was performed, a phenomenon of insufficient cleaning effect occurred. When investigating the cause, it was found that there was a problem with the oxygen gas flow rate. The present invention is for dealing with such problems, and aims to provide a method for manufacturing a submount characterized by controlling the ashing process.

Means for Solving the Problems

[0005] The method for manufacturing a submount according to an embodiment is a method for manufacturing a submount having a metal thin film on the surface of a ceramic substrate, and includes a plasma ashing step of plasma ashing the surface of the ceramic substrate, and a thin film forming step of forming a metal thin film on the plasma-ashed surface. The step of plasma ashing is characterized by being within the range of RF power of 200 W or more and 900 W or less, plasma irradiation time of 30 seconds or more and 120 minutes or less, and oxygen gas flow rate of 30 ccm or more and 1000 ccm or less.

Brief Description of the Drawings

[0006]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0007] The method for manufacturing a submount according to the embodiment is a method for manufacturing a submount having a metal thin film on the surface of a ceramic substrate. The method includes a plasma ashing step of plasma ashing the surface of the ceramic substrate and a thin film forming step of forming a metal thin film on the plasma-ashed surface. The plasma ashing step is characterized in that the RF power is in the range of 200 W or more and 900 W or less, the plasma irradiation time is in the range of 30 seconds or more and 120 minutes or less, and the oxygen gas flow rate is in the range of 30 ccm or more and 1000 ccm or less. An example of a submount is shown in FIG. 1. In the figure, 1 is the submount, 2 is the ceramic substrate, 3 is the Ti film, 4 is the Pt film, 5 is the Au film, and 6 is the solder layer. In FIG. 1, a three-layer structure of the Ti film 3 / Pt film 4 / Au film 5 is illustrated, but it is not limited thereto. Also, an example in which metal films are provided on both sides of the ceramic substrate 2 is shown, but a structure in which a metal film is provided on only one side may also be acceptable. The submount 1 has a metal thin film provided on the surface of the ceramic substrate 2. The metal thin film preferably has one or more selected from titanium (Ti), platinum (Pt), and gold (Au). Also, it preferably has a three-layer structure of the Ti film 3 / Pt film 4 / Au film 5. The Ti film 3 is a material with good adhesion to the ceramic substrate 2. Also, the Au film 5 is a material with good adhesion to the solder layer 6, and due to its characteristics, it is also excellent as a mounting property and a conductor film. Also, since the Au film 5 is difficult to oxidize, it is a material that serves as a protective film. In particular, it is a material with good adhesion to AuSn solder. Also, the Pt film 5 is an effective material as a barrier layer to avoid the mutual diffusion of the Ti film 3 and the Au film 5 due to the thermal history during mounting and to avoid an adverse effect on the mounting property. Also, the Pt film 5 is a material with good adhesion to the Ti film 3 and the Au film 5. For this reason, it is preferable to have a three-layer structure of the Ti film 3 / Pt film 4 / Au film 5.

[0008] Also, the solder layer 6 preferably has at least an AuSn layer. The AuSn solder can melt Sn and react with the Au film 5 to achieve bonding. Also, the AuSn solder has a high melting point of about 280°C. Therefore, even if a semiconductor element such as an optical semiconductor element generates heat, it is possible to suppress the AuSn solder layer from starting to melt. Also, the solder layer 6 may be a multilayer of AuSn layers with different compositions. Also, the solder layer 6 may have a structure in which an AuSn solder and a metal film are laminated. Examples of the metal film when laminating the AuSn solder and the metal film include a Pt film and an Au film.

[0009] Also, the ceramic substrate 2 is preferably one selected from an aluminum nitride substrate, a silicon nitride substrate, an aluminum oxide substrate, an aldyl substrate, a sapphire substrate, and a diamond substrate. The aldyl substrate is a substrate in which aluminum oxide and zirconium oxide are mixed. Also, the ceramic substrate 2 preferably has a thickness in the range of 0.2 mm or more and 1 mm or less. Also, the ceramic substrate 2 may have internal wiring. Also, the ceramic substrate 2 may have a multilayer structure equipped with internal wiring. The internal wiring is a conductive structure using through-holes or the like. The aluminum nitride substrate has a high thermal conductivity of 160 to 260 W / m·K, but the three-point bending strength is about 300 to 550 MPa. Also, the silicon nitride substrate has a thermal conductivity of about 80 to 120 W / m·K, but the three-point bending strength is as high as 600 MPa or more. The aluminum nitride substrate and the silicon nitride substrate are called nitride-based ceramic substrates. Also, the aluminum oxide substrate has a thermal conductivity of 20 to 40 W / m·K and a three-point bending strength of about 400 to 500 MPa. The aldyl substrate has a three-point bending strength of about 500 to 600 MPa. The aluminum oxide substrate and the aldyl substrate are called oxide-based ceramic substrates. Among ceramic substrates, an aluminum nitride substrate is preferred. Since the aluminum nitride substrate has a high thermal conductivity, it has good heat dissipation performance. Also, the submount may sometimes be manufactured by taking a large number of them. The ceramic substrate provided with a metal thin film is cut so that the long side is 0.5 mm or more and 10 mm or less, and processed into a small submount. Since the strength of the aluminum nitride substrate is about 300 to 550 MPa, it is easy to perform cutting processing. Moreover, since the thermal conductivity is as high as 160 W / m·K or more, the heat dissipation performance is also good.

[0010] Also, it is preferable that the surface roughness Ra of the ceramic substrate 2 is 0.5 μm or less. The surface on which the metal thin film is provided is preferably a flat surface. For this reason, it is preferable that the surface roughness Ra of the ceramic substrate 2 is 0.5 μm or less, and further preferably 0.1 μm or less. For this reason, it is most preferable to use an aluminum nitride substrate having a surface roughness Ra of 0.5 μm or less. Note that the surface roughness Ra shall be measured by a method in accordance with JIS-B-0601 (2013).

[0011] The manufacturing method of the submount according to the embodiment has a plasma ashing step of plasma ashing the surface of the ceramic substrate and a thin film forming step of forming a metal thin film on the plasma-ashed surface. The process flow of the manufacturing method of the submount is illustrated in FIG. 2. FIG. 2 is in the order of the first ashing step → lithography step → metal thin film forming step on the front surface → metal thin film forming step on the back surface → annealing step → lithography step → second ashing step → solder layer forming step on the front surface → third ashing step → solder layer forming step on the back surface. The manufacturing method of the submount according to the embodiment is not limited to this process order. If necessary, an inspection step, a water washing step, a dicing step, or the like may be performed. Also, the notations of the front surface and the back surface are based on the surface on which the film is first formed as the front surface and the opposite surface as the back surface. Similarly, the surface on which the solder layer is first formed is the front surface, and the opposite surface is the back surface. First, it has a plasma ashing process for plasma-ashing the surface of the ceramic substrate. The plasma ashing process is characterized by an RF power of 200 W or more and 900 W or less, a plasma irradiation time of 30 seconds or more and 120 minutes or less, and an oxygen gas flow rate of 30 ccm or more and 1000 ccm or less.

[0012] Ashing is a process for cleaning the surface on which a film formation process is to be performed. When film formation processes are performed on both sides of the ceramic substrate, ashing processes are to be performed on both sides. Also, plasma ashing is an ashing process using plasma. The plasma may be one generated by non-ionizing radiation of oxygen gas. Non-ionizing radiation includes microwaves, visible light, and the like.

[0013] The plasma ashing process for plasma-ashing the surface of the ceramic substrate is called the first ashing process. The first ashing process has the effect of cleaning the surface of the ceramic substrate. On the surface of the ceramic substrate, there are attached organic substances in the atmosphere, organic substances during polishing for surface smoothing, and the like. If film formation is performed with the organic substances remaining, it will cause film formation defects. Therefore, it is effective to perform an ashing process on the surface of the ceramic substrate. The ashing process may be performed at the location where the metal thin film of the ceramic substrate is provided. Also, when a large number are processed, it is preferable to perform the ashing process on the entire surface of the ceramic substrate. The first ashing process is characterized by an RF power of 200 W or more and 900 W or less, a plasma irradiation time of 30 seconds or more and 120 minutes or less, and an oxygen gas flow rate of 30 ccm or more and 1000 ccm or less. RF power refers to the power (W, watt) of an RF device for generating an RF (Radio Frequency) electric field. When the RF power is within the range of 200 W or more and 900 W or less, the surface of the ceramic substrate can be cleaned without being damaged. If the RF power is less than 200 W, the cleaning effect is insufficient. Also, if it exceeds 900 W, not only is there no further effect, but it may also have an adverse impact on the surface of the ceramic substrate. Therefore, the RF power is preferably within the range of 200 W or more and 900 W or less, and more preferably within the range of 250 W or more and 700 W or less.

[0014] Also, the plasma irradiation time is within the range of 30 seconds or more and 120 minutes or less. The plasma irradiation time refers to the time during which the surface of the ceramic substrate is irradiated with plasma. It is the time during which plasma is generated in the plasma ashing processing chamber. By having the plasma irradiation time within the range of 30 seconds or more and 120 minutes or less, a cleaning effect can be obtained. If the plasma processing time is less than 30 seconds, the cleaning effect is insufficient. Also, if the plasma processing time exceeds 120 minutes, not only is no further effect obtained, but it also becomes a factor in increasing costs. Therefore, the plasma irradiation time is preferably within the range of 30 seconds or more and 120 minutes or less, and more preferably within the range of 40 seconds or more and 100 minutes or less.

[0015] In addition, the oxygen gas flow rate is in the range of 30 ccm or more and 1000 ccm or less. The unit of flow rate, ccm, means cc (cm3) / min. The oxygen gas flow rate is the flow rate of the oxygen gas supplied into the ashing processing chamber. The oxygen gas is for generating oxygen plasma. The amount of generated oxygen plasma changes with the supply amount of oxygen. Even when RF power is applied, if the oxygen gas is insufficient, the amount of generated oxygen plasma will decrease. If the oxygen gas flow rate is less than 30 ccm, the amount of oxygen gas is insufficient. Also, if it exceeds 1000 ccm, not only will no further effect be obtained, but it will also be a factor in cost increase. In addition, the efficiency of surface cleaning will decrease. Also, to efficiently generate plasma, it is necessary to control the inside of the ashing processing chamber to a predetermined vacuum degree. If the oxygen gas flow rate is large, it may have an adverse effect on the predetermined vacuum degree inside the ashing processing chamber. Therefore, the oxygen gas flow rate is preferably in the range of 30 ccm or more and 1000 ccm or less, and more preferably in the range of 50 ccm or more and 700 ccm or less. In addition, the oxygen gas is preferably in the range of 99.9 vol% or more and 100 vol% or less of oxygen. To efficiently generate oxygen plasma, it is preferable that the purity of the oxygen gas is high. Therefore, the purity of the oxygen gas is preferably in the range of 99.9 vol% or more and 100 vol% or less, and more preferably in the range of 99.99 vol% or more and 100 vol% or less.

[0016] Also, the plasma ashing process is preferably performed at 180°C or lower. Performing it at 180°C or lower means performing it within the range from normal temperature to 180°C or lower. To enhance the effect of plasma ashing, it is also effective to form an oxide film on the surface of the ceramic substrate. Also, by providing an oxide film, the chemical resistance of the surface of the ceramic substrate can be improved. For example, in the resist removal process, an alkaline chemical solution may be used. The surface of the ceramic substrate may become rough with the alkaline chemical solution. Therefore, it is assumed that an oxide film may be provided. When applying an oxide film, it is preferable to perform the plasma ashing process within the range of 80°C or higher and 180°C or lower. Also, when an oxide film is provided on the surface of the ceramic substrate, the adhesion with the Ti film is improved. Further, when it is not necessary to provide an oxide film, it is preferably within the range from room temperature to less than 80°C. In addition, if the plasma ashing process is performed at a temperature exceeding 180°C, the ceramic substrate may be altered. For example, when treating an aluminum nitride substrate at a temperature exceeding 180°C, the oxidation treatment may proceed excessively and the thermal conductivity may decrease. Therefore, it is preferable to perform the plasma ashing process at 180°C or lower.

[0017] Also, the plasma ashing process is preferably performed in a vacuum. Atmospheric pressure is 1.01×10 5 Pa (=1 atm). Vacuum indicates a pressure less than atmospheric pressure (1.01×10 5 Pa). Also, it is preferably within the range of 20 Pa or higher and 800 Pa or lower in a vacuum. 20 - 800 Pa is a region called low vacuum and medium vacuum. By performing it in a vacuum, the generation amount of oxygen plasma can be controlled. If it is in a vacuum less than 20 Pa or exceeding 800 Pa, the generation of oxygen plasma may not be stable. Therefore, the degree of vacuum in the plasma ashing process is preferably within the range of 20 Pa or higher and 800 Pa or lower, and more preferably within the range of 30 Pa or higher and 650 Pa or lower.

[0018] By performing the first ashing process as described above, the surface of the ceramic substrate can be cleaned. Also, after the first ashing process, if necessary, a cleaning process using a liquid may be performed. The cleaning process using a liquid has the effect of removing dust on the surface cleaned in the first ashing process. The liquid is preferably an organic substance such as acetone. Also, a method of immersing in a liquid cleaning tank for 5 minutes or more and 60 minutes or less can be mentioned. Also, a method of immersing in a plurality of liquid cleaning tanks may be performed. By using a plurality of liquid cleaning tanks, the influence of dust remaining in the cleaning tank can be suppressed. Also, the cleaning process using a liquid may be performed while applying ultrasonic waves. By performing while applying ultrasonic waves, it becomes easier to remove dust.

[0019] Next, a thin film forming process is performed. Also, the thin film forming process is preferably a process of forming a single-layer film or a multi-layer film mainly composed of one selected from titanium, platinum, and gold. Examples of the thin film forming process include PVD methods such as evaporation method, sputtering method, and ion plating method. Among these, the evaporation method is preferably used. The evaporation method is a method of evaporating a material to form a film. Also, the evaporation process is preferably vacuum evaporation performed in a vacuum of 10 -3 Pa or less. When it is the vacuum evaporation method, the evaporation temperature of the material can be lowered. Also, it has the effect of removing unnecessary gases in the film formation chamber. Also, the thin film forming process shall be performed in the order of Ti film, Pt film, and Au film. Also, the thickness of each film shall be in the range of 0.05 μm or more and 10 μm or less. Also, the thin film forming process shall be applied to both the front and back surfaces of the ceramic substrate. It may be possible to form a film from the front surface or from the back surface.

[0020] Also, the thin film forming process may include a resist coating process for patterning. The surface on which the semiconductor element is mounted needs to pattern the thin film. Therefore, a resist is applied to the locations where it is not desired to form the thin film. The applied resist is hardened by heating or exposure. This heating process and exposure treatment are also included in the resist coating process. After the resist coating process, the thin film forming process may be performed. Also, the resist coating process is called lithography. Also, the surface on which the resist coating process is performed may be called the lithography surface. Also, after the thin film forming process, a process of removing the resist is performed as needed. Examples of the method of removing the resist include using a chemical solution or the like. The process of removing the resist is called lift-off. Patterning is performed by lithography → thin film forming process → lift-off. In lithography → thin film forming process → lift-off, the ashing process may be performed multiple times. Note that the process of ashing the surface of the ceramic substrate 2 is called the first ashing process. Also, the process of ashing the metal thin film on the surface side, which will be described later, is called the second ashing process. Also, the process of ashing the metal thin film on the back side, which will be described later, is called the third ashing process. When patterning the metal thin film, the ashing process may be performed multiple times. Even if the ashing process is performed multiple times, the ashing process for the metal thin film on the surface side is the second ashing process. Similarly, even if the ashing process is performed multiple times, the ashing process for the metal thin film on the back side is the third ashing process. Also, after the thin film forming process, an annealing process may be performed as needed. By performing the annealing process, the adhesion of the Ti film / Pt film / Au film can be improved. Next, a resist coating process for patterning is performed on the surface of the Ti film / Pt film / Au film as necessary. A solder layer is formed on the surface of the Ti film / Pt film / Au film. The solder layer is formed at the location where the semiconductor element is to be mounted. By performing the resist coating process, the location where the solder layer is to be formed can be patterned. After applying the resist, heat treatment or exposure treatment is to be performed. This heat treatment and exposure treatment are also to be included in the resist coating process. Also, when patterning is required on both sides, the resist coating process is to be performed on both sides.

[0021] Next, the metal thin film provided by the thin film forming process may have a second plasma ashing process and a process of providing a solder layer mainly composed of one or two of gold or tin on the surface of the metal thin film after the second plasma ashing process. Also, on the surface opposite to the surface where the second plasma ashing process is performed, there may be a third plasma ashing process and a process of providing a solder layer mainly composed of one or two of gold or tin on the surface of the metal thin film after the third plasma ashing process. The ashing process on the metal film on the surface side is called the second ashing process, and the ashing process on the metal thin film on the back side is called the third ashing process. For convenience, the ashing process on the metal thin film on the surface side is performed first, but the ashing process on the metal thin film on the back side may also be performed first. When the ashing process on the metal thin film on the back side is performed first, it will be performed in the order of the third ashing process → the second ashing process. Also, the second plasma ashing process is preferably performed within the range of RF power of 200 W or more and 900 W or less, plasma irradiation time of 30 seconds or more and 120 minutes or less, and oxygen gas flow rate of 30 ccm or more and 1000 ccm or less.

[0022] When the RF power is in the range of 200 W or more and 900 W or less, a cleaning effect can be obtained without damaging the metal thin film. If the RF power is less than 200 W, the cleaning effect may be insufficient. Also, if it is too high, exceeding 900 W, not only will there be no further effect, but it may also have an adverse impact on the surface of the ceramic substrate. In addition, when the RF power exceeds 900 W, the temperature of the ceramic substrate rises. When the temperature of the ceramic substrate rises, there is a possibility that the AuSn solder layer 6 described later and the Au film (antioxidant layer) on its surface will react. Then, the Au film may lose its antioxidant effect. For this reason, the RF power is preferably in the range of 200 W or more and 900 W or less, and more preferably in the range of 250 W or more and 700 W or less. Also, the plasma irradiation time is preferably in the range of 30 seconds or more and 120 minutes or less, and more preferably in the range of 40 seconds or more and 100 minutes or less. Also, the oxygen gas flow rate is preferably in the range of 30 ccm or more and 1000 ccm or less, and more preferably in the range of 50 ccm or more and 700 ccm or less. The conditions of the RF power, plasma irradiation time, and oxygen gas flow rate are the same as those in the first ashing process. Also, when ashing the surface coated with the resist (lithography surface), it is preferable to keep the RF power low and shorten the plasma irradiation time. Specifically, it is preferable to set the RF power in the range of 200 W or more and 350 W or less. Also, it is preferable to set the plasma irradiation time in the range of 30 seconds or more and 5 minutes or less. This is to prevent the resist from being removed or the pattern shape from being deformed by the ashing process. If the RF power exceeds 350 W or the plasma irradiation time exceeds 3 minutes, the resist may be removed. Also, in the second ashing process, the purity of the oxygen gas is preferably in the range of 99.9 vol% or more and 100 vol% or less, and more preferably in the range of 99.99 vol% or more and 100 vol% or less. Also, for the second plasma ashing process, it is preferably performed at 180°C or lower. Also, for the second plasma ashing process, it is preferably performed in a vacuum. Also, the vacuum is preferably in the range of 20 Pa or more and 800 Pa or less.

[0023] Also, a second ashing process is performed to provide a solder layer on the cleaned metal thin film. The solder layer is mainly composed of one or two of gold and tin. Further, the solder layer is preferably AuSn solder. By providing an AuSn solder layer 6 on the Au film 5, the bondability is improved. Further, by performing a second plasma ashing process, dust on the Au film 5 can be removed. In particular, a resist coating process for patterning may be performed on the surface side. By performing the second ashing process, the residue of the resist can be removed. Also, the solder layer 6 is mainly composed of one or two of gold and tin. The main component means that it occupies 50% by mass or more and 100% by mass or less of the solder layer. Further, in the case of an AuSn alloy, it is preferable that the total of AuSn is in the range of 50% by mass or more and 100% by mass or less. Also, the solder layer 6 may be a laminated film of an Au layer and a Sn layer. Further, AuSn layers having different composition ratios may be laminated. Also, a Pt layer or an Au layer may be provided between the AuSn layers. Also, a Pt layer may be provided between the metal thin film and AuSn. Also, if necessary, an Au film may be provided on the surface of the solder layer 6. The Au film on the surface of the solder layer 6 has an antioxidant effect. When the solder layer 6 is an AuSn alloy, the Sn component is easily oxidized. By providing an Au film, an antioxidant effect can be obtained. When the solder layer 6 is oxidized, it causes poor bonding when mounting the semiconductor element.

[0024] Also, the third plasma ashing process is preferably performed within the range of RF power of 200 W or more and 900 W or less, plasma irradiation time of 30 seconds or more and 120 minutes or less, and oxygen gas flow rate of 30 ccm or more and 1000 ccm or less. When the RF power is in the range of 200 W or more and 900 W or less, a cleaning effect can be obtained without damaging the metal thin film. If the RF power is less than 200 W, the cleaning effect may be insufficient. Also, if it is too high exceeding 900 W, the metal thin film may be damaged. Further, when the RF power exceeds 900 W, the temperature of the ceramic substrate becomes high. When the temperature of the ceramic substrate becomes high, there is a possibility that the AuSn solder layer 6 described later and the Au film (antioxidant layer) on its surface may react. Then, the Au film may lose its antioxidant effect. For this reason, the RF power is preferably in the range of 200 W or more and 900 W or less, and more preferably in the range of 250 W or more and 700 W or less. Also, the plasma irradiation time is preferably in the range of 30 seconds or more and 120 minutes or less, and more preferably in the range of 40 seconds or more and 100 minutes or less. Also, the oxygen gas flow rate is preferably in the range of 30 ccm or more and 1000 ccm or less, and more preferably in the range of 50 ccm or more and 700 ccm or less. The conditions of the RF power, plasma irradiation time, and oxygen gas flow rate are the same as those in the first ashing process. Also, in the third ashing process, when performing the ashing process on the surface coated with the resist (lithography surface), it is preferable to lower the RF power and shorten the plasma irradiation time. Specifically, the RF power is preferably in the range of 200 W or more and 350 W or less. Also, the plasma irradiation time is preferably in the range of 30 seconds or more and 5 minutes or less. This is to prevent the resist from being removed by the ashing process. If the RF power exceeds 350 W or the plasma irradiation time exceeds 3 minutes, the resist may be removed. Also, in the third ashing process, the purity of the oxygen gas is preferably in the range of 99.9 vol% or more and 100 vol% or less, and more preferably in the range of 99.99 vol% or more and 100 vol% or less. Also, for the third plasma ashing process, it is preferably performed at 180°C or lower. Also, for the third plasma ashing process, it is preferably performed in a vacuum. Also, the vacuum is preferably in the range of 20 Pa or more and 800 Pa or less.

[0025] Also, a third soldering process is performed to provide a solder layer on the cleaned metal thin film. The solder layer is mainly composed of one or two of gold and tin. Also, the solder layer is preferably AuSn solder. By providing an AuSn solder layer 6 on the Au film 5, the bonding property is improved. Further, by performing a third plasma ashing process, dust on the Au film 5 can be removed. In particular, a resist coating process for patterning may be performed on the surface side. By performing the third ashing process, the residue of the resist can be removed. Also, the solder layer 6 is mainly composed of one or two of gold and tin. The main component occupies 50% by mass or more and 100% by mass or less of the solder layer. Also, in the case of an AuSn alloy, it is preferable that the total of AuSn is within the range of 50% by mass or more and 100% by mass or less. Also, the solder layer 6 may be a laminated film of an Au layer and a Sn layer. Also, AuSn layers with different composition ratios may be laminated. Also, a Pt layer may be provided between the AuSn layers. Also, if necessary, an Au film may be provided on the surface of the solder layer 6. The Au film on the surface of the solder layer 6 has an antioxidant effect. When the solder layer 6 is an AuSn alloy, the Sn component is easily oxidized. By providing an Au film, an antioxidant effect can be obtained.

[0026] Through the above steps, a submount with a metal film and a solder layer provided on both sides of a ceramic substrate can be manufactured. Also, the metal thin film and the solder layer may have a structure provided only on one side. In this case, the third plasma ashing process may not be performed. Also, dicing processing shall be performed as necessary. The dicing process is a process of cutting the ceramic substrate into small pieces. It shall be cut into the size required for the submount. For example, it can be cut to a size of 5 mm or less on each side. Obtaining many submounts by cutting into small sizes is called multi-taking. By performing multi-taking, mass productivity is improved. Also, the dicing process includes methods using a cutting blade and laser processing, etc. The submount according to the embodiment controls the ashing process, so it has excellent adhesion to the metal thin film and solder layer. Therefore, even if dicing processing is performed, film peeling can be suppressed. Through the above processes, a submount can be manufactured.

[0027] Also, a semiconductor device can be manufactured by performing a process of mounting a semiconductor element on the submount obtained by the above method for manufacturing a submount. The process of mounting the semiconductor element may be before or after the dicing process. Also, the semiconductor element may be an optoelectronic semiconductor element. Examples of optoelectronic semiconductor elements include light-emitting diodes, laser diodes, photodiodes, etc. Optoelectronic semiconductor elements are often small elements about 1 to 3 mm in size. By making the submount for mounting the optoelectronic semiconductor element also small with a size of 5 mm or less on each side, space can be saved. Also, optoelectronic semiconductor elements generate heat when emitting light. By using an aluminum nitride substrate with a thermal conductivity of 160 W / m·K or more as the ceramic substrate, heat dissipation is also good. Since the bonding strength of the metal thin film is improved, the bonding property of the optoelectronic semiconductor element is also good. Therefore, the yield of the semiconductor device is also improved.

[0028] (Example) (Examples 1 to 5, Comparative Examples 1 to 2) As a ceramic substrate, an aluminum nitride substrate was prepared. The aluminum nitride substrate used had a thermal conductivity of 180 W / m·K and a three-point bending strength of 400 MPa. Also, the aluminum nitride substrate used had dimensions of 50 mm in length × 50 mm in width × 0.3 mm in thickness. Further, the aluminum nitride substrate used had a surface roughness Ra of 0.05 μm or less. A metal thin film having a three-layer structure of a Ti film, a Pt film, and an Au film was provided on the aluminum nitride substrate. The metal thin film was provided on both sides of the aluminum nitride substrate. The film thicknesses of the Ti film, the Pt film, and the Au film were unified to 0.1 μm each. Also, an AuSn solder layer was provided on the metal thin film on both sides. The film thickness of the AuSn solder layer was unified to 2 μm.

[0029] The following steps were performed: aluminum nitride substrate → first ashing process → process of depositing the Ti film, the Pt film, and the Au film → second ashing process (front side) → process of depositing the AuSn solder layer (front side) → third ashing process (back side) → process of depositing the AuSn solder layer (back side). Note that the second ashing process (front side) is the ashing process for the surface on which the semiconductor element is to be mounted. The surface on which the semiconductor element is to be mounted may be coated with a resist to impart a pattern shape as necessary. Also, a water washing process may be performed as necessary. The first ashing process, the second ashing process, and the third ashing process were performed under the conditions shown in the table. Also, the ashing process was a plasma ashing process using oxygen plasma. Also, the oxygen gas used had an oxygen purity of 99.9 vol% or more and 100 vol% or less. Also, when the temperature is "room temperature", it indicates a state where temperature control is not performed. Also, when the pressure is "atmospheric pressure", it indicates a state where pressure control is not performed.

[0030]

Table 1

[0031] After performing the first ashing process, the steps of depositing a Ti film, a Pt film, and an Au film were carried out. The yield when forming the metal thin film was measured. The yield was measured by measuring the bonding strength of the metal thin film. The bonding strength was measured by a tape test. The tape test was carried out using a tape corresponding to 80% of the area of the metal thin film. The tape was attached to the center of the metal thin film. When the tape was peeled off, those in which the metal thin film peeled off by 5% or more in area ratio were identified as "defective products". 100 tests were carried out for each, and the defect occurrence rate was obtained. The results are shown in Table 2.

[0032]

Table 2

[0033] Next, the step of forming a solder layer on the metal thin film was carried out. The ashing process on the metal thin film on the surface side was defined as the second ashing process, and the ashing process on the metal thin film on the back side was defined as the third ashing process. Also, a lithography process was carried out on the surface side. That is, pattern formation was carried out on the surface side. Also, the solder layer was made of an AuSn alloy. Also, an Au film was provided as an antioxidant film on the solder layer. The conditions are as shown in the table.

[0034]

Table 3

[0035]

Table 4

[0036] Through the above steps, a submount was manufactured. For the obtained submount, the bonding strength of the solder layer was measured. The bonding strength was measured by a tape test. For the solder layer, when the tape was attached and peeled off, those in which the solder layer peeled off by 5% or more in area ratio were regarded as "defective products". 100 submounts were manufactured for each, and the defect occurrence rate was measured. The results are shown in Table 5.

[0037]

Table 5

[0038] Next, the obtained submount was diced into a size of 5 mm in length and 5 mm in width. A dicing machine (rotary blade) was used for the dicing process. The rate of film peeling during the dicing process was measured. The results are shown in Table 6.

[0039]

Table 6

[0040] As can be seen from the table, it was found that the submount according to the example can suppress the occurrence of film peeling during the dicing process. It can be seen that the adhesion of the metal thin film and the solder layer is excellent. On the other hand, in Comparative Example 1, since the ashing process (first ashing process) on the ceramic substrate was not performed, the occurrence of film peeling increased. Also, as in Comparative Example 2, when the conditions of plasma ashing were different, the occurrence of film peeling also increased. It was confirmed that the method of the embodiment is effective for improving the yield.

[0041] As described above, several embodiments of the present invention have been illustrated. However, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, changes, etc. can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are included in the invention described in the claims and its equivalent scope. Also, the above-described embodiments can be implemented in combination with each other.

Explanation of Reference Numerals

[0042] 1... Submount 2... Ceramic substrate 3... Ti film 4... Pt film 5... Au film 6... Solder layer

Claims

1. In a method for manufacturing a submount having a metal thin film on the surface of a ceramic substrate, a first ashing step of plasma ashing the surface of the ceramic substrate, and a metal thin film step of having a metal thin film on the surface of the ceramic substrate that has undergone the first ashing step, wherein the first ashing step is within a range of RF power of 200 W or more and 900 W or less, plasma irradiation time of 8 minutes or more and 120 minutes or less, and oxygen gas flow rate of 30 ccm or more and 1000 ccm or less. A method for manufacturing a submount, characterized in that.

2. The method for manufacturing a submount according to claim 1, wherein the ceramic substrate is an aluminum nitride substrate having a surface roughness Ra of 0.5 μm or less.

3. The method for manufacturing a submount according to any one of claims 1 to 2, wherein the oxygen gas is within a range of 99.9 vol% or more and 100 vol% or less of oxygen.

4. The method for manufacturing a submount according to any one of claims 1 to 3, wherein the plasma ashing step is performed at 180 °C or lower.

5. The method for manufacturing a submount according to any one of claims 1 to 4, wherein the plasma ashing step is performed at normal pressure or in a vacuum.

6. The method for manufacturing a submount according to any one of claims 1 to 5, wherein the thin film forming step is a step of forming a single-layer film or a multilayer film mainly composed of one selected from titanium, platinum, and gold.

7. The method for manufacturing a submount according to any one of claims 1 to 6, wherein the thin film forming step has a resist coating step for patterning.

8. For the metal thin film provided by the thin film forming step, a second plasma ashing step and a step of providing a solder layer mainly composed of one or two of gold and tin on the surface of the metal thin film that has undergone the second plasma ashing step, The step of performing the second plasma ashing is performed within a range of RF power of 200 W or more and 900 W or less, plasma irradiation time of 30 seconds or more and 120 minutes or less, and oxygen gas flow rate of 30 ccm or more and 1000 ccm or less. A method for manufacturing a submount according to any one of claims 1 to 7, characterized in that.

9. A third plasma ashing process is performed on the surface opposite to the surface where the second plasma ashing process has been performed, and a step of providing a solder layer mainly composed of one or two of gold or tin is provided on the surface of the metal thin film where the third plasma ashing process has been performed. The method for manufacturing a submount according to claim 8, wherein the step of performing the third plasma ashing is performed within a range of an RF power of 200 W or more and 900 W or less, a plasma irradiation time of 30 seconds or more and 120 minutes or less, and an oxygen gas flow rate of 30 ccm or more and 1000 ccm or less.

10. The method for manufacturing a submount according to any one of claims 1 to 9, wherein the ashing process after lithography is the second ashing process or the third ashing process.

11. The method for manufacturing a submount according to any one of claims 1 to 10, wherein the ashing process on the lithography surface is within a range of an RF power of 200 W or more and 350 W or less, a plasma irradiation time of 30 seconds or more and 5 minutes or less, and an oxygen gas flow rate of 30 ccm or more and 1000 ccm or less.

12. A method for manufacturing a semiconductor device, comprising a step of mounting a semiconductor element on a submount obtained by the method for manufacturing a submount according to any one of claims 1 to 11.

Images