Surface treatment method

Abstract

To provide a surface treatment method.SOLUTION: Disclosed herein is a surface treatment method. Atmospheric plasma is provided, and an object to be treated is placed in the atmospheric plasma atmosphere, where a residual oxygen level in the atmospheric plasma atmosphere is lower than a prescribed value. A soldering process is integrated into an atmospheric plasma treatment system without any flux protection, and it is ensured that the oxygen level in the atmosphere is reduced to an extremely low level, which can meet the high requirements for surface soldering performance.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present application is in the field of semiconductor manufacturing. The present application relates to a surface treatment method, and in particular to a method for cleaning a surface by using plasma. [Background technology]

[0002] The manufacturing process of semiconductor devices requires the use of soldering techniques. The common objects processed are often some metal materials, and these metal materials are easily oxidized in the presence of oxygen. The oxide film formed after oxidation can lead to joint defects such as poor wetting and ductility, poor bonding performance and increased voids. Furthermore, when the metal materials melt under high temperature conditions, the oxide film will become thicker. The thickened oxide film will mix between the joined objects after bonding, causing poor conductivity.

[0003] At present, the commonly used soldering process is flux soldering. Although the flux can reduce the surface tension of the workpiece and remove the oxide film on the surface of the workpiece, various types of flux still have unavoidable drawbacks. One drawback of flux soldering is the formation of corrosive flux residue around the solder spot, which can lead to long-term reliability issues and is disadvantageous in terms of heat exchange. Another drawback of flux soldering is the problem of internal voids. Due to the evaporation and vaporization of the flux, internal voids are almost inevitable. In order to reduce internal voids and flux residues, a vacuum reflow oven is used to suck out the flux residues. However, the vacuum process has the following drawbacks: first, some parts will degas excessively in the vacuum, so the target vacuum pressure needs to be reached very slowly. Second, heat cannot be transferred in vacuum via convection, but only via infrared radiation or thermal conduction. Third, the price of vacuum equipment is very expensive. For example, Torch Technology Co., Ltd. estimates that vacuum equipment costs six to seven times more than non-vacuum equipment.

[0004] The treatment of metal surfaces is especially important because soldering requires high soldering activity on the metal surfaces. Take the insulated gate bipolar transistor (IGBT) as an example, which is widely used in many high power scenarios, such as wind power generation, high speed trains, electric vehicles, ships, etc. The trend in IGBT applications is toward higher power density, higher switching frequency, and smaller volume. Therefore, the reliability of IGBTs is very important. Moreover, voids that inevitably appear during the soldering process will seriously affect the reliability of IGBTs during operation. A typical IGBT package module is shown in FIG. 1. In the figure, 101 represents the main solder layer, 102 represents the DBC, 103 represents the secondary solder layer, 104 represents the insulating layer, 105 represents the bonding copper, and 106 represents the heat dissipation substrate. The main function of the heat dissipation substrate is to quickly transfer the heat generated during the IGBT switching process. The heat dissipation substrate is usually made of copper. The thickness of the heat dissipation board is usually 3-8mm. The main function of direct bond copper (DBC) is to ensure the electrical insulation, heat conduction and current transfer ability between the chip and the heat dissipation board. The soldering between the chip and DBC is called the primary soldering. Due to the further requirement of heat dissipation, the DBC will be soldered directly onto the heat dissipation board, which is called the secondary soldering. The secondary soldering has a larger soldering area than the primary soldering, and therefore more voids may be generated. For some extremely demanding industries such as new energy field, the individual voids of the IGBT should be less than 1%, and the overall voids should be less than 1.5%. It can be seen that ensuring a stable and low voids is an urgent requirement of IGBT module packaging.

[0005] In this respect, how to design a new surface treatment method to eliminate the above-mentioned defects and shortcomings in the prior art and to realize the high requirements of soldering quality in some technical fields is an urgent problem to be solved by those skilled in the art. Summary of the Invention [Means for solving the problem]

[0006] This application relates to a surface treatment method, and in particular to an improved method for cleaning metal surfaces without the use of fluxes and organic solderability preservatives (OSPs) so that the treated surfaces meet reliability requirements after soldering. The improved method of this application includes using atmospheric pressure plasma to treat the metal surfaces.

[0007] In order to achieve the above-mentioned object of the present invention, a surface treatment method is disclosed in the present application, which includes the following steps: providing atmospheric pressure plasma, and placing a workpiece in the atmospheric pressure plasma atmosphere, where a residual oxygen level in the atmospheric pressure plasma atmosphere is lower than a predetermined value.

[0008] According to another aspect of the present application, the atmospheric pressure plasma is generated by supplying argon, helium, hydrogen, nitrogen, or a combination thereof.

[0009] According to another aspect of the present application, the atmospheric pressure plasma is generated by supplying argon and hydrogen, or nitrogen and hydrogen.

[0010] According to another aspect of the present application, hydrogen occupies between 1% and 10% of the volume of the atmospheric pressure plasma, and preferably hydrogen occupies between 1% and 4% of the volume of the atmospheric pressure plasma.

[0011] According to another aspect of the present application, the predetermined value is 200 ppm, preferably 100 ppm, more preferably 50 ppm, and most preferably 10 ppm.

[0012] According to another aspect of the present application, the surface of the workpiece is not coated with an organic solderability preservative.

[0013] According to another aspect of the present application, the surface treatment method further includes the step of placing the workpiece into a soldering system after the atmospheric plasma treatment: wherein no flux is used in the soldering system.

[0014] According to another embodiment of the present application, the atmospheric pressure plasma sweeps the workpiece at a speed of 10 mm / s to 300 mm / s, preferably 25 mm / s to 150 mm / s, and more preferably 25 mm / s to 100 mm / s.

[0015] According to another aspect of the present application, the treatment method causes the surface of the treated object to have a porosity of less than 1%.

[0016] According to another aspect of the present application, the atmospheric plasma is generated by an atmospheric plasma generator, and the atmospheric plasma generator includes a plasma injection torch having a series of nozzles arranged in such a way that the workpiece can be covered by the atmospheric plasma atmosphere.

[0017] According to another aspect of the present application, the atmospheric pressure plasma is projected perpendicularly or obliquely to the surface of the workpiece.

[0018] The technical solution provided by this application is particularly applicable to large area metal surface soldering in demanding fields such as IGBT, and has the following advantages: (1) It is a "green" solution that does not require the use of environmentally unfriendly and health-hazardous compounds such as fluxes and formic acid. (2) The plasma treatment does not leave any residue, avoiding reliability hazards caused by flux or formic acid residue. (3) Plasma treatment of the surface can greatly reduce porosity and significantly reduce internal porosity on metal surfaces. (4) Compared with the vacuum plasma method, the method of the present application allows the process flow to be integrated into the same assembly line without causing interruption of the assembly line. Furthermore, the vacuum plasma method requires that the DBC and the heat sink substrate are placed in a vacuum chamber in batches. This process inevitably exposes the cleaned DBC surface to oxygen atmosphere, which aggravates the surface oxidation and causes interruption of the assembly line.

[0019] The advantages and spirit of the present application may be further understood from the following detailed description of the invention and the accompanying drawings. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 shows a typical IGBT package module. [Diagram 2] FIG. 2 shows a flow chart for treating a surface with atmospheric plasma in one embodiment of the present application. [Figure 3a-3c] 3a-3c show schematic diagrams of solder spots on DBC of a plasma jet torch in one embodiment of the present application at different sweep speeds and different ROLs, respectively. [Figure 4a-4d] 4a-4d show schematic diagrams of a solder spot with voids obtained after flux soldering and atmospheric pressure plasma treatment, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Specific embodiments of the present application are described in detail below with reference to the accompanying drawings. However, it should be understood that the present application is not limited to the implementation forms described below, and the technical concepts of the present application can be implemented in combination with other well-known technologies or other technologies having the same functions as those well-known technologies.

[0022] In the description of specific embodiments which follow, a number of directional terms will be used for explanatory purposes in order to clearly illustrate the structure and manner of operation of the present application, however, it should be understood that terms such as "front", "rear", "left", "right", "outer", "inner", "outer", "inner", "axial", "radial", and the like are terms of convenience rather than terms of limitation.

[0023] In the following description of specific embodiments, it should be understood that the orientation or positional relationships indicated by terms such as "length," "width," "upper," "lower," "front," "rear," "left," "right," "vertical," "top," "bottom," "inner," and "outer" are based on the orientation or positional relationships shown in the accompanying drawings, and are intended only to facilitate and simplify the description of this application without indicating or implying that the referenced devices or elements must have a particular orientation or be constructed or operated in a particular orientation, and therefore should not be understood as limitations of this application.

[0024] In addition, the terms "first" and "second" are used only for descriptive purposes rather than restricting chronological order, quantity or importance, and should not be interpreted as indicating or implying relative importance or as implicitly defining the number of the indicated technical features, but are only intended to distinguish one technical feature from another technical feature of the present technical solution. Thus, features defined by "first" and "second" may explicitly or implicitly include one or more features. In the specification of this application, "plurality" means two or more unless expressly and specifically stated otherwise. Similarly, modifiers such as "a" appearing in this specification do not indicate a quantitative definition, but describe a technical feature that did not appear in the preceding text. Similarly, unless modified by a specific quantitative measure, nouns in this specification should be considered to include both the singular and the plural, i.e., a technical solution may include only one of the technical features in question, but may also include a plurality of the technical features. Similarly, modifiers such as "approximately" and "about" appearing before a number herein typically include the number, and the specific meaning should be understood in light of the context.

[0025] In this application, it should be understood that "at least one" means one or more, and "multiple" means more than one. The phrase "and / or" is used to describe an association relationship between related objects and indicates that three types of relationships may exist; for example, "A and / or B" may refer to three situations: only A is present, only B is present, and both A and B are present, where A and B may be singular or plural. The symbol (" / ") generally means that the preceding related object and the following related object have an "or" relationship. The phrase "at least one of the following" and the like means any combination of these items (including any combination of single or multiple items). For example, at least one of a, b, or c may mean: a, b, c, "a and b," "a and c," "b and c," or "a and b and c," where a, b, and c may be singular or plural.

[0026] The terms "unit," "member," "object," and "module" described in this specification indicate a unit used to process at least one function or operation, and may be implemented by hardware components or software components or a combination thereof.

[0027] Unless expressly indicated otherwise, each aspect or embodiment defined in this specification may be combined with any other aspect or embodiment, and in particular any preferred or advantageous feature indicated may be combined with any other preferred or advantageous feature indicated.

[0028] Explanation of terms As used herein, plasma, also referred to as electric plasma, is an ionized gaseous substance consisting of positive and negative ions generated by ionization of atoms and atomic groups after some electrons are stripped away. The motion of plasma is mainly governed by electromagnetic forces and exhibits important collective behavior. Atmospheric pressure plasma refers to electric plasma generated at or near one atmosphere. Both atmospheric pressure plasma and vacuum plasma have high energy particles, but atmospheric pressure plasma does not require the installation of a closed reaction chamber and vacuum pump system as vacuum plasma does. Thus, compared to vacuum plasma, atmospheric pressure plasma eliminates the need for a large vacuum chamber and vacuum pump, and has an absolute advantage in terms of equipment cost. The surface treatment temperature in this application may be in the range of about 25°C to about 150°C. The plasma may optionally include at least one gas selected from argon, helium, nitrogen, hydrogen, or a combination thereof. In some non-limiting implementations, hydrogen may occupy 1% to 10%, preferably about 2% to 8%, and more preferably 4% to 7% of the volume of the atmospheric pressure plasma.

[0029] As used herein, contacting the atmospheric plasma with the surface to be treated can be accomplished by any suitable means, such as using a plasma jet to sweep the surface. The sweep speed can be varied as needed to target the efficiency required for a particular process. For example, the sweep speed can be within the range of about 1 to about 500 mm / s (such as about 10 to about 300 mm / s, 25 to 300 mm / s, 25 to 250 mm / s, 25 to 150 mm / s, 50 to 150 mm / s, and 50 to 100 mm / s), including all ranges and subranges therebetween.

[0030] Similarly, the length of time that the atmospheric plasma is in contact with the surface to be treated (which may be referred to as the residence time) may vary depending on the sweep rate and the desired surface properties. As a non-limiting example, the residence time may be within the range of 1 second to several minutes (such as about 1 second to about 20 minutes, about 10 seconds to about 10 minutes, about 30 seconds to about 9 minutes, about 1 minute to about 8 minutes, about 2 minutes to about 7 minutes, about 3 minutes to about 6 minutes, or about 4 minutes to about 5 minutes, including all ranges and subranges therebetween).

[0031] As used herein, the device used to generate plasma in this application is the OpenAir TM Plasma jet torch or Atomflo TM A plasma machine can be used, specifically, the reconstructed processing parameters can be accurately estimated according to the sweep speed and the distance from the plasma torch to the surface to be processed.

[0032] As used herein, natural oxidation (NO) refers to exposing the workpiece to air at room temperature for two weeks to simulate the effects of natural oxidation of the workpiece during the transfer process.

[0033] As used herein, deep oxidation (DO) refers to placing a citric acid cleaned DBC in a nitrogen atmosphere at 250° C. and 50 ppm for 10 minutes to simulate the surface condition of the DBC after the first solder during IGBT packaging.

[0034] In one implementation, atmospheric pressure plasma is generated by supplying argon, helium, hydrogen, nitrogen, or combinations thereof (such as argon and hydrogen, nitrogen and hydrogen, helium and hydrogen, etc.).

[0035] As used herein, "clean surface," "surface cleaning," "cleaning," and the like refer to cleaning organic and inorganic contaminants on a treated surface such that more effective and stronger mechanical and electrical connections are created between metal materials.

[0036] As used herein, solder spot analysis may be performed by using X-ray inspection techniques known in the art, where X-rays penetrate the interior of the package and can directly inspect the quality of the solder spots.

[0037] The following embodiment takes the treatment of contaminants and oxides on the DBC surface during the IGBT packaging process as an example. In a nitrogen atmosphere, the embodiment uses atmospheric pressure plasma to treat the DBC surface instead of other chemical methods.

[0038] The solder used in flux soldering is Sn3O5, which has a melting point of 217°C.

[0039] The surface of DBC is usually copper, so it is easily oxidized. Organic solderability preservatives are a common protective measure in the prior art. OSP is a layer of organic copper composite film that is grown on the surface of copper by using chemical methods. OSP is cheap and has good solderability initially, but after being used for a period of time, its solderability deteriorates significantly, its life is insufficient, and OSP is vulnerable to surface discoloration, non-uniform film thickness, excessive (too thick or too thin) film thickness, and other issues. If there is a hole in the OSP directly above the copper surface, the copper surface will start to oxidize from the hole, resulting in poor assembly. Furthermore, the thicker the OSP, the better it protects the copper foil. However, it also requires a correspondingly more active flux to remove the OSP.

[0040] In this embodiment, atmospheric pressure plasma is used to treat the surface of the workpiece, and the requirements of wetting, spreadability and porosity are reached, on the premise that the surface of the DBC is not coated by any organic solderability preservatives while no flux is used.

[0041] In this embodiment, the process of surface treatment using atmospheric plasma is shown in FIG. 2. The workpiece moves toward the reflow soldering device by a conveyor device. An atmospheric plasma treatment chamber is placed in front of the reflow soldering device and can provide the atmospheric plasma atmosphere. Here, any known plasma generator (such as a plasma jet torch) can be used to provide the atmospheric plasma atmosphere. A typical plasma jet torch includes a series of nozzles arranged in such a way that it can be ensured that the area to be treated is covered by the plasma atmosphere. The arrangement of the nozzles can be freely selected by those skilled in the art.

[0042] As shown in FIG. 2, 201 and 206 represent the transport platform for placing and fixing the workpiece. The whole atmospheric plasma processing system includes transition chambers 202 and 205, atmospheric plasma processing chamber 203 and reflow soldering chamber 204. Among them, transition chambers 202 and 205 and atmospheric plasma processing chamber 203 need to use inert gas (such as nitrogen) to replace the atmosphere to maintain the required residual oxygen level. Atmospheric plasma processing chamber 203 is equipped with an atmospheric plasma generator, where the atmospheric plasma generator is connected to a gas source through a pipeline. The reflow soldering process is completed in reflow soldering chamber 204. The specific reflow soldering process can be performed according to methods well known to those skilled in the art and is not limited in this application.

[0043] The atmospheric plasma generator in the atmospheric plasma processing chamber 203 can generate any gas suitable for surface treatment (nitrogen, argon, helium, or a mixture of these with hydrogen).

[0044] It has been unexpectedly discovered that the residual oxygen level (ROL) in the entire working environment of the above atmospheric plasma processing system must be lower than a predetermined value. In previous studies, the degree to which the residual oxygen level needs to be reduced has not been fully realized. It is expected that the lower the residual oxygen level, the more ideal it is; that is, the closer the residual oxygen level is to 0 ppm, the more ideal it is. Thus, the residual oxygen level should be less than 300 ppm, preferably less than 200 ppm, more preferably less than 100 ppm, and optimally less than 50 ppm, even less than 10 ppm, or even less than 5 ppm. An oxygen analyzer is configured in the atmospheric plasma processing chamber to monitor the residual oxygen level. As an example, a fuel cell oxygen analyzer can be used.

[0045] As an example, a nitrogen atmosphere may be utilized to protect the entire working environment. Initially, the atmosphere of the working environment is replaced with nitrogen. After the ROL is reduced to about 200 ppm, the ROL may be rapidly reduced to, for example, 10 ppm or less by continuing to use nitrogen for replacement or by using hydrogen-containing plasma combustion to consume the remaining oxygen.

[0046] In this embodiment, the soldering temperature in the reflow soldering chamber is set at 250°C.

[0047] The sweep rate range of the plasma jet torch can be estimated. Too fast sweep rate may not provide enough plasma intensity, while too slow sweep rate may result in too high temperature and too few units per hour (UPH). As an example, the sweep rate of the plasma jet torch may be between 10 and 250 mm / sec. In other words, the plasma treatment time per unit area (inversely proportional to the sweep rate) should be optimized. Long-term treatment will rapidly increase the surface temperature of DBC and cause re-oxidation. However, short-term treatment cannot remove the oxide film on the DBC surface. Even a thin oxide film may cause undesirable solder failure.

[0048] Surface wetting is an indicator of whether the soldering is good or not. If there are any firmly attached contaminants (such as oxide films) on the treated surface, they will act as a barrier to the metal connection and thus hinder the surface wetting. It is difficult to directly measure the contact angle of the solder spot on the metal surface, since the solder spot may not be a regular circle. The area of ​​the solder spot will be used as follows to evaluate the surface wetting. The larger the area of ​​the solder spot and the fuller and more uniform the solder spot is, the better the surface wetting will be.

[0049] According to the process shown in Figure 2, DBC was treated with different sweep speeds and different ROL, and the solder spot images in Figures 3a, 3b, and 3c were obtained. As shown in Figure 3a, the ROL in the atmospheric plasma treatment chamber was maintained to be less than 5 ppm, and the sweep speed of the plasma injection torch was 25 mm / s. In Figure 3b, the ROL in the atmospheric plasma treatment chamber was in the range of about 130-180 ppm, and the sweep speed of the plasma injection torch was 25 mm / s. In Figure 3c, the ROL in the atmospheric plasma treatment chamber was maintained to be less than 5 ppm, and the sweep speed of the plasma injection torch was 150 mm / s, 100 mm / s, and 50 mm / s from left to right, respectively.

[0050] A lower sweep rate means a longer treatment time and a higher likelihood of achieving good surface wetting. However, the solder spot in Fig. 3b shows severe shrinkage. A possible explanation is that a re-oxidation process occurred on the treated surface in Fig. 3b due to the higher ROL. Unexpectedly, the solder spot obtained at the faster sweep rate is uniform and sufficient, as shown in Fig. 3c. It is usually thought that an excessively fast sweep rate will result in an insufficient treatment time of the DBC surface. However, the results of this application show that when the ROL is low enough, the oxygen is always very thin, so the effect of the sweep rate is not obvious.

[0051] Internal voids is another important evaluation criterion for good soldering. Two indicators will be considered: the first is the void ratio, which represents the area of ​​all voids in the target area divided by the area of ​​all solder spots; the second indicator is the percentage of solder spots with voids, which represents the number of solder spots with voids divided by the number of all solder spots.

[0052] To achieve this goal, by using deeply oxidized DBC as the soldering substrate, we compared the porosity of solder spots after flux soldering and atmospheric plasma treatment. The results are shown in Table 1 below. In flux soldering, the flux model number is SAC305 (purchased from Shanghai Huaqing Welding Materials Tech Co., Ltd.), the flux percentage is 0.2%-5%, and the activity level is ROL0. The atmospheric plasma treatment is carried out in air with ROL less than 50ppm. Each group is set by 75 solder spots. Among the solder spots with voids, the solder spot with the least voids and the solder spot with the most voids are selected as an example, as shown in Figures 4a and 4d.

[0053] [Table 1]

[0054] Table 1 shows that compared with flux soldering, atmospheric plasma treatment can significantly reduce the porosity and the percentage of solder spots with voids. After flux soldering, 100% of the solder spots had voids, and the average porosity was 2.7%. However, the DBC treated by atmospheric plasma had a significantly reduced porosity, and the quality of the solder spots was higher.

[0055] Therefore, plasma treatment showed better performance not only in terms of porosity but also in terms of the number of void solder spots. This means that flux soldering can lead to a rapid increase in porosity if the soldering area is increased, which is not the case for atmospheric plasma treatment. This application provides a new surface treatment method. The soldering process is integrated into an atmospheric plasma treatment system without any flux protection, and it is ensured that the oxygen level in the atmosphere is reduced to an extremely low level that can meet the high requirements of surface soldering performance.

[0056] What is described in this specification is only the preferred specific embodiments of the present application, and the above embodiments are only used to describe the technical solutions of the present application without limiting the present application. All technical solutions that can be obtained by those skilled in the art through logical analysis, reasoning or limited experiments according to the concept of the present application should be within the scope of the present application. [Explanation of symbols]

[0057] 101 Main solder layer 102 DBC 103 Secondary solder layer 104 Insulating layer 105 Bonded copper 106 Heat dissipation board 201 Transport Platform 202 Transition room 203 Atmospheric pressure plasma processing chamber 204 Reflow Soldering Room 205 Transition Room 206 Transport Platform

Claims

1. providing an atmospheric pressure plasma; and A surface treatment method comprising the steps of placing an object to be treated in an atmospheric pressure plasma atmosphere, The surface treatment method, wherein the residual oxygen level in the atmospheric plasma atmosphere is lower than a predetermined value.

2. 2. The surface treatment method according to claim 1, wherein the atmospheric pressure plasma is generated by supplying argon, helium, hydrogen, nitrogen, or a combination thereof.

3. 3. The surface treatment method according to claim 2, wherein the atmospheric pressure plasma is generated by supplying argon and hydrogen, or nitrogen and hydrogen.

4. 4. The surface treatment method according to claim 3, wherein the hydrogen occupies 1% to 10% of the volume of the atmospheric pressure plasma, and preferably the hydrogen occupies 1% to 4% of the volume of the atmospheric pressure plasma.

5. 5. The method of any one of claims 1 to 4, wherein said predetermined value is 200 ppm, preferably 100 ppm, more preferably 50 ppm, and most preferably 10 ppm.

6. 5. The method of claim 1, wherein the surface of the workpiece is not coated with an organic solderability preservative.

7. The surface treatment method according to any one of claims 1 to 4, further comprising the step of placing the workpiece in a soldering system after the atmospheric plasma treatment, wherein no flux is used in the soldering system.

8. 5. The surface treatment method according to claim 1, wherein the atmospheric pressure plasma sweeps the workpiece at a speed of 10 mm / s to 300 mm / s, preferably 25 mm / s to 150 mm / s, and more preferably 25 mm / s to 100 mm / s.

9. 5. The surface treatment method according to claim 1, wherein the porosity of the surface of the object to be treated is less than 1%.

10. 5. The surface treatment method according to claim 1, wherein the atmospheric pressure plasma is emitted perpendicularly or obliquely to the surface of the workpiece.

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