Techniques for atmospheric pressure plasma to remove oxidation from metal contacts to improve electrical and mechanical bonding

Abstract

A method and apparatus for plasma processing of electrical interconnects of an electronic assembly of an integrated circuit, such as a flip chip, is disclosed herein. A method of interconnect bonding includes applying, by an atmospheric pressure plasma applicator, a plasma including activated hydrogen to a metal contact supported on a substrate to remove oxidation and create a newly deoxygenated metal surface. The metal contact can be bonded to the other by reflow or thermocompression bonding. An inert gas environment may be formed within an enclosure for application of the plasma and / or bonding of the metal contacts.

Description

TECHNICAL FIELD

[0001] The present invention relates to a plasma apparatus and a method for removing oxidation from copper, tin, tin-silver alloy and indium for metal-metal bonding using the plasma apparatus. In particular, the present invention relates to the reduction of oxidized copper, oxidized tin, oxidized tin-silver and oxidized indium to improve mechanical and electrical bonding in electronic assemblies (e.g., flip-chip). BACKGROUND

[0002] Ionized gas plasmas have a wide range of applications in material processing. Plasmas used for material processing are typically weakly ionized, meaning that a small fraction of the molecules in the gas are charged and balanced by free electrons, such that the total sum of charge is zero. In addition to ions and free electrons, these plasmas contain reactive species that can clean, activate, etch, and deposit thin films onto surfaces. The temperature of these weakly ionized gases is typically less than 250°C, so that heat sensitive substrates are not damaged. The physical and chemical properties of weakly ionized plasmas are described in several textbooks. See, for example, Lieberman and Lichtenberg, "Principles of Plasma Discharges and Materials Processing", (John Wiley & Sons, Inc., New York, 1994) and Raizer, Y.P., "Gas Discharge Physics", (Springer-Verlag, Berlin (1991).

[0003] According to the literature, weakly ionized plasmas are generated in a vacuum with a pressure between 0.001 and 1.0 Torr (see Lieberman and Lichtenberg (1994)). An electric power is applied between two electrodes to dissociate and ionize the gas. The power can be provided as direct current (DC), alternating current (AC), radio frequency (RF) power, or microwave (MW) power. The electrodes can be constructed to provide capacitive or inductive coupling to excite and sustain the plasma. In the former case, two conducting electrodes are placed in a vacuum chamber filled with a small amount of gas. One of the electrodes is powered by an RF generator or biased, while the other electrode is grounded. In the latter case, the RF power is provided by an antenna in a coil wrapped around the insulated walls of the chamber. The oscillating electric field from the coil penetrates into the gas to cause ionization.

[0004] Over the past twenty-five years, atmospheric pressure plasma has been developed as an alternative to vacuum plasma. These plasmas can process objects of any size and shape because they do not need to be contained within a vacuum chamber. This significantly reduces process costs. Many different atmospheric pressure plasma devices have been developed (Schutze, et al., “Theatmospheric-pressure plasma jet: A review and comparison to other plasma sources”, IEEE Trans. Plasma Sci. 26, 1685-1694 (1998)). The control of these plasmas is determined by the method of ionization process. At atmospheric pressure, the gas density is extremely high, and the ionization reaction can easily run away from control, generating high-temperature arcs, which is unusable in many material handling applications.

[0005] Atmospheric pressure plasmas exist in three common types. These include dielectric barrier discharge (DBD), torch discharge, and radio frequency inert gas discharge. DBD has long been used to process rolls of plastic films, where material flows continuously between electrodes. In some cases, DBD can be deployed as a downstream device to process 3D objects using reactive gases flowing from between the electrodes. Torch discharge and radio frequency inert gas discharge are used to process substrates placed downstream. In some cases, ions and electrons flow from the torch to contact the substrate. On the other hand, in inert gas discharge, ions and electrons are confined in the gap between the electrodes, resulting in the substrate being exposed to a beam of neutral reactive material (see, for example, Cheng et al., U.S. Patent No. 9,406,485, August 2, 2016). An automaton scans the plasma beam above the substrate surface, which is less than one centimeter below the plasma shell. Unlike vacuum plasma, only the area on the sample surface that needs to be treated is exposed to the reactive gaseous material.

[0006] Dielectric barrier discharges typically operate in air (see Goldman and Sigmond, “Corona and Insulation”). IEEE Transactions on Electrical Insulation , EI-17, no. 2, 90-105(1982) and Eliasson and Kogelschatz, "Nonequilibrium Volume Plasma Chemical Processing", IEEE Transactions on Plasma Science, 19, 1063-1077, (1991)). A 10 kV power supply operating at approximately 20 kHz provides the voltage required to decompose the gas. A dielectric barrier layer covers one of the electrodes to prevent the formation of a high-current arc. During operation, charge accumulates on the surface of the insulator and discharges in the form of tiny “micro-arcs” in each AC cycle. The micro-discharges occur randomly in space and time, lasting from 10 to 100 ns. The electron density is very high inside the micro-discharge and extremely low outside. Therefore, it is impossible to measure the average electron density and electron temperature of the entire gas volume between the electrodes. Note that the substrate placed downstream of the DBD cannot be uniformly processed by the plasma on a microscale. Furthermore, the discharge generates an electrical interaction with the substrate, making it difficult to process components containing metals.

[0007] The torch is generated by forming an electric arc between closely spaced energized and grounded electrodes. Fauchais and Vardelle described this structure in their article, “Thermal Plasmas”, IEEE Transactions on Plasma Science, 25, 1258-1280 (1997). Air is passed between the electrodes and ionized by applying a 10 kV AC power supply. The electric arc is a thermal plasma with a core temperature of several thousand degrees. However, gas is blown across the arc at a sufficiently high velocity, making the overall gas temperature low enough to handle heat-sensitive materials, including polymers. Plasmaflume from PlasmaTreat... TM This is an example of such a structure. It utilizes a rotating conical electrode whose rapidly rotating arc passes through a flowing gas volume, maintaining an average core temperature below 700 K. A plasma stream containing ions and electrons is ejected from the end of the outer shell and processes an object placed not far below.

[0008] Atmospheric pressure inert gas plasmas driven by 13.56 or 27.12 MHz radio frequency power operate differently from DBD and torch plasmas. These plasmas are weakly ionized capacitive discharges (see Jeong et al., “Etching Materials with an Atmospheric-Pressure Plasma Jet”). Plasma Sources Science Technol ., 7, 282-285 (1998); Babayan et al., “Deposition of SiliconDioxide Films with an Atmospheric-Pressure Plasma Jet,” Plasma Sources Science Technol., 7, 286-288, (1998); Moravej, et al., “Physics of High-Pressure Helium and Argon Radio-Frequency Plasmas”, J. Appl. Phys., vol. 96, p. 7011 (2004); Babayan and Hicks, U.S. Patent No. 7,329,608, February 12, 2008; Babayan and Hicks, U.S. Patent No. 8,328,982, December 11, 2012; and Cheng et al., U.S. Patent No. 9,406,485, August 2, 2016). Ions and electrons uniformly fill the gas volume between the metal electrodes, forming a collision sheath at the boundary to bounce electrons and maintain the plasma. The average electron density and temperature in the RF inert gas plasma have been determined to be 10 11 Up to 10 12 cm -3 And 1 to 2 eV. The center gas temperature ranges from 323 to 573 K, depending on the RF power level. Molecular gases are fed in helium and argon at concentrations of 0.1 to 5.0 vol%.

[0009] The processing of semiconductor materials (e.g., silicon) in vacuum plasma involves inserting them into a chamber providing energized and grounded electrodes, supplying power to these electrodes to induce a discharge, and then running the plasma for several minutes to modify the material surface. Silicon substrates are provided in the form of thin wafers with diameters of 100, 150, 200, and 300 mm. Vacuum chambers are specifically designed to accommodate wafers of a given size. One method for cleaning the silicon surface is to feed oxygen and argon into the chamber. High-energy free electrons in the plasma convert some oxygen molecules into oxygen atoms and other reactive substances, which attack organic contaminants on the surface and convert them into gaseous carbon dioxide. Furthermore, positively charged argon ions (Ar... + Bombarding the substrate physically sputters surface contaminants. After a few minutes of oxygen plasma treatment in a vacuum chamber, the silicon wafer surface becomes clean and activated for other semiconductor processing steps.

[0010] Another method for cleaning the surface of semiconductor materials is to insert the substrate into a vacuum chamber and excite plasma with a feed gas containing a mixture of hydrogen and argon. In this case, high-energy free electrons will generate H atoms and Ar atoms. +Ions bombard the substrate surface and remove metal oxide contaminants, including, but not limited to, copper oxide, silver oxide, tin oxide, or indium oxide. To prevent the introduction of flammable hydrogen mixtures into the chamber and create safety hazards, the plasma feed gas must contain less than 4.0% H2 in argon. At the operating pressure of 0.05 to 0.50 Torr in the vacuum chamber, the resulting hydrogen concentration is only 2 to 20 millitorr. Therefore, the supply of active hydrogen atoms and the hydrogen atom etching rate of the metal oxides will be extremely low, necessitating the removal of most oxide contaminants by sputtering.

[0011] Another drawback of processing materials with vacuum plasma is that the chamber becomes contaminated during repeated processing of silicon wafers. The chamber slowly fills with particles (i.e., contaminants) ranging in size from 0.01 to 10.0 micrometers in diameter. This issue has been documented in many open-source documents (e.g., see GS Selwyn, et al., J. Vac. Sci. Technol. A 7, 2758 (1989); ibid., 8, 1726 (1990); MJ McCaughey and MJ Kushner, Appl. Phys. Lett. 55, 951 (1989); RN Nowlin and R. N Carlile, J. Vac. Sci. Technol. A9, 2824 (1990); GS Selwyn, Jpn. J. Appl. Phys. 32, 3068 (1993); and SJ Choi, et al., Plasma Sources Sci. Technol. 4, 418 (1994); and RL Merlino and J. A. Goree, Physics Today 1 (July 2004)). When the plasma is turned on, the particles become negatively charged, resulting in an electric field in the chamber, causing the particles to float above the wafer surface. When the plasma is turned off, the particles fall onto the wafer, forming a contaminant layer. Those skilled in the art will understand that particles present during wafer handling can damage solid-state devices. The semiconductor industry is obsessed with eliminating them and has spent billions of dollars building cleanrooms free of particles larger than 0.01 micrometers in diameter. Contaminated plasma chambers must be cleaned regularly to reduce particle contamination. Furthermore, after plasma immersion, the wafer may require wet cleaning to remove any particles adhering to it. All of this drives up the manufacturing cost of semiconductor devices.

[0012] Flip-chip interconnect bonding is a process that greatly benefits from the removal of metal oxides via hydrogen plasma. In this process, a chip with a two-dimensional array of solder balls or bumps (the distance between the centers of the solder balls or bumps is less than 200 micrometers) is placed on another chip or substrate with a two-dimensional array of metal pads or bumps having the same spatial arrangement. The chips are then heated and pressed together, forming metal-to-metal interconnects on each ball or bump (JH Lau, “Semiconductor Advanced Packaging,” (Springer Nature, Singapore). (2021). Before bonding, the oxide layer on the metal surface must be removed to ensure the formation of ohmic contacts with high mechanical strength and minimal resistance. Metal oxides on balls or bumps are typically removed by adding organic flux to the solder. During reflow, the organic flux reacts with the metal oxide to form a compound that sublimates from the surface, leaving the bare metal. However, the flux leaves organic residues during reflow, which must be removed by a cleaning step. When the distance or pitch between balls or bumps is less than 50 micrometers, it is no longer possible to remove these organic residues by cleaning. In this case, hydrogen plasma removal of metal oxides would be an excellent alternative to flux. However, using vacuum plasma for this process is ineffective because, as mentioned above, the hydrogen radical concentration is too low, resulting in an excessively slow etching rate; sputtering must be used instead. Sputtering is not a self-limiting process and can easily damage semiconductor materials. Furthermore, flip chips with ball grid arrays must be transferred from vacuum to a thermosetting bonding machine, where the metal balls may oxidize before they are properly bonded together.

[0013] Atmospheric pressure inert gas plasma supplied with hydrogen can be used to remove oxidation from metals, such as removing copper oxide from copper and indium oxide from indium (see, for example, Cheng et al., U.S. Patent No. 9,406,485, August 2, 2016, and Schulte, U.S. Patent No. 8,567,658, October 29, 2013). This process can be used to remove oxidation from ball grid arrays or microbump arrays prior to flip chip bonding. The advantage of atmospheric pressure hydrogen and inert gas plasma is that the concentration of hydrogen radicals is much higher than in a vacuum, and sputtering that could damage the semiconductor device does not occur. However, metal balls or bumps may be re-oxidized during the transfer from the plasma to the thermosetting bonder. Schulte (U.S. Patent No. 8,567,658, October 29, 2013) teaches the need for a passivation step using an "activated chemical passivator" to prevent re-oxidation during the transfer from the plasma processing step to the bonding step. Depending on the combination of metal elements used in the ball or microbump array, the passivation step may or may not effectively achieve mechanically robust metal interconnects with minimal resistance.

[0014] In view of the foregoing, there is a need for an apparatus and method capable of rapidly removing oxides from an array of metal interconnects using hydrogen plasma at atmospheric pressure, and then transferring the deoxidized metal balls or bumps on the chip and / or substrate to a bonding machine, where they are bonded together to form a high-density interconnect array. Furthermore, this apparatus and method must be suitable for material handling and must not damage the semiconductor chip due to ion bombardment, electrostatic discharge, particulate contamination, or ultraviolet light exposure. In particular, there is a need for an apparatus and method capable of removing oxides of copper, silver, tin, and indium from the array of metal balls or bumps, and subsequently thermoforming them to form mechanically robust and conductive metal interconnects. Such an apparatus and method has specific applications in flip-chip bonding of two-dimensional arrays involving densely spaced interconnects. These and other needs are met by embodiments of the invention described below. Summary of the Invention

[0015] This document discloses a method and apparatus for plasma treatment and bonding of electrical interconnects for integrated circuit packaging (e.g., flip chips). The interconnect bonding method includes applying plasma containing activated hydrogen to a first metal contact supported on a substrate using an atmospheric pressure plasma applicator to remove oxidation and create a newly deoxidized metal surface. The first metal contact is then bonded to a second metal contact by applying mechanical force, heat, or both, such as by thermocompression bonding (TCB). An inert gas environment is formed within an enclosure for applying the plasma, and a second inert gas environment is formed within an enclosure for bonding the first metal contact to the second metal contact. The substrates used for the first and second metal contacts can be selected from, but are not limited to, the group consisting of, but not limited to: semiconductor dies (also referred to as integrated circuits or chips), semiconductor wafers, such as 300mm silicon wafers having integrated circuits thereon, organic or plastic substrates, ceramic substrates, lead frames, and printed circuit boards (PCBs).

[0016] In another embodiment of the invention, the application of plasma to remove oxidized plasma from the first metal contact point supported on the substrate and the bonding of the first metal contact point on the first substrate to the second metal contact point on the second substrate are performed in a closed system under an inert gas environment throughout the process. The substrate may be selected from, but is not limited to, the group consisting of, semiconductor bare dies, semiconductor wafers, plastic substrates, ceramic substrates, lead frames, and printed circuit boards (PCBs).

[0017] In another embodiment of the invention, the method of interconnect bonding includes applying plasma containing activated hydrogen to a first metal contact point and a second metal contact point supported on a substrate using an atmospheric pressure plasma applicator to remove oxidation from both metal contact points and create a newly deoxidized metal surface. The first metal contact point is then bonded to the second metal contact point by applying mechanical force, heat, or both, for example by TCB. An inert gas environment is formed within the enclosure used to apply the plasma to the two metal contact points and to bond the first metal contact point to the second metal contact point.

[0018] A typical embodiment of the present invention includes a method for joining interconnects, the method comprising the steps of: applying plasma containing activated hydrogen to at least one first metal contact point supported on a first substrate using an atmospheric pressure plasma applicator to remove oxidation and create a newly deoxidized metal surface on the first metal contact point; joining the first metal contact point to the second metal contact point by applying mechanical force, heat, or both to the first metal contact point in contact with a second metal contact point supported on a second substrate, and forming an inert gas environment in which the plasma is applied and the joining of the first metal contact point to the second metal contact point is performed, wherein the joining of the first metal contact point to the second metal contact point forms a metal interconnect between the first metal contact point and the second metal contact point.

[0019] In this embodiment, the first and second substrates may be chips, and the metal contacts on the chips are joined together to form a chip-on-a-chip (CoC) package; or, the first substrate may be a chip, and the second substrate may be a wafer, and the metal contacts are joined together to form a wafer-on-a-chip (CoW) package; or, the first and second substrates may be wafers, and the metal contacts on the wafers are joined together to form a wafer-on-a-wafer (WoW) package; or, the first substrate may be a chip, and the second substrate may be a board, and the metal contacts are joined together to form a chip-on-board (CoB) package. These and other interconnect package designs will be apparent to those skilled in the art. Embodiments of the present invention may further encompass apparatus embodiments consistent with any method embodiments described herein.

[0020] In another embodiment, the inert gas environment may contain less than 500 parts per million (ppm) of oxygen, preferably less than 100 ppm. Furthermore, the inert gas environment can be generated by mounting the atmospheric pressure plasma applicator, the first substrate supporting the first metal contact point, and the second substrate supporting the second metal contact point within a volume, and purging the volume with an inert gas flow, wherein the inert gas flow replaces the initial air inside the volume. The inert gas flow may be selected from the group consisting of argon and nitrogen.

[0021] In other embodiments, the plasma can be applied by sweeping an atmospheric pressure plasma applicator across the first substrate supporting the first metal contact point. Furthermore, activated hydrogen can be formed by supplying argon gas with a hydrogen concentration of 0.1% to 2.0% to the plasma. The contact between the first and second metal contacts for bonding can occur at a temperature between 20 and 250°C. Additionally, both the first and second metal contacts may each comprise a metal or metal alloy selected from the group consisting of: cobalt, nickel, copper, rhodium, palladium, silver, iridium, platinum, gold, indium, tin, antimony, lead, and bismuth.

[0022] In some embodiments, the first metal contact on the first substrate may comprise a two-dimensional array of metal bumps, the center-to-center distance between the metal bumps being 2 to 100 micrometers. During plasma application and bonding of the first metal contact to the second metal contact on the second substrate, the first substrate may be held separately in the inert gas environment. In another embodiment, both the first and second substrates may be held in the inert gas environment throughout the entire process of plasma application and bonding of the first metal contact on the first substrate to the second metal contact on the second substrate. Alternatively, plasma containing activated hydrogen may be applied to the second metal contact on the second substrate using an atmospheric pressure plasma applicator, and the second substrate may be held in the inert gas environment throughout the entire process of plasma application and bonding of the first and second metal contacts. In a typical embodiment, the first metal contact on the first substrate and the second metal contact on the second substrate may comprise a flip-chip assembly.

[0023] In another embodiment, the first metal contact point on the first substrate and the second metal contact point on the second substrate may each include a two-dimensional array of metal bumps, the center-to-center distance between the metal bumps being 2 to 100 micrometers, and joining the first metal contact point to the second metal contact point includes aligning and joining the metal bumps of the first metal contact point with the metal bumps of the second metal contact point. Furthermore, the two-dimensional array of the second metal contact points may include an under-bump metallization layer containing metal pads.

[0024] In one embodiment, the atmospheric pressure plasma applicator may include an atmospheric pressure plasma head fixed to a cover plate having a slotted channel for the plasma to pass through to the first metal contact point supported by the first substrate. The cover plate may include at least one purge port for an inert gas to pass through to create an inert gas environment. The cover plate may include at least one sampling port passing through it for measuring and confirming an oxygen concentration below 500 ppm. The plasma is applied by scanning through the slotted channel across the first metal contact point on the first substrate. The cover plate may be located on top of an enclosed volume in which the first substrate is inserted, wherein the distance from the top of the substrate to the slotted channel does not exceed 10 mm. It will be apparent to those skilled in the art that the enclosed volume should be sufficient to accommodate the substrate, but not larger than the volume required for the plasma head mounted on the cover plate to scan the substrate surface at a distance of 0.5 to 10.0 mm. After processing the metal contact points on the first substrate, the apparatus embodied in the present invention has the means to move the first substrate onto the second substrate, align the metal contact points on the first substrate with the metal contact points on the second substrate, and join them together by applying mechanical force, heat or both to form a metal interconnect, and all steps are performed in an inert environment with an oxygen concentration of less than 500 ppm.

[0025] In another embodiment, the atmospheric pressure plasma applicator may include an atmospheric pressure plasma head fixed to the bottom of an enclosed volume, wherein a plasma beam is guided upward through a slotted channel. The plasma exhaust gas flows upward through the slotted channel and brushes over the first metal contact point on the first substrate. In this case, the first substrate is attached to a joint mounted on a cover plate. The first substrate protrudes into the enclosed volume at a distance of no more than 10 mm from the slotted channel, preferably between 1 and 4 mm. The apparatus embodied in this invention includes a tool for sweeping the first substrate over a plasma beam containing activated hydrogen to remove oxidation from the first metal contact point, aligning the first metal contact point on the first substrate with the second metal contact point on a second substrate, and joining them together by applying mechanical force, heat, or both to form a metal interconnect, all steps being performed in an inert environment with an oxygen concentration of less than 500 ppm. Attached Figure Description

[0026] Referring now to the accompanying drawings, where the same symbols denote corresponding parts throughout the drawings:

[0027] Figure 1 An exemplary plasma process for removing a metal oxide layer from a metal is shown;

[0028] Figure 2 This is a schematic diagram of an exemplary apparatus for exposing a substrate to an activating gas from atmospheric pressure hydrogen and argon plasma, wherein air has been removed from the processing area by local inert gas purging;

[0029] Figure 3A This is a photo of solder bumps before the tin oxide on the surface was removed using atmospheric pressure hydrogen and argon plasma;

[0030] Figure 3B This is a photo of solder bumps after the tin oxide has been removed using atmospheric pressure hydrogen and argon plasma;

[0031] Figure 4A It is the X-ray emission spectrum of the 3d peak of tin on the oxidized tin surface;

[0032] Figure 4B The X-ray emission spectrum of the tin 3d peak of tin has been scanned using atmospheric pressure hydrogen and argon plasma.

[0033] Figure 4C The X-ray emission spectrum of the tin 3d peak on the tin surface was obtained by scanning with atmospheric pressure hydrogen and argon plasma and then allowing it to stand in air at room temperature for 30 hours.

[0034] Figure 5AOptical micrographs showing solder bumps after heating to 250°C in an inert gas environment.

[0035] Figure 5B Optical micrographs showing solder bumps after being heated to 250°C in an inert gas environment and then exposed to atmospheric pressure hydrogen and argon plasma at 250°C.

[0036] Figure 6A This illustrates poor metal-metal bonding caused by a thin layer of tin oxide that forms rapidly when air is allowed between plasma processing and reflow bonding;

[0037] Figure 6B This demonstrates a strong metal-to-metal bond when there is no air exposure between treatment with atmospheric pressure hydrogen and argon plasma and reflow bonding of the microbump array to the copper substrate.

[0038] Figure 7A This is an optical micrograph of an indium bump heated to 165°C in an inert gas environment;

[0039] Figure 7B This is an optical microscopic image of an indium bump after being treated with atmospheric pressure hydrogen and argon plasma at 165°C in an inert gas environment.

[0040] Figure 8 Compare the relative shear strength with the relative bonding force applied to a two-dimensional indium bump array thermo-pressed at room temperature;

[0041] Figure 9 The relative shear strengths of the indium-to-UBM bond assemblies are shown for those without plasma treatment (control group), those with hydrogen plasma treatment only on the indium bumps, and those with hydrogen plasma treatment on both the indium bumps and the UBM.

[0042] Figure 10 The relative shear strength of the indium-to-indium microbump assembly is shown when exposed to air for different times between hydrogen plasma cleaning and thermo-press bonding.

[0043] Figure 11 This is a schematic diagram of an apparatus for treating substrates or bare crystals with atmospheric pressure hydrogen and argon plasma while they are kept in an inert gas purging environment.

[0044] Figure 12A and 12B The top and isometric views of a recessed volume are shown, which is used to mount a bare crystal in a device for atmospheric pressure hydrogen and argon plasma processing in an inert gas environment.

[0045] Figure 13A and 13BThe assembled cover plate and plasma head are shown in top and bottom isometric views, respectively.

[0046] Figure 14 This is a schematic diagram of an apparatus for treating silicon wafers with atmospheric pressure hydrogen and argon plasma in an inert gas purging environment.

[0047] Figure 15 This shows a layered cross-sectional view of the apparatus used for atmospheric pressure hydrogen and argon plasma treatment of wafers in an inert gas purging environment.

[0048] Figure 16 The diagram shows a cross-sectional view of an apparatus for atmospheric pressure hydrogen and argon plasma processing, wherein the silicon wafer is mounted inside the inert gas purging environment;

[0049] Figure 17 Showing a side view of an apparatus for atmospheric pressure hydrogen and argon plasma processing, wherein the silicon wafer is mounted inside the inert gas purging environment;

[0050] Figure 18 An enlarged side view of the apparatus used for atmospheric pressure hydrogen and argon plasma processing of wafers is shown, highlighting the inert gas purge flow.

[0051] Figure 19 Showing a top view and a side view of the wafer holder, wherein the inert purging airflow enters from the left and flows uniformly across the wafer from left to right;

[0052] Figure 20 The diagram shows the motion path of a 100mm linear plasma head and cover plate performing a full scan on a 300mm silicon wafer. Detailed Implementation

[0053] Detailed description including preferred embodiments

[0054] 1.0 Overview

[0055] The embodiments described below are merely examples for practicing the invention. Those skilled in the art will understand how the described examples can be readily applied to a wide range of other applications. It is noteworthy that the atmospheric pressure plasma deoxidation process and apparatus described herein can be applied to semiconductor packaging applications, including, but not limited to, flip-chip bonding. As is known in the art, flip-chip bonding offers advantages compared to other interconnect processes. This method uses the entire area of ​​the bare die, rather than just the periphery of the integrated circuit for connection. Therefore, flip-chip bonding provides higher density interconnects and shorter interconnect paths compared to wire bonding, enabling faster device manufacturing. Efficient and cost-effective integrated circuit manufacturing processes that can be used to improve the quality and performance of interconnects such as those described herein are of significant value for electronics production.

[0056] Typically, all bonding in a flip chip package is completed in a single process, rather than sequentially as individual connections. The solder bumps used for flip chip bonding are usually composed of low-melting-point metals, typically indium, tin, or other suitable alloys. After the chip is aligned and placed, electrical connections are created using thermocompression bonding (TCB) at temperatures above or below the solder's melting point. The presence of metal oxides on the solder bump surface hinders both techniques, as it prevents the solder bump from fully melting during reflow or increases the force required to break through the surface oxides used for TCB.

[0057] Organic fluxes are often used to remove metal oxides during solder bump connections. However, corrosive residues left by the flux can damage nearby components and / or cause short circuits between interconnects. Therefore, these residues must be washed away after bonding. With the trend towards smaller solder bumps and increasingly smaller gaps between bumps, the removal of solder residues has become a major obstacle due to the difficulty in introducing wet chemical solutions into the micro-volume between the flip chip and the substrate. In particular, high surface tension prevents liquids from penetrating into these tiny spaces.

[0058] Flip-chip bonding that does not use organic flux embedded in the solder is becoming an attractive option for smaller pitch dimensions. One such example is the removal of metal oxides from the solder bump surface by contact with formic acid vapor. Here, formic acid reacts with the metal oxides to form metal formate, which then sublimates from the surface. Unfortunately, this process requires temperatures above 200°C to achieve a significant reaction rate. Furthermore, treatment with formic acid vapor leaves organic residues and may corrode the solder surface and other exposed metals during the reaction.

[0059] The atmospheric pressure plasma process described in this article uses activated hydrogen to react with metal oxides, converting them into metal and water vapor that exits the system. This process can be performed at lower temperatures than the formic acid vapor process described above. Furthermore, no organic residues are generated, and no metal components in the integrated circuit are corroded. By performing the process in an inert gas environment of argon or nitrogen, metal oxides are removed, and no further oxidation occurs before the metal contacts are joined together to form an interconnect.

[0060] 2.0 Apparatus and method for removing surface oxides from tin and tin / silver alloys

[0061] Figure 1This diagram illustrates an exemplary metal oxide removal process. Argon gas with a concentration of 0.1 to 2.0 vol% hydrogen molecules is fed into an atmospheric pressure plasma head, which is driven at radio frequency power (e.g., 27.12 MHz). This radio frequency power causes the gas to ionize within the head, generating positive and negative ions and free electrons. The free electrons collide with H2 molecules, causing them to dissociate into free radicals. . As shown in step (1). In step (2), hydrogen radicals flow out from the plasma device and contact the metal oxides on the spheres or bumps of the substrate. In step (3), the hydrogen radicals react with the oxygen contained in the metal oxides to generate water, H2O, leaving the bare metal. In the case of tin oxide on tin or tin-silver alloys, according to the stoichiometry of the reaction, the hydrogen radicals react with tin oxide, SnO or SnO2 to generate tin (Sn) metal and H2O. Any silver oxide present will also be reduced to silver metal. In addition, if there are residual organic contaminants on the surface, the hydrogen radicals can remove them rapidly. The metal contact points on the substrate are exposed to reactive hydrogen for a sufficient time of 0.01 seconds to 1.0 minutes, typically in the range of 0.1 to 3.0 seconds. During the contact of the metal contact points on the substrate (which may optionally be a chip or wafer), atmospheric pressure plasma can be scanned over the substrate, bare die or wafer so that the metal oxide surface on the spheres or bumps is uniformly contacted with the hydrogen radicals.

[0062] Hydrogen is particularly suitable for metal oxide removal processes. However, other hydrogen-containing molecules, such as ammonia (NH3) or hydrogen sulfide (H2S), can be used, as will be apparent to those skilled in the art. Atmospheric pressure plasmas suitable for embodiments of the invention include those that generate high concentrations of ground-state atoms, radicals, or metastable molecules (containing active hydrogen) downstream of the plasma head.

[0063] exist Figure 2The diagram illustrates an exemplary apparatus for removing metal oxides by combining an inert gas purging environment with activated hydrogen from plasma. The apparatus consists of a cover plate 4 directly attached to a plasma head 5. A feed gas, containing argon and 0.1% to 2.0% hydrogen, enters the plasma head 5 at inlet 11 and is converted into a weakly ionized plasma, generating free electrons, ions, and an activated gas containing at least a suitable concentration of hydrogen radicals. The activated gas 8 flows out of the plasma head 5 and contacts a substrate 6. The cover plate 4 is flush with the plasma head 5 and mounted above the substrate 6, maintaining a small gap 7 between the plasma head 5 and the substrate 6. Air in the gas volume of the small gap 7 is rapidly replaced by the activated gas flow. A suitable gap spacing is 0.1 to 5.0 mm, preferably between 0.1 and 2.0 mm. Additional purge gas can be introduced through purge holes 9 and 10 in the cover plate 4. This purge gas is selected from the group consisting of inert gases, argon, and nitrogen. The purge gas helps the activating gas displace air in the gas volume of gap 7, thereby maintaining an inert gas environment during the removal of metal oxides from the metal contact point by the activated hydrogen. An inert gas environment is defined as an environment in which the oxygen concentration is low enough that exposure to activated hydrogen generated during the plasma process will not cause re-oxidation of the metal contact point. Examples of an inert gas environment are oxygen concentrations below 500 ppm, preferably below 100 ppm. Exemplary purge orifices 9 and 10 may comprise elongated slits or one or more orifices through which the purge gas flows around the activating gas 8.

[0064] A slight positive pressure is established within gap 7, allowing sufficient airflow to expel all air from around cover plate 4. This enables the plasma reduction process to proceed in an inert gas environment with an oxygen concentration below 500 ppm. This low-oxygen environment facilitates the removal of metal oxides and prevents re-oxidation of the metal, especially when the substrate 6 is heated.

[0065] In practice, the substrate to be treated can be transported through this device while the environment around the substrate is continuously exchanged, maintaining a low-oxygen environment before subsequent reflow or pressurized bonding operations. The device containing atmospheric pressure hydrogen and argon plasma can be operated as a standalone unit for removing tin oxide from tin or tin / silver alloy bumps. After the tin and / or silver oxide is removed, the substrate is transferred to the bonding operation under ambient conditions. In another preferred embodiment of the invention, the device containing the plasma head is integrated with the bonding operation, so that the substrate can be maintained in an inert gas environment during plasma treatment, transfer to the bonding area, and bonding of the metal contacts.

[0066] exist Figure 3A and 3BThe images show optical micrographs of solder bumps before and after plasma oxide removal. Image 3A shows solder bumps covered in tin oxide, evidenced by their yellowish and rough, mottled appearance. After removing the tin oxide with activated hydrogen from atmospheric plasma, the solder bumps became smooth and had a highly reflective metallic silver color.

[0067] An exemplary embodiment of the present invention involves treating a tin metal film with atmospheric pressure argon and hydrogen plasma in a purging environment. The tin film has a thin, native oxide layer on its surface. The atmospheric plasma head is mounted on a cover plate with a purge gas, such as... Figure 2 As shown. The head and cover plate are then fixed to the scanning automation machine so that the activating gas can sweep across the sample in the absence of air. In this example, a linear plasma source is used, through which activating hydrogen flows out from a slit with a width of 100 mm. The distance between the exit slit and the tin surface is approximately 1.0 mm. The plasma source scans over the substrate at different scan rates ranging from 0.25 to 10.0 mm / s at room temperature. 40.0 LPM of argon and 0.40 LPM of hydrogen are fed into the plasma source per minute. A radio frequency power of 500 W (27.12 MHz) is applied to the plasma head to excite and sustain the gas discharge.

[0068] Figure 4A , 4B The figures shown in 4C are Sn3d on the surface of the tin film after the formation of the primary oxide, after the primary oxide was reduced by hydrogen plasma, and after the primary oxide was aged in air for 30 hours following plasma reduction. 3 / 2 and 3D 5 / 2 The X-ray emission spectrum of the peak. Figure 4B and 4C In the middle, Sn 3d 5 / 2 The peaks have been deconvoluted to reveal two separate peaks at 486.7 eV and 485.0 eV. The peaks for tin oxide, SnO, and SnO2 are very close together, therefore the single broad peak at 486.7 eV is attributed to tin oxide (SnO2). X The peak at 485.0 eV is generated by metallic tin (Sn). A significant change is observed between the tin peak before and after argon and hydrogen plasma treatment. Before plasma treatment, the light emission of metallic tin is almost negligible (if any). After plasma treatment, a large peak at 285.0 eV is observed due to metallic tin. These results are very consistent with previous studies on surfaces exposed to hydrogen plasma in vacuum.

[0069] Table 1 provides information from deconvolution Sn 3d 5 / 2The peak area was used to estimate the relative fraction of metallic tin to tin oxide. The results showed that hydrogen plasma reduction at atmospheric pressure increased the metallic tin fraction from 3.0% to 40.8%. This increase in metallic tin indicates a significant reduction in the thickness of the oxide surface layer. This thinner oxide layer may be more easily broken through when tin or tin-silver bumps are thermocompressed together. Significant oxide regeneration occurred during 30 hours of exposure to air after hydrogen plasma treatment. In this case, the detected metallic tin fraction decreased from 40.8% to 25.4%. These results suggest that the time between oxide removal with hydrogen plasma and bonding should be kept as short as possible, preferably less than one hour.

[0070] Table 1, through Sn 3d 5 / 2 The deconvolution of the light emission peaks determines the composition of different tin surfaces.

[0071]

[0072] Figure 5A and 5B This image shows a solder bump captured by an optical microscope. Figure 5A The image shows solder bumps that have been heated to 250°C in an inert gas environment. Figure 5B The image shows a solder bump that has been treated with atmospheric pressure hydrogen and argon plasma at 250°C, while the sample was kept in an inert gas environment. The plasma treatment was performed using a 100-mm-wide linear beam operating at 500 W, 40 L / min argon, and 0.4 LPM H₂. The gap between the plasma head and the sample was 1.5 mm. Figure 5A The solder bumps in the sample were severely oxidized, as evidenced by their rough, flat tops and numerous small bumps. In contrast, Figure 5B The visible bumps exhibit a smooth, spherical shape and a highly reflective surface. Strong light reflection near the center of the bumps, surrounded by dark rings created by the large-angle sides, confirms this spherical shape. These results demonstrate the ability of atmospheric pressure hydrogen and argon plasma treatment to completely remove metal oxides from solder bumps.

[0073] The surface tension involved during solder reflow is much smaller than the forces applied during thermoforming. Unlike thermoforming, the solder bump surface must be completely free of oxides for the reflow process to proceed correctly. Therefore, the substrate must be kept in an inert gas environment throughout the process of removing oxides by activated hydrogen in plasma and bonding the solder to the substrate. These findings are described in… Figure 6A and 6BIn this experiment, the first substrate comprises a two-dimensional array of copper pillars 100 with tin caps 101, while the second substrate comprises a bare silicon die coated with a copper thin film 104. After treating both substrates with activated hydrogen from atmospheric pressure hydrogen and argon plasma, the first substrate is placed on top of the second substrate, and the package is heated to above 200°C. Note that the first substrate is placed top down so that the tin caps 101 are in direct contact with the copper film 104. Figure 6A The image shows the results obtained during hydrogen plasma cleaning and reflow in an inert gas environment, but during the transfer from one process to another in air. In this case, the solder slightly wets the copper film, thus forming a weak solder bond 102 with the copper surface. Figure 6B The image shows the results obtained when the hydrogen plasma cleaning, reflow process, and transfer steps are all performed in an inert gas environment. Here, the solder completely wets and bonds to the copper, forming an intermetallic Sn layer between the two substrates. x Cu y Connector 103. The latter case is required for manufacturing suitable microbump interconnects for flip chips.

[0074] 3.0 Method for removing surface oxides from indium

[0075] Another embodiment of the invention involves removing surface oxides from the indium bumps prior to bonding. It has been found that surface passivation is not necessary for forming interconnect bonding. The surface indium oxide layer is removed by exposure to atmospheric pressure hydrogen and argon plasma in an inert gas environment. Indium metal has a melting point of 157°C. Due to its low melting point, indium can replace tin when semiconductor packages are heat-sensitive (e.g., in some image sensors). However, indium oxide has a melting point of 1910°C, so any surface oxides on the indium metal mean that solder bumps will not form intermetallic bonding unless the oxides are removed. Once the oxides are reduced, like tin, a strong electrical and mechanical connection can be established using reflow bonding or thermoforming bonding.

[0076] Indium solder bumps were placed on a hot plate and heated to 160°C. At this temperature, metallic indium melts, but the surface oxide layer remains solid. This oxide layer forms a thin film on the surface inside the molten indium, preventing the solder bumps from flowing back and changing shape. To remove this surface oxide, a gas stream containing 32.4 LPM of argon and 7.0 LPM of syngas (5% hydrogen in 95% argon) was fed into a 100 mm linear plasma head at atmospheric pressure. A radio frequency power of 500 W was applied to the electrodes, causing the plasma to ignite and be sustained. The plasma source was then scanned over the indium microbumps on the hot plate at a distance of 2.0 mm and a scan rate of 1.0 mm / s.

[0077] Optical microscopic images of indium microbumps before and after exposure to atmospheric pressure plasma. Figure 7A and 7B The image shown is before plasma treatment. Figure 7A ) and afterwards Figure 7B (Image of bumps) Figure 7A The results show that even when heated above the melting temperature, the indium bumps retain the same rough texture and flat surface as the initially deposited material. This indicates that the indium oxide surface layer prevents the reflow of indium metal. In contrast, Figure 7B It displays a smooth sphere with a bright reflective top and dark sides. This morphological change indicates that the activated hydrogen in the plasma successfully removed indium oxide and allowed the indium metal to flow back and form a sphere.

[0078] In another example of the invention, prior to die bonding of flip chips at room temperature, indium oxide is removed from indium microbumps using atmospheric pressure hydrogen and argon plasma. No passivation step is required in this case. The indium interconnect array is bonded together in two configurations: indium-to-indium bump bonding and indium bump-to-UBM (under-bump metallization) bonding. The plasma process is compared to an industry-standard method that exposes indium bumps to formic acid vapor in an oven at a temperature between 200 and 225°C. In this case, indium oxide is removed by forming indium formate that can sublimate from the metal surface. After the metal oxide is removed, the dies containing the indium microbump array are aligned on top of each other, or on a second die containing under-bump metallization, and bonded together by thermocompression (i.e., using TCB).

[0079] The plasma process was performed in an inert gas environment purged with argon to maintain an oxygen concentration below 500 ppm. 32.4 LPM of argon and 7.0 LPM of syngas (5% hydrogen in 95% argon) were fed into a 100 mm linear plasma head, powered by 300 W RF power (27.12 MHz). Each bare die was scanned twice using the plasma head at 1.0 mm / s. The samples were then removed from the inert gas environment, transferred to a bonding machine in air, and bonded within 30 minutes. During the plasma scanning process, the surface temperature of the indium microbump array did not exceed 70°C. The ability to remove indium oxide at such low temperatures is a significant advantage of this invention.

[0080] Figure 8The maximum shear strength of the indium-indium bonded samples is shown. All shear strength measurements were compared with control values ​​obtained using a formic acid-treated bare die and another bare die without any oxide removal treatment. A relative bond force of 1.0 is the maximum force that TCB can apply, and it gives a relative shear strength of 1.0. With formic acid reflow, if this bond force is reduced by 50%, the shear strength will decrease by 90%. In contrast, when the indium microbump array is cleaned with atmospheric pressure hydrogen and argon plasma before bonding, the bond force can be reduced by 50% or 75%, while the relative shear strength is 1.5 or 1.35 times higher than the control group. The plasma removal process is an improvement over industry-standard cleaning processes in terms of the mechanical shear strength of interconnects and the reduction of the force required to achieve bonding.

[0081] Interconnect assemblies with UBMs (Under-Bump Metal Layers) where an indium-bumped substrate is directly bonded to another substrate were also tested. The UBM material consists of stacked metal layers with gold contact layers. Figure 9 The maximum shear stress is shown for three different bonding scenarios. The control group uses initially plated indium bumps bonded to the UBM. As shown in the figure, treating the UBM is crucial for achieving a strong bond with the indium bumps. When indium is treated with plasma alone, the relative shear strength increases from 1.0 to only 1.2. When both the indium bumps and the UBM pad are treated with atmospheric pressure hydrogen and argon plasma, the relative shear strength increases from 1.0 to 6.4. Furthermore, for the latter sample, the observed failure mode is not due to interconnect bonding failure between the indium bumps and the UBM pad, but rather mechanical failure leading to the silicon chip breaking into fragments. Using plasma treatment is crucial for maximizing the bonding strength between indium and UBM. Due to the inertness of gold, pretreatment of the UBM gold surface is typically not required when bonding indium bumps to the UBM pad. However, these results show that plasma treatment is important not only for removing oxide metals from indium but also for removing organic contaminants from gold.

[0082] This embodiment of the invention offers an unexpected improvement over prior art. A novel result is that the aforementioned shear strength properties are maintained even after a prolonged shutdown, where the substrate is exposed to air for several hours prior to thermoforming. Figure 10 The relative shear strength values ​​of indium-indium bond assemblies treated with argon and hydrogen plasma and then held in air for different times before TCB are shown. The relative shear strength of the interconnect did not decrease significantly during a downtime of up to 13.5 hours.

[0083] The invention described herein utilizes a single-step hydrogen and argon plasma process to remove indium oxide coating from indium microbumps. Prior art reports indicate that if the sample is exposed to air, a nitrogen passivation surface needs to be generated to maintain improved bonding performance (Schulte, U.S. Patent No. 8,567,658, October 29, 2013). Figure 10 The results shown demonstrate that passivation of the indium bumps is unnecessary.

[0084] 4.0 Apparatus for removing metal oxides from metal contact points on substrates, bare dies, and wafers

[0085] This section describes some exemplary inert gas purging enclosures, combined with suitable plasma applicators and inert gas purging environments for electronic assemblies, particularly flip chips. As previously described, plasma is applied to prepare the electronic solder joints of integrated circuits (chips) so that interconnects can be bonded to one package or another integrated circuit.

[0086] Figure 11 An exemplary purge enclosure is shown, comprising a plasma applicator and a plasma device for treating a substrate or bare die (i.e., a chip) with atmospheric pressure hydrogen and argon plasma. A plasma head 5 is attached to a cover plate 4, which is suspended a short distance above a recessed plate 12. The short distance between the cover plate 4 and the recessed plate 12 is designed so that the plasma head 5 can scan over the substrate or bare die, but the plates do not contact each other. A suitable distance is 0.5 mm, but other distances can be used and will be apparent to those skilled in the art. The scanning is performed by attaching the plasma head 5 and the cover plate 4 to a suitable XYZ scanning automaton with an adjustable scan rate in the X and Y directions, which can be precisely selected between 0 and 1,000 mm / s. First, the plasma head 5 is moved to the left side of the recessed plate 12 so that the hydrogen and argon gas flow entering the recessed volume purges away all air, leaving a residual oxygen content of less than 500 ppm. The plasma is then activated, and plasma head 5 is moved from left to right to fully scan over the substrate or bare die, removing any metal oxides from the array of solder balls, bumps, or metal contact pads. After the process is complete, the plasma may or may not be shut down, depending on the throughput and interconnect bonding operation configuration.

[0087] Figure 12A and 12B Describing a concave plate 12, which forms a closed volume 22 in which a bare crystal 6 has been placed to be exposed to atmospheric pressure hydrogen and argon plasma. The concave plate 12 includes four walls defining the closed volume 22. The thickness of these walls is designed to be 0.1 to 5.0 mm greater than the thickness of the bare crystal 6. This fabricates... Figure 2 The small gap mentioned in the text 7.

[0088] Figure 13A and 13B The images show a top and bottom isometric view of the atmospheric pressure plasma head 5, fixed to the cover plate 4. Hydrogen and argon gas enter the plasma head 5 through gas inlet 11. The plasma head 5 has two fittings 13 through which water flows for recirculation via channels within the device. The water is typically maintained at 60°C, which helps to keep the plasma process at a constant temperature throughout operation. Other temperatures between 0 and 100°C can be used, as will be apparent to those skilled in the art. Figure 13A The plasma head 5 shown is also equipped with an optical sensor 14 to detect light emission from the plasma, thereby monitoring plasma performance. The plasma mounting plate 23 includes two holes 15 on each side of the plasma head, serving as inert gas purging inlets. The inert gas is selected from the group consisting of argon and nitrogen. Alternatively, one of the holes 15 can be used to sample the gas and monitor residual oxygen concentration. Bottom view, as shown... Figure 13B The image shows the exit slit 24 through which the activating gas flows out from the plasma head 5. Also shown is the orifice 15 for introducing inert gas purging. The exit slit 24 is at least as wide as the bare crystal, such that when the plasma head scans over the bare crystal, the entire surface is treated with activating gas from the atmospheric pressure hydrogen and argon plasma.

[0089] Figure 14 An apparatus for processing a wafer with hydrogen and argon plasma at atmospheric pressure in an inert gas environment is shown. A plasma head 5 is fixed to a large cover plate 4 suspended above a wafer holder 17. The wafer holder 17 contains a concave circular plate 16 on which a wafer is inserted. The cover plate 4 must be at least twice the diameter of the wafer so that the plasma head 5 can scan a distance in the X and Y directions sufficient to ensure the wafer is completely scanned by the plasma while maintaining the inert gas environment. For example, if the diameter of the silicon wafer containing integrated circuits is 300 mm, the cover plate 4 needs to be at least 600 mm wide × 600 mm long.

[0090] Figure 15 A layered cross-section of the apparatus used for processing wafers is shown. The plasma head 5 is mounted inside the cover plate 4 such that the outlet slit from which the activating gas flows is flush with the bottom of the cover plate 4. The concave circular plate 16 in the wafer holder 17 can be clearly seen in this cross-section.

[0091] Figure 16 This shows a cross-sectional view of an apparatus for processing a wafer. In this figure, a silicon wafer 18 has been placed on a concave circular plate on a wafer holder 17. The depth of the concave circular plate is substantially the same as the thickness of the wafer, such that the surface of the wafer holder is at the same height as the surface of the wafer.

[0092] Figure 17This shows a side view of an apparatus for processing wafers with hydrogen and argon plasma at atmospheric pressure in an inert gas environment. The wafer holder 17 is configured with a gas inlet 19 for allowing inert gas to flow into the purge volume 20. The purge volume 20 is defined by a small gap between the wafer 18 and the cover plate 4. This small gap... Figure 2 The gap 7 indicated in the text is the same. Figure 18 This is a side view of the device, magnified to better show the inert purge airflow 25 above the wafer. Arrows indicate the direction in which the inert purge airflow 25 enters through inlet 19 and flows from left to right through the gap and over wafer 18. Cover plate 4 and plasma head 5 are suspended above wafer holder 17 and can scan above wafer 18 without contacting the walls of wafer holder 17. Other suitable designs, obvious to those skilled in the art, are conceivable, and embodiments of the invention can be achieved by minimizing the total purge volume 20 while allowing air to be purged away quickly and efficiently by the inert airflow.

[0093] Figure 19 Further explanation of the wafer holder 17 is provided. This top view shows a wafer 18 mounted on the wafer holder 17, with some additional space between the wafer 18 and the wall 26. This additional space is to ensure that the purge gas flow 25 over the wafer 18 is not disturbed. The purge gas enters the purge volume through a series of orifices 27 distributed on the back side of the wafer holder 17. These orifices are configured to generate a uniform inert gas flow rate across the diameter of the wafer 18. The gas flows from left to right and exits the wafer holder 17 through the toothed wall 21 on the right side.

[0094] Figure 20 This is a schematic diagram illustrating the path of plasma head 5 scanning over wafer 18. For illustrative purposes, it is assumed that the wafer diameter is 300 mm and the plasma head is configured to generate a 100 mm wide linear beam of activating gas. Initially, the plasma head is located at the upper left corner of the wafer holder. A purge flow is activated to remove air and create an inert gas environment, wherein the inert gas is selected from the group consisting of argon and nitrogen. Next, in step 1, the plasma is activated and scans the wafer along the left third of the wafer surface. The scan rate ranges from 0 to 1,000 mm / s and is determined by the exposure time required to remove oxidation from the surface of the metal contact points. In step 2, the plasma head moves to the center of the wafer holder. Then, in step 3, the plasma head scans along the middle third of the wafer. In step 4, the plasma head moves to the right side of the wafer holder, and finally, in step 5, the plasma head scans along the right third of the wafer. Through this process, the entire wafer is uniformly treated with atmospheric pressure hydrogen and argon plasma. Many other scan paths can be designed, and will be apparent to those skilled in the art.

[0095] The description of the preferred embodiments of the invention concludes here. The foregoing description, including the preferred embodiments of the invention, is presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible within the scope of the foregoing teachings. Additional variations of the invention may be devised without departing from the inventive concept set forth in the following claims.

Claims

1. A method for joining interconnects, comprising the following steps: Plasma containing activated hydrogen is applied to at least one first metal contact point supported on a first substrate by an atmospheric pressure plasma applicator to remove oxidation and create a new deoxidized metal surface on the first metal contact point. The first metal contact point is joined to the second metal contact point by applying mechanical force, heat or both to the first metal contact point that is in contact with a second metal contact point supported on a second substrate. and An inert gas environment is formed, in which the plasma is applied and the first metal contact point and the second metal contact point are joined. The engagement of the first metal contact point and the second metal contact point forms a metal interconnect between the first metal contact point and the second metal contact point.

2. The method of claim 1, wherein the inert gas environment contains less than 500 parts per million (ppm) of oxygen.

3. The method of claim 1, wherein the inert gas environment is generated by mounting the atmospheric pressure plasma applicator, the first substrate supporting the first metal contact point, and the second substrate supporting the second metal contact point within a volume, and purging the volume with an inert gas flow, wherein the inert gas flow replaces the initial air inside the volume.

4. The method of claim 2, wherein the inert gas flow is selected from the group consisting of argon and nitrogen.

5. The method of claim 1, wherein the plasma is applied by sweeping the atmospheric pressure plasma applicator across the first substrate supporting the first metal contact point.

6. The method of claim 1, wherein the activated hydrogen is formed by supplying argon gas having a hydrogen concentration of 0.1% to 2.0% to the plasma.

7. The method of claim 1, wherein the contact between the first metal contact point and the second metal contact point for engagement occurs at a temperature between 20 and 250°C.

8. The method of claim 1, wherein the first metal contact and the second metal contact each comprise a metal or metal alloy selected from the group consisting of: cobalt, nickel, copper, rhodium, palladium, silver, iridium, platinum, gold, indium, tin, antimony, lead, and bismuth.

9. The method of claim 1, wherein the first metal contact point comprises a two-dimensional array of metal bumps, the center-to-center distance between the metal bumps being 5 to 100 micrometers.

10. The method of claim 1, wherein the first metal contact point remains in the inert gas environment throughout the application of the plasma and its bonding to the second metal contact point.

11. The method of claim 1, wherein the plasma containing the activated hydrogen is also applied to the second metal contact point supported on the second substrate by the atmospheric pressure plasma applicator, and the second substrate is held in the inert gas environment.

12. The method of claim 1, wherein the first metal contact and the second metal contact comprise a flip-chip assembly.

13. The method of claim 1, wherein the first metal contact point and the second metal contact point each comprise a two-dimensional array of metal bumps, the center-to-center distance between the metal bumps being 5 to 100 micrometers, and joining the first metal contact point to the second metal contact point comprises aligning and joining the metal bumps of the first metal contact point with the metal bumps of the second metal contact point.

14. The method of claim 13, wherein the two-dimensional array of the first metal contact points comprises an integrated circuit.

15. The method of claim 13, wherein the two-dimensional array of the second metal contact points comprises a bump under-metallization layer containing metal pads.

16. The method of claim 1, wherein the atmospheric pressure plasma applicator comprises an atmospheric pressure plasma head fixed to a cover plate having a slotted channel for the plasma to pass through to the first metal contact point supported by the first substrate.

17. The method of claim 16, wherein the cover plate includes at least one purge hole through which an inert gas passes to form the inert gas environment.

18. The method of claim 16, wherein the cover plate includes at least one sampling hole passing through it.

19. The method of claim 16, wherein the plasma is applied by scanning the slotted channel through the first metal contact point on the first substrate.

20. The method of claim 16, wherein the cover plate encloses a volume in which the plasma is applied to the first contact point.

21. The method of claim 20, wherein the volume includes a height of no more than 10 mm from the slotted channel.

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