Oxidation removal from metal contacts using atmospheric pressure plasma for improved electrical and mechanical coupling.

Abstract

A method and apparatus for plasma treatment of electrical interconnections of electronic assemblies for integrated circuits such as flip chips are disclosed. The interconnection coupling method includes applying a plasma containing active hydrogen via an atmospheric pressure plasma applicator to metal contacts supported on a substrate to remove oxidation and create a newly deoxidized metal surface. The metal contacts can be coupled to each other by reflow or thermocompression bonding. An inert gas environment may be formed in a sealed space for applying the plasma and / or coupling the metal contacts.

Description

[Technical Field]

[0001] Background of the Invention 1. Field of Invention The present invention relates to a plasma apparatus for removing oxidation from copper, tin, tin-silver alloys, and indium for metal-metal bonding, and to a method of using the plasma apparatus. More specifically, the present invention relates to the reduction of copper oxide, tin oxide, tin-silver oxide, and indium oxide for improving mechanical and electrical coupling in electronic assemblies such as flip chips.

[0002] 2. Description of Prior Art Ionized gas plasmas have been recognized as having a wide range of applications in materials processing. Plasmas used in materials processing are generally weakly ionized, meaning that only a small fraction of the molecules in the gas are charged, and this is balanced by free electrons so that the sum of the charges is zero. In addition to ions and free electrons, these plasmas contain reactive species that can clean, activate, etch, and deposit thin films on surfaces. The temperature of these weakly ionized gases is usually below 250°C, so heat-sensitive substrates are not damaged. The physics and chemistry of weakly ionized plasmas are described in several textbooks. For example, see Lieberman and Lichtenberg, "Principles of Plasma Discharges and Materials Processing" (John Wiley & Sons, Inc., New York, 1994), and Raizer, YP, "Gas Discharge Physics" (Springer-Verlag, Berlin (1991)).

[0003] According to the literature, weakly ionizing plasma is generated in a vacuum at a pressure of 0.001–1.0 Torre (see Lieberman and Lichtenberg (1994)). Power is applied across two electrodes to decompose and ionize the gas. The electricity can be supplied as direct current (DC), alternating current (AC), radio frequency (RF) power, or microwave (MW) power. The electrodes can be constructed to provide either capacitive or inductive coupling to generate and maintain the plasma. In the former case, two conductive electrodes are placed inside a vacuum chamber filled with a small amount of gas. One electrode is powered or biased by an RF generator, and the other is grounded. In the latter case, the RF power is supplied through an antenna wound in a coil around the insulating wall of the chamber. The oscillating electric field from the coil penetrates the gas, inducing ionization.

[0004] Over the past 25 years, atmospheric pressure plasmas have been developed as an alternative to vacuum plasmas. These plasmas can process objects of any size and shape because they do not require loading into a vacuum chamber. This significantly reduces process costs. Various atmospheric pressure plasma devices have been developed (Schutze, et al., "The atmospheric-pressure plasma jet: A review and comparison to other plasma sources," IEEE Trans.Plasma Sci.26, 1685-1694 (1998)). The effectiveness of these plasmas depends on how the ionization process is controlled. At atmospheric pressure, the extremely high gas density makes ionization reactions prone to runaway reactions and the generation of high-temperature arcs, which are often unsuitable for material processing.

[0005] There are typically three types of atmospheric pressure plasmas. These include dielectric barrier discharge (DBD), torch, and RF rare gas discharge. DBD has long been used to process rolls of plastic film in which the material passes continuously between electrodes. In some examples, DBD can be deployed as a downstream device, allowing 3D objects to be processed with reactive gases flowing out from between the electrodes. Torch and RF rare gas discharge are used to process substrates placed downstream. In some cases, ions and electrons flow out of the torch and come into contact with the substrate. In contrast, with rare gas discharge, ions and electrons are confined in the gap between the electrodes, so the substrate is exposed to a beam of neutral reactive species (see, e.g., Cheng, et al., U.S. Patent No. 9,406,485 (August 2, 2016)). A robot is used to scan the plasma beam over the substrate surface, positioned less than 1 centimeter below the plasma housing. In contrast to vacuum plasma, only the area on the sample surface that needs to be processed is exposed to the reactive gas species.

[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 10kV power supply operating at approximately 20kHz provides the voltage necessary to decompose the gas. A dielectric barrier covers one of the electrodes, preventing the formation of a high-current arc. During operation, charge accumulates on the surface of the insulator and discharges as small "microarcs" within each AC cycle. Micro-discharges occur randomly in space and time and last for a period of 10-100ns. Inside the micro-discharge, the electron density is high, but outside it is extremely low. As a result, it is not possible to measure the average electron density and electron temperature of the entire gas volume between the electrodes. It should be noted that substrates placed downstream of DBD will not be uniformly processed by the plasma at the microscale. Furthermore, the discharge may electrically interact with the substrate, making it difficult to process components containing metal.

[0007] A torch is generated by forming an arc between closely spaced energized electrodes and a ground electrode. This structure is described by Faucais and Vardelle in the paper “Thermal Plasmas,” IEEE Transactions on Plasma Science, 25, 1258-1280 (1997). Air passes between the electrodes and is ionized by applying 10 kV AC power. The arc is a thermal plasma with a neutral temperature of several thousand degrees. Nevertheless, it is possible to blow the gas through the arc at a speed fast enough so that the overall gas temperature is low enough to process heat-sensitive materials, including polymers. Plasmaflume™ by PlasmaTreat is an example of this type of structure. It maintains an average neutral temperature below 700 K by utilizing a rotating conical electrode that rapidly rotates the arc through a flowing gas volume. The plasma streamer, containing ions and electrons, shoots out from the edge of the housing and processes an object placed a short distance below.

[0008] Atmospheric pressure noble gas plasmas driven by high-frequency power of 13.56 or 27.12 MHz behave differently from DBD and torch plasmas. These plasmas are weakly ionized capacitive discharges (Jeong et al., "Etching Materials with an Atmospheric-Pressure Plasma Jet," Plasma Sources Science Technol., 7, 282-285 (1998), Babayan et al., "Deposition of Silicon Dioxide 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 See 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, and a collision sheath is formed at the boundary to repel electrons and maintain the plasma. The mean electron density and temperature in the RF, noble gas plasma are 10, respectively. 11 ~10 12 cm -3 The voltage is determined to be 1-2 eV. Depending on the RF power level, the neutral gas temperature range is 323-573 K. The molecular gas is supplied as helium or argon at a concentration of 0.1-5.0 volume%.

[0009] Semiconductor materials such as silicon are processed in a vacuum plasma by inserting them into a chamber equipped with energizing and grounding electrodes, applying power to the electrodes to generate a discharge, and then operating the plasma for several minutes to modify the material surface. Silicon substrates are supplied as thin wafers of modest sizes with diameters of 100, 150, 200, and 300 mm. The vacuum chamber is specially designed to fit wafers of a given size. One method of cleaning the silicon surface is to supply oxygen and argon to the chamber. Free electrons with energy in the plasma convert some of the oxygen molecules into oxygen atoms and other reactive species, which attack organic contaminants on the surface and convert them into gaseous carbon dioxide. Furthermore, the surface can be physically sputtered of contaminants by impacting the substrate with positively charged argon ions (Ar+). After several minutes of oxygen plasma treatment in the vacuum chamber, the silicon wafer surface is clean and activated for other semiconductor processing steps.

[0010] Another method for cleaning the surface of semiconductor materials involves placing the substrate in a vacuum chamber and generating a plasma with a supply gas containing a mixture of hydrogen and argon. In this case, energetic free electrons generate H atoms and Ar+ ions, which collide with the substrate surface and remove metal oxide contaminants, including but not limited to copper oxide, silver oxide, tin oxide, or indium oxide. To prevent safety hazards from introducing a flammable hydrogen mixture into the chamber, the supply gas to the plasma must contain less than 4.0% H2 in argon. At operating pressures of 0.05–0.50 Torre in the vacuum chamber, the resulting hydrogen concentration is only 2–20 millitorre. Therefore, the supply of active hydrogen atoms, and consequently the rate of etching of metal oxides by hydrogen atoms, is extremely low, and most of the oxide contaminants must be removed by sputtering.

[0011] Another drawback of vacuum plasma treatment of materials is that repeated processing of silicon wafers contaminates the chamber. The chamber slowly fills with particles (i.e., contaminants) ranging in size from 0.01 to 10.0 microns in diameter. This issue has been documented in numerous publications (see, for example, GSSelwyn, et al., J.Vac.Sci.Technol.A7,2758(1989), ibid.,8,1726(1990), MJMcCaughey and MJKushner, Appl.Phys.Lett.55,951(1989), RNNowlin and RN Carlile, J.Vac.Sci.Technol.A9,2824(1990), GSSelwyn, Jpn.J.Appl.Phys.32,3068(1993), and SJChoi, et al., Plasma Sources Sci.Technol.4,418(1994), and RLMerlino and JAGoree, Physics Today 1(July 2004)). When the plasma is turned on, particles become negatively charged and, as a result of the electric field in the chamber, suspend on the wafer surface. When the plasma is turned off, the particles jump down onto the wafer, forming a layer of contaminants. It is well known to those skilled in the art that particles present during wafer processing can kill solid devices. The semiconductor industry is obsessed with eliminating them, spending billions of dollars to build cleanrooms free of particles larger than 0.01 microns in diameter. To reduce particle contamination, contaminated plasma chambers must be cleaned regularly. Furthermore, wafers may need to be wet-cleaned after plasma immersion to remove particles adhering to them. All of this drives up the manufacturing costs of semiconductor devices.

[0012] A manufacturing process that greatly benefits from the removal of metal oxides via hydrogen plasma is flip-chip interconnect coupling. In this process, one chip having a two-dimensional array of solder balls or bumps, with the distance between the centers of the balls or bumps being less than 200 microns, is placed on another chip or substrate having a two-dimensional array of multiple metal pads or bumps having the same spatial arrangement. The chips are then heated and pressed together to form intermetallic interconnects between each ball or bump (JHLau, "Semiconductor Advanced Packaging," Springer). (Nature, Singapore, 2021). To ensure ohmic contact that is mechanically strong and exhibits minimal electrical resistance, the oxide film on the metal surface must be removed before bonding. Oxidation of the metal on the balls or bumps is usually removed by incorporating an organic flux into the solder. During reflow, the organic flux reacts with the metal oxide to form a compound, which sublimes from the surface, leaving the base metal. However, the flux generates organic residues during reflow that must be removed by a cleaning step. If the distance or pitch between the balls or bumps is less than 50 microns, cleaning can no longer remove these organic residues. In this case, hydrogen plasma removal of the metal oxide would be a good alternative to flux. However, using vacuum plasma for this process is inefficient because, as mentioned above, the hydrogen radical concentration is too low for the etching rate to be fast enough, and sputtering must be used instead. Sputtering is not a self-limiting process and can be prone to damaging semiconductor materials. Furthermore, the flip-chip with the ball grid array must be transferred from vacuum to a thermal compression bonder, and the metal balls may re-oxidize before they can be bonded together.

[0013] Using atmospheric pressure noble gas plasma supplied with hydrogen, oxidation can be removed from metals, such as copper oxide from copper and indium oxide from indium (see, e.g., 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 before flip-chip bonding. The advantage of atmospheric pressure hydrogen and noble gas plasma is that the concentration of hydrogen radicals is much higher than in a vacuum, preventing sputtering that can damage semiconductor devices. Nevertheless, metal balls or bumps can re-oxidize during transfer from the plasma to the thermal compression bonder. Schulte (U.S. Patent No. 8,567,658 (October 29, 2013)) teaches that a passivation step using an "activated chemical passivator" is necessary to prevent re-oxidation during the transition from the plasma processing step to the bonding step. Depending on the combination of metallic elements used in the ball or microbump array, the passivation step may or may not be effective in achieving mechanically strong metallic interconnections with minimal electrical resistance.

[0014] Considering the above, there is a need for an apparatus and method capable of rapidly removing oxidation from a metal interconnect array using hydrogen plasma at atmospheric pressure, and then transferring the deoxidized metal balls or bumps onto a chip and / or substrate to a bonder, where the metal balls or bumps are bonded to form a high-density interconnect array. Furthermore, this apparatus and method must be suitable for material processing and must not damage the semiconductor chip through ion bombardment, electrostatic discharge, particle contamination, or exposure to ultraviolet light. In particular, there is a need for an apparatus and method that allows for the removal of copper, silver, tin, and indium oxides from an array of metal balls or bumps, enabling the formation of mechanically strong, conductive metal interconnects by subsequent thermocompression bonding. Such an apparatus and method has a particular application in flip-chip bonding with a two-dimensional array of dense interconnects. These and other needs are met by embodiments of the present invention described below. [Overview of the Initiative]

[0015] A method and apparatus for plasma treatment and coupling of electrical interconnects for integrated circuit packages such as flip chips are disclosed. The interconnect coupling method includes applying a plasma containing active hydrogen to a first metal contact supported on a substrate via an atmospheric pressure plasma applicator to remove oxidation and create a newly deoxidized metal surface. The first metal contact is then coupled to a second metal contact by applying mechanical force, heat, or both, such as thermocompression bonding (TCB). An inert gas environment is formed within the housing for applying the plasma, and a second inert gas environment is formed within the housing for coupling the first metal contact to the second metal contact. Substrates for the first and second metal contacts may be selected from the group consisting of, but are not limited to, semiconductor dies (also known as integrated circuits or chips), semiconductor wafers such as 300 mm silicon wafers having integrated circuits, organic or plastic substrates, ceramic substrates, lead frames, and printed circuit boards (PCBs).

[0016] In another embodiment of the present invention, the application of plasma is carried out in a housing where an inert gas environment is maintained throughout the process in order to remove oxidation from a first metal contact supported on a substrate and to couple the first metal contact on the first substrate to a second metal contact on a second substrate. The substrate may be selected from the group consisting of, but are not limited to, semiconductor dies, semiconductor wafers, plastic substrates, ceramic substrates, lead frames, and printed circuit boards (PCBs).

[0017] In another embodiment of the present invention, the interconnection coupling method includes applying a plasma containing active hydrogen via an atmospheric pressure plasma applicator to a first metal contact supported on a substrate and a second metal contact supported on a substrate to remove oxidation from both metal contacts and create a newly deoxidized metal surface. The first metal contact is then coupled to the second metal contact by applying mechanical force, heat, or both, for example, by a TCB. An inert gas environment is formed within the housing to apply the plasma to both metal contacts and to couple the first metal contact to the second metal contact.

[0018] A typical embodiment of the present invention includes a method for interconnecting a first metal contact to a second metal contact supported on a second metal contact via an atmospheric pressure plasma applicator, the steps being to remove oxidation and create a newly deoxidized metal surface on the first metal contact; to connect the first metal contact to a second metal contact supported on a second substrate, thereby applying mechanical force, heat, or both to the first metal contact by bringing it into contact with the second metal contact; and to create an inert gas environment in which the application of the plasma is carried out and the coupling of the first metal contact to the second metal contact is carried out, wherein coupling the first metal contact to the second metal contact forms a metal interconnect between the first metal contact and the second metal contact.

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

[0020] In further embodiments, 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, and the second substrate supporting the second metal contact inside a volume to be purged by an inert gas flow, where the air inside the volume is first displaced by the inert gas flow. The inert gas flow may be selected from the group including argon and nitrogen.

[0021] In other embodiments, the application of plasma may be performed by passing the atmospheric pressure plasma applicator through the entire first substrate supporting the first metal contact. Furthermore, active hydrogen may be formed by supplying hydrogen gas to the plasma at a concentration of 0.1–2.0% in argon gas. Contact of the first metal contact with the second metal contact for bonding may occur at a temperature of 20–250°C. Furthermore, the first and second metal contacts may each contain a metal or metal alloy selected from the group including 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 include a two-dimensional array of metal bumps with a center-to-center distance between the metal bumps of 2 to 100 microns. During plasma application and coupling of the first metal contact to the second metal contact on the second substrate, the first substrate can be kept separately in an inert gas environment. In another embodiment, the first and second substrates can be kept in an inert gas environment through plasma application and coupling of the first metal contact on the first substrate to the second metal contact on the second substrate. A plasma containing active hydrogen can also be applied to the second metal contact on the second substrate via an atmospheric pressure plasma applicator, and the second substrate can be kept in an inert gas environment through plasma application and coupling of the first metal contact to the second metal contact. In a typical embodiment, the first metal contact on the first substrate and the second metal contact on the second substrate may include a flip-chip assembly.

[0023] In further embodiments, a first metal contact on a first substrate and a second metal contact on a second substrate may each include a two-dimensional array of metal bumps with a center-to-center distance between the metal bumps of 2 to 100 microns, and coupling the first metal contact to the second metal contact includes aligning and coupling the metal bumps of the first metal contact with the metal bumps of the second metal contact. Furthermore, the two-dimensional array of the second metal contact may include underbump metallization including metal pads.

[0024] In one embodiment, the atmospheric pressure plasma applicator may include an atmospheric pressure plasma head attached to a cover plate having a slotted passage for the plasma to pass to a first metal contact supported by a first substrate. The cover plate may include at least one purge hole for an inert gas to pass therethrough to form an inert gas environment. The cover plate may include at least one sampling hole through the cover plate to measure and confirm that the oxygen concentration is less than 500 ppm. Application of the plasma may be performed by scanning the slotted passage across the first metal contact on the first substrate. The cover plate can be disposed above a sealed volume into which the first substrate is inserted, and the distance from the top of the substrate to the slotted passage is 10 millimeters or less. It should be apparent to those skilled in the art that the sealed volume should be sufficient to accommodate the substrate, but should not be larger than the volume required for the plasma head attached to the cover plate to scan across the substrate surface at a distance of 0.5 to 10.0 millimeters. After processing the metal contact on the first substrate, the apparatus embodied in the present invention moves the first substrate onto a second substrate, aligns the metal contact on the first substrate with the metal contact on the second substrate, and joins them together by applying mechanical force, heat, or both, to form metal interconnects in all steps performed in an inert environment having an oxygen concentration of less than 500 ppm.

[0025] In another embodiment, the atmospheric pressure plasma applicator may include an atmospheric pressure plasma head attached to the bottom of the sealed volume with the plasma beam directed upward through a slotted passage. The plasma exit gas flows through the slotted passage and lightly touches the first metal contact on the first substrate. In this case, the first substrate is attached to a bond head attached to the cover plate. The first substrate protrudes into the sealed volume at a distance of 10 millimeters or less, preferably between 1 and 4 millimeters, from the slotted passage. The apparatus embodied in the present invention passes through the first substrate between plasma beams containing active hydrogen, removes oxidation from the first metal contact, then aligns the first metal contact on the first substrate with the second metal contact on the second substrate, and joins them together by applying mechanical force, heat, or both, to form metal interconnects in all steps performed in an inert gas environment having an oxygen concentration of less than 500 ppm.

Brief Description of the Drawings

[0026] Referring now to the drawings, like reference numerals represent corresponding parts throughout. [Figure 1] An exemplary plasma process for removing a metal oxide film from a metal is shown. [Figure 2] [[ID=十三]]A schematic diagram of an exemplary apparatus for exposing a substrate to active gases from atmospheric pressure hydrogen and argon plasma with air removed from the processing area by local inert gas purge. [Figure 3A] A view of a tin solder bump before removing the surface tin oxide with atmospheric pressure hydrogen and argon plasma. [[ID=十七]] [Figure 3B] A view of a tin solder bump after removing tin oxide with atmospheric pressure hydrogen and argon plasma. [Figure 4A] An X-ray photoelectron spectrum of the Sn 3d peak of the oxidized tin surface. [Figure 4B] An X-ray photoelectron spectrum of the Sn 3d peak of tin scanned with atmospheric pressure hydrogen and argon plasma. [Figure 4C]This is the X-ray photoelectron spectrum of the tin 3d peak on a tin surface scanned with atmospheric pressure hydrogen and argon plasma, and then remaining in air at room temperature for 30 hours. [Figure 5A] This image shows an optical microscope image of a tin solder bump after heating to 250°C in an inert gas environment. [Figure 5B] The images show optical microscope images of tin solder bumps after heating to 250°C in an inert gas environment and then exposing the inert gas environment to atmospheric pressure hydrogen and argon plasma at 250°C. [Figure 6A] It exhibits weak metal-metal bonding resulting from a thin tin oxide film that forms rapidly when air exposure is permitted between plasma processing and reflow bonding. [Figure 6B] This demonstrates robust metal-to-metal bonding when there is no air exposure between treatment with atmospheric pressure hydrogen and argon plasma and the reflow bonding of the microbump array to the copper substrate. [Figure 7A] This is an optical microscope image of an indium bump after heating to 165°C in an inert gas environment. [Figure 7B] This is an optical microscope image of an indium bump after treatment with hydrogen and argon plasma at atmospheric pressure at 165°C in an inert gas environment. [Figure 8] This study compares the relative bonding force and relative shear strength applied to a two-dimensional indium bump array bonded by thermal compression at room temperature. [Figure 9] The relative shear strengths of indium-UBM bonded assemblies with no plasma treatment (control), hydrogen plasma treatment of indium bumps only, and hydrogen plasma treatment of both indium bumps and UBMs are shown. [Figure 10] This shows the relative shear strength of indium vs. indium microbump assemblies when exposed to air for different durations between hydrogen plasma cleaning and thermocompression bonding. [Figure 11] This is a schematic diagram of an apparatus for atmospheric pressure hydrogen and argon plasma treatment of a substrate or die held in an inert gas purging environment. [Figure 12]Figures A and B show top and isometric views, respectively, of a concave volume for mounting a die within a device for atmospheric pressure hydrogen and argon plasma processing in an inert gas environment. [Figure 13] A and B show isometric top and bottom views, respectively, of the assembled cover plate and plasma head. [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. [Figure 15] This diagram shows a hierarchical cross-sectional view of an apparatus for treating wafers with atmospheric pressure hydrogen and argon plasma in an inert gas purging environment. [Figure 16] A cutaway diagram of the apparatus for atmospheric pressure hydrogen and argon plasma processing is shown, in which the silicon wafer is mounted inside an inert gas purging environment. [Figure 17] This shows a side view of an apparatus for atmospheric pressure hydrogen and argon plasma processing, in which the silicon wafer is mounted inside an inert gas purging environment. [Figure 18] This image shows an enlarged side view of the apparatus for atmospheric pressure hydrogen and argon plasma processing of wafers, with the inert gas purge flow rate highlighted. [Figure 19] The images show a top view and a side view of a wafer holder, in which an inert purge gas flow enters from the left and flows uniformly from left to right across the wafer. [Figure 20] A 100mm linear plasma head and cover plate are moved to show a path that completely scans a 300mm silicon wafer. Detailed description including preferred embodiments

[0027] 1.0 Overview The embodiments described below are for illustrative purposes only and illustrate examples of how to carry out the present invention. Those skilled in the art will understand how the described embodiments can be readily applied to a wide range of other applications. In particular, the atmospheric pressure plasma deoxidation process and apparatus described can be applied to semiconductor packaging applications, including but not limited to flip-chip coupling. As is known in the art, flip-chip coupling offers advantages compared to other interconnection processes. This method uses the entire area of ​​the die rather than making connections only around the periphery of the integrated circuit. Thus, flip-chip coupling provides higher density interconnections, and the interconnection paths are shorter compared to wire bonds, which enables faster device speeds. An efficient, cost-effective integrated circuit manufacturing process that can be used to improve the quality and performance of interconnections such as those described herein has significant value to electronic equipment production.

[0028] Typically, all bonding in a flip-chip package is completed in a single process rather than making individual connections sequentially. Solder bumps for flip-chip bonding are typically made of a metal with a low melting point, often indium, tin, or other suitable alloys. After the chips are aligned and positioned, the electrical connections are made by heating the solder above its melting point, or by using thermocompression bonding (TCB) at a temperature below its melting point. The presence of metal oxides on the solder bump surface hinders both such techniques, either by preventing complete melting of the solder bump in the case of reflow, or by increasing the force required to break down the surface oxides of the TCB.

[0029] Organic fluxes are often used to remove metal oxides when making solder bump connections. However, fluxes leave corrosive residues that can damage nearby components or cause short circuits between interconnects. Therefore, these residues must be washed away after bonding. As solder bumps tend to become smaller and the pitch size between bumps decreases, removing solder residue has become a major obstacle due to the difficulty of introducing wet chemical solutions into the microscopic volume between the flip chip and the substrate. In particular, high surface tension prevents the liquid from penetrating these narrow spaces.

[0030] Flip-chip bonding without embedded organic flux in the solder is becoming an attractive option for smaller pitch sizes. One such example is the removal of metal oxides from the solder bump surface by contact with formic acid vapor. Here, the formic acid and metal oxide react with each other to form a metal formate that sublimates from the surface. Unfortunately, this process requires temperatures above 200°C to achieve a significant reaction rate. Furthermore, treatment with formic acid vapor may still leave organic residues that can corrode the solder surface and other exposed metals during the reaction.

[0031] The atmospheric pressure plasma process embodied herein uses activated hydrogen gas to react with metal oxides, converting them into water vapor emanating from the metal and the system. This process can occur at lower temperatures than the formic acid vapor process described above. Furthermore, no organic residues are produced, and no corrosion occurs on any metal components within the integrated circuit. By carrying out the process in an inert gas environment of argon or nitrogen, the metal oxides are removed, and no further oxidation occurs before the metal contacts bond to each other to form interconnects.

[0032] 2.0 Apparatus and method for removing surface oxides from tin and tin / silver alloys Figure 1 shows a schematic diagram of an exemplary metal oxide removal process. Hydrogen molecules at a concentration of 0.1–2.0 volume percent are supplied in argon to an atmospheric pressure plasma head driven by, for example, a high-frequency power of 27.12 MHz. The high-frequency power ionizes the gas inside the head, generating positive and negative ions, as well as free electrons. The free electrons collide with H2 molecules, which dissociate into radicals H· as shown in step (1). The hydrogen radicals emanate from the plasma device and come into contact with the metal oxide on the ball or bump of the substrate in step (2). In step (3), the hydrogen radicals react with oxygen contained in the metal oxide to produce water, H2O, leaving the metal behind. For tin oxide or tin-silver alloys, the hydrogen radicals react with tin oxide, SnO or SnO2, to produce tin (Sn) metal and H2O according to the stoichiometry of the reaction. Any silver oxide present is also reduced to metallic silver. Furthermore, if residual organic contaminants are present on the surface, these can be rapidly removed by hydrogen radicals. The time required to expose the metal contacts on the substrate to reactive hydrogen species can be 0.01 seconds to 1.0 minute, and is generally in the range of 0.1 to 3.0 seconds. Alternatively, during contact of the metal contacts on the substrate, which may be a chip or wafer, atmospheric pressure plasma may be scanned over the substrate, die, or wafer to ensure uniform contact between the metal oxide surface on the ball or bump and the hydrogen radicals.

[0033] Hydrogen gas is particularly suitable for metal oxide removal processes. However, other hydrogen-containing molecules, such as ammonia (NH3) or hydrogen sulfide (H2S), may also be used, as will be apparent to those skilled in the art. Suitable atmospheric pressure plasmas for embodiments of the present invention include those that generate high concentrations of ground-state atoms, radicals, or metastable molecules containing active hydrogen downstream of the plasma head.

[0034] Figure 2 shows a schematic diagram of an exemplary apparatus for removing metal oxides by combining an inert gas purging environment with an active hydrogen gas from a plasma. The apparatus consists of a cover plate 4 directly mounted to a plasma head 5. A supply gas containing argon and 0.1–2.0% hydrogen enters the plasma head 5 at an inlet 11 and is converted into a weakly ionized plasma, generating free electrons, ions, and an active gas containing at least a suitable concentration of hydrogen radicals. The active gas 8 flows out from the plasma head 5 and comes into contact with the substrate 6. The cover plate 4 and the plasma head 5 are coplanar with each other and mounted on the substrate 6, maintaining a small gap 7 between the plasma head 5 and the substrate 6. Air is rapidly displaced from the gas volume in the small gap 7 by the active gas flow. A suitable gap is 0.1–5.0 mm, preferably 0.1–2.0 mm. Additional purge gas may be introduced through purge holes 9 and 10 in the cover plate 4. The purge gas is selected from the group of inert gases, argon, and nitrogen. The purge gas assists the active gas in displacing air from the gas volume within the gap 7, thereby maintaining an inert gas environment while the active hydrogen removes metal oxides from the metal junctions. An inert gas environment is defined as an environment where the oxygen concentration is sufficiently low that it does not cause re-oxidation of the metal junctions after exposure to the active hydrogen generated by the plasma process. An example of an inert gas environment is when the oxygen concentration is less than 500 ppm, preferably less than 100 ppm. Exemplary purge holes 9 and 10 may include elongated slits or one or more holes through which the purge gas flows around the active gas 8.

[0035] A slight positive pressure is established within the gap 7 so that there is sufficient gas flow to expel all the air from around the cover plate 4. This allows the plasma reduction process to be carried out in an inert gas environment with an oxygen concentration of less than 500 ppm. The low-oxygen environment promotes the removal of metal oxides and prevents re-oxidation of the metal, especially when the substrate 6 is heated.

[0036] In practice, the substrate to be processed may be transported through the apparatus while the air surrounding the substrate is continuously replaced, maintaining a low-oxygen environment before the subsequent reflow or compression bonding operation. Apparatus including atmospheric pressure hydrogen and argon plasma may be operated as a separate unit for removing tin oxide from tin or tin / silver alloy bumps. After removal of tin and / or silver oxide, the substrate is transferred to the bonding operation under ambient conditions. In another preferred embodiment of the present invention, apparatus including a plasma head is integrated with the bonding operation so that the substrate can be kept in an inert gas environment during plasma treatment, transfer to the bonding area, and bonding of the metal contacts.

[0037] Figures 3A and 3B show optical microscope images of tin solder bumps before and after the removal of plasma oxides, respectively. Image 3A clearly shows a solder bump covered with tin oxide, as is evident from its yellow color and rough appearance. After removing the tin oxide from the atmospheric plasma using activated hydrogen, the solder bump is smooth and has a highly reflective metallic silver color.

[0038] An exemplary embodiment of the present invention is the processing of a tin metal film using atmospheric pressure argon and hydrogen plasma in a purged environment. A thin native oxide film is present on the surface of the tin film. The atmospheric plasma head is attached to a cover plate with a purge gas, as shown in Figure 2. The head and cover plate are then attached to a scanning robot to allow the active gas to pass over the sample without air. In this embodiment, a linear plasma source is used, through which the active hydrogen gas flows out of a 100 mm wide slit. The distance between the outlet slit and the tin surface is approximately 1.0 mm. The plasma source scans the substrate at room temperature at different scanning speeds ranging from 0.25 to 10.0 mm / second. The plasma source is supplied with 40.0 liters (LPM) of argon and 0.40 LPM of hydrogen per minute. 500 W (27.12 MHz) of high-frequency power is supplied to the plasma head to generate and maintain a gas discharge.

[0039] Figures 4A, 4B, and 4C show the Sn 3d on the surface of the tin film after the formation of native oxide, after hydrogen plasma reduction of the native oxide, and 30 hours after aging the native oxide in air following plasma reduction. 3 / 2 and 3D 5 / 2 The peak X-ray photoelectron spectra are shown in Figures 4B and 4C. 5 / 2 The peak has been restored to its original shape to show the presence of two distinct peaks at 486.7 eV and 485.0 eV. Since the peaks for tin oxide, SnO and SnO2, are very close together, the single broad peak at 486.7 eV represents tin oxide (SnO2). X The peak at 485.0 eV is attributed to metallic tin (Sn). A significant change can be observed between the tin peaks before and after treatment with argon and hydrogen plasma. Before plasma treatment, photon emission from metallic tin is barely apparent, if present. After plasma treatment, a large peak at 285.0 eV due to metallic tin can be observed. These results are in good agreement with previous studies of surfaces exposed to hydrogen plasma in a vacuum.

[0040] Table 1 shows the Sn 3D restored to its original form. 5 / 2 The estimated relative fractions of metallic tin and tin oxide, derived from the peak area, are shown. These results indicate that hydrogen plasma reduction at atmospheric pressure increases the proportion of metallic tin from 3.0% to 40.8%. The increase in metallic tin indicates a significant reduction in the thickness of the surface oxide film. This thinner oxide film can be more easily broken when tin or tin-silver bumps are thermocompressed together. Significant regrowth of the oxide film occurred during the 30 hours of exposure of the tin film to air after hydrogen plasma treatment. In this case, the detected proportion of metallic tin decreases from 40.8% to 25.4%. These results suggest that the time between oxide removal and bonding by hydrogen plasma should be as short as possible, preferably less than one hour. [Table 1]

[0041] Figures 5A and 5B show images of tin solder bumps taken with an optical microscope. The image in Figure 5A is of a tin solder bump heated to 250°C in an inert gas environment. In contrast, the image in Figure 5B is of a tin solder bump treated with atmospheric pressure hydrogen and argon plasma at 250°C while maintaining the sample in an inert gas environment. The plasma process was carried out using a 100 mm wide linear beam running at 500 W, 40 L / min of argon and 0.4 LPM of H2. The gap between the plasma head and the sample was 1.5 mm. The tin solder bump in Figure 5A is strongly oxidized, as is evident from its rough, flat top with multiple small bumps. In contrast, the bump seen in Figure 5B has a smooth spherical shape with a highly reflective surface. The spherical shape is evidenced by the strong light reflection near the center of the bump, surrounded by a dark ring caused by the high-angle sides. These results demonstrate the ability of atmospheric pressure hydrogen and argon plasma processes to completely remove metal oxides from tin solder bumps.

[0042] The surface tension associated with the reflow of tin solder is much smaller than the force applied during thermocompression bonding. Different from thermocompression bonding, in order for the reflow process to function, the tin solder bump surface must be free of oxides. Therefore, the substrate must be maintained in an inert gas environment during the removal of oxides by active hydrogen from the plasma and the bonding of the tin solder to the substrate. These findings are shown in FIGS. 6A and 6B. In this experiment, the first substrate included a two-dimensional array of copper pillars 100 with tin caps 101, while the second substrate included a silicon die covered with a thin film of copper 104. After both substrates were treated with active hydrogen from atmospheric pressure hydrogen and argon plasma, the first substrate was placed on top of the second substrate, and the package was heated above 200°C. Note that due to the first substrate being placed on top and bottom, the tin cap 101 is in direct contact with the copper film 104. In FIG. 6A, the results obtained when the hydrogen plasma cleaning and reflow are performed in an inert gas environment, but the transfer from one process to the other is performed in air are shown. In this case, the tin solder slightly wets the copper film, forming a weak tin bond 102 to the copper surface. In FIG. 6B, the results obtained when all of the hydrogen plasma cleaning, reflow process, and transfer steps are performed in an inert gas environment are shown. Here, the tin solder is completely wet, bonds to the copper, and forms an intermetallic Sn x Cu y -Cu joint 103. This latter case is necessary for proper microbump interconnects for flip chips.

[0043] 3.0 Method for removing surface oxides from indium Another embodiment of the present invention involves removing surface oxides from indium bumps before bonding. It has been found that surface passivation is not necessary for interconnect bonding to occur. The indium oxide film on the surface 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. Because indium has a low melting point, it may be substituted with tin when the semiconductor package is heat sensitive, as in the case of some image sensors. However, indium oxide has a melting point of 1910°C, and therefore any oxides on the surface of the indium metal mean that the solder bumps will not form intermetallic bonds unless these oxides are removed. Once the oxides are reduced, strong electrical and mechanical connections can be made using reflow bonding or thermocompression bonding, as with tin.

[0044] An indium solder bump was placed on a hot plate and heated to 160°C. At this temperature, the metallic indium melts, but the surface oxide film remains solid. The oxide film forms a thin layer on the surface inside the molten indium, preventing the solder bump from reflowing and changing shape. To remove the surface oxide, a gas stream containing 32.4 LPM of argon and 7.0 LPM of forming gas (95% argon and 5% hydrogen) was supplied to a 100 mm linear plasma head at atmospheric pressure. 500 W of high-frequency power was applied to the electrodes to ignite and maintain the plasma. The plasma source was then scanned over the indium microbump on the hot plate at a distance of 2.0 mm and a scanning speed of 1.0 mm / s.

[0045] Optical microscope images of indium microbumps were obtained before and after exposure to atmospheric pressure plasma. Figures 7A and 7B show the bumps before (Figure 7A) and after (Figure 7B) plasma treatment. Figure 7A shows that even when heated above the melting temperature, the indium bumps retain a rough texture and flat surface consistent with the deposited material. This indicates that the indium oxide film on the surface prevented the reflow of the indium metal. In contrast, Figure 7B shows a smooth, rounded sphere with a bright, reflective top and dark sides. This change in morphology clearly demonstrates that active hydrogen from the plasma successfully removed the indium oxide, allowing the indium metal to reflow and form a sphere.

[0046] In another example of the present invention, indium oxide was removed from indium microbumps using atmospheric pressure hydrogen and argon plasma immediately before flip-chip die bonding at room temperature. In this case, a passivation step was not required. The indium interconnect arrays were bonded together in two configurations: indium-indium bump bonding and indium bump-UBM (underbump metallization) bonding. This plasma process was compared to an industry standard method of exposing indium bumps to formic acid vapor in an oven at temperatures between 200°C and 225°C. In this case, indium oxide is removed by forming indium formate, which sublimates from the metal surface. After removing the metal oxides, dies containing the indium microbump arrays were aligned on top of each other, or on top of a second die containing underbump metallization, and bonded together by thermal compression (i.e., using TCB).

[0047] The plasma process was carried out in an inert gas environment purged with argon to maintain an oxygen concentration of less than 500 ppm. A 100 mm linear plasma head was supplied with 32.4 LPM of argon and 7.0 LPM of forming gas (95% argon and 5% hydrogen) and powered with 300 W of RF power (at 27.12 MHz). Each die was scanned twice with the plasma head at 1.0 mm / second. The sample was then removed from the inert gas environment, transferred to a bonder in air, and bonded within 30 minutes. During plasma scanning, 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 unique advantage of the present invention.

[0048] Figure 8 shows the maximum shear strength of indium-bonded samples. All shear strength measurements are referenced against control values ​​obtained using one die treated with formic acid and the other die without the oxide removal process. A relative bonding force of 1.0 is the maximum force that TCB can apply, which gives a relative shear strength of 1.0. In the case of formic acid reflow, a 50% reduction in bonding force results in a 90% reduction in shear strength. In contrast, cleaning the indium microbump array before bonding using atmospheric pressure hydrogen and argon plasma can reduce the bonding force by 50% or 75%, resulting in relative shear strengths 1.5 or 1.35 times higher than the control. The plasma removal process is an improvement over industry-standard cleaning processes in both the mechanical shear strength of the interconnects and the reduced force required to achieve bonding.

[0049] Interconnection assemblies in which a substrate with indium bumps was directly bonded to UBM (Underbump Metallization) of another substrate were also tested. The UBM material consisted of laminated metal layers with a gold contact layer. Figure 9 shows the maximum shear stress for three different bonding scenarios. The control used as-deposited indium bumps bonded to the UBM. As shown in the figure, treatment of the UBM was essential to achieve strong bonding to the indium bumps. When only the indium was treated with plasma, the relative shear strength increased from 1.0 to only 1.2. When both the indium bumps and the UBM pads were treated with atmospheric pressure hydrogen and argon plasma, the relative shear strength increased from 1.0 to 6.4. Furthermore, in the case of these latter samples, the observed failure mode was not due to failure of the interconnection bond between the indium bumps and the UBM pads, but rather a mechanical failure resulting in the silicon chip shattering into fragments. The use of plasma processes was essential to maximize the bonding strength between indium and UBM. The bonding of indium bumps to UBM pads is often performed without pretreatment of the gold surface of the UBM due to the inert properties of gold. Nevertheless, these results indicate that the plasma process is essential not only for the removal of oxide metals from indium but also for the removal of organic contaminants from gold.

[0050] This embodiment of the present invention provides an unexpected improvement over the prior art. One novel result is that the shear strength performance described above can be maintained even after significant out-times when the substrate is stored in contact with air for extended periods before thermocompression bonding. Figure 10 shows the relative shear strength values ​​of indium-to-indium bond assemblies treated with argon and hydrogen plasma and then held in air for different times before TCB. The relative shear strength of the interconnects does not decrease significantly even with an out-time of up to 13.5 hours.

[0051] The invention described herein utilizes a single-step hydrogen and argon plasma process to remove the indium oxide film from indium microbumps. Prior art has reported the need to create a nitrogen passivation surface to maintain improved bonding performance when the sample is exposed to air (Schulte, U.S. Patent No. 8,567,658 (October 29, 2013)). The results presented in Figure 10 clearly demonstrate that such passivation of indium bumps is not necessary.

[0052] 4.0 Apparatus for removing metal oxides from metal contacts on substrates, dies, and wafers This section describes several exemplary inert gas-purged enclosures that incorporate plasma applicators and inert gas-purged environments particularly suitable for flip-chip electronic assembly. As previously mentioned, plasma is applied to prepare the electronic solder joints of an integrated circuit (chip) for interconnecting and coupling to either a package or another integrated circuit.

[0053] Figure 11 shows an exemplary purging enclosure combined with a plasma applicator for processing a substrate or die (i.e., a chip) with atmospheric pressure hydrogen and argon plasma. The plasma head 5 is mounted on a cover plate 4 suspended slightly above the recess plate 12. The short distance between the cover plate 4 and the recess plate 12 is designed so that the plasma head 5 can be scanned over the substrate or die without these plates contacting each other. A suitable distance is 0.5 mm, but other distances may be adopted and will be apparent to those skilled in the art. Scanning is achieved by mounting the plasma head 5 and cover plate 4 on a suitable XYZ scanning robot having adjustable scanning speeds in the X and Y directions that can be precisely selected between 0 and 1,000 mm / sec. Initially, the plasma head 5 is moved to the left side of the recess plate 12 so that the gas flow of hydrogen and argon into the recess volume purges all the air and supplies a residual amount of oxygen of less than 500 ppm. Next, the plasma is started, and the plasma head 5 moves from left to right, scanning the substrate or die completely to remove metal oxides from the array of metal solder balls or bumps or metal contact pads. Depending on the throughput and configuration of the interconnection coupling operation, the plasma may or may not be turned off after the process is complete.

[0054] Figures 12A and 12B show a recessed plate 12 that forms a sealed volume 22 in which the die 6, which is exposed to atmospheric pressure hydrogen and argon plasma, is placed. The recessed plate 12 includes four walls that define the sealed volume 22. The thickness of these walls is designed to be 0.1 to 5.0 mm greater than the thickness of the die 6. This creates a small gap 7 as shown in Figure 2.

[0055] Figures 13A and 13B show isometric top and bottom views, respectively, of the atmospheric pressure plasma head 5 mounted on the cover plate 4. Hydrogen and argon gases enter the plasma head 5 at the gas inlet 11. The plasma head 5 has two fixtures 13 through which water flows, recirculating channels inside the device. The water is typically maintained at 60°C, which helps keep the plasma process at a constant temperature throughout the operation of the plasma process. Other temperatures between 0°C and 100°C may be used, as will be apparent to those skilled in the art. The plasma head 5 shown in Figure 13A also includes an optical sensor 14 that detects the emission of light from the plasma, thereby monitoring plasma performance. The plasma mounting plate 23 includes two holes 15 on either side of the plasma head that serve as inert gas purge inlets. The inert gases are selected from the group including argon and nitrogen. Alternatively, one of the holes 15 may be used to sample the gas and monitor the residual oxygen concentration. Figure 13B, a bottom view, shows an outlet slit 24 through which the active gases exit the plasma head 5. The diagram also shows a hole 15 for introducing an inert gas purge. The exit slit 24 is at least the width of the die, so that the entire surface is treated with active gases from atmospheric pressure hydrogen and argon plasma as the plasma head scans over the die.

[0056] Figure 14 shows an apparatus for processing wafers with atmospheric pressure hydrogen and argon plasma in an inert gas environment. The plasma head 5 is mounted on a large cover plate 4 suspended above the wafer holder 17. The wafer holder 17 includes a concave circular plate 16 into which the wafer is inserted. The cover plate 4 must be at least twice the diameter of the wafer in order to allow the plasma head 5 to scan over a distance in the X and Y directions sufficient to ensure that the wafer is fully scanned by the plasma while maintaining an inert gas environment. For example, if the diameter of a silicon wafer containing an integrated circuit is 300 mm, the cover plate 4 must be at least 600 mm wide × 600 mm long.

[0057] Figure 15 shows a cross-sectional view of the wafer processing apparatus. The plasma head 5 is mounted on the cover plate 4 such that the outlet slit from which the active gas flows out is coplanar with the bottom of the cover plate 4. The concave circular plate 16 inside the wafer holder 17 is clearly visible in the cross-sectional view.

[0058] Figure 16 shows a cutaway view of the apparatus for processing wafers. In this drawing, the silicon wafer 18 is placed on a concave circular plate on a wafer holder 17. Since the depth of the concave circular plate is substantially the same as the thickness of the wafer, the surface of the wafer holder is at the same height as the surface of the wafer.

[0059] Figure 17 shows a side view of an apparatus for processing wafers with atmospheric pressure hydrogen and argon plasma in an inert gas environment. The wafer holder 17 consists of a gas inlet 19 for introducing inert gas into a 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 is the same gap 7 identified in Figure 2. Figure 18 is a side view of the apparatus, enlarged to better show the inert purge gas flow 25 on the wafer. The arrows indicate the direction in which the inert purge gas flow 25 enters from the inlet 19, flows down into the gap from left to right, and flows over the wafer 18. The cover plate 4 and plasma head 5 are suspended above the wafer holder 17 and can scan over the wafer 18 without contacting the walls of the wafer holder 17. As will be apparent to those skilled in the art, other suitable designs may be envisioned to achieve embodiments of the present invention by minimizing the total purge volume 20 while rapidly and efficiently purging the air with an inert gas flow.

[0060] Figure 19 provides a further example of a wafer holder 17. The top view shows a wafer 18 mounted in the wafer holder 17 with an additional space between the wafer 18 and the wall 26. The additional space exists 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 small holes 27 that extend across the back surface of the wafer holder 17. These small holes are configured to generate a uniform flow velocity of the inert gas across the entire diameter of the wafer 18. The gas flows from left to right and exits the wafer holder 17 through the sawtooth wall 21 on the right side.

[0061] Figure 20 is a schematic diagram showing the path for scanning the plasma head 5 on wafer 18. For illustrative purposes, it is assumed that the wafer has a diameter of 300 mm, and the plasma head is configured to generate a linear beam of active gas with a width of 100 mm. First, the plasma head is positioned in the upper left corner of the wafer holder. The purge flow is turned on to remove air and create an inert gas environment, where the inert gas is selected from the group including argon and nitrogen. Next, in step 1, the plasma is turned on and the wafer is scanned to the left third of the wafer surface. The scanning speed can be in the range of 0 to 1,000 mm / s and is determined by the exposure time required to remove oxidation from the surface of the metal contacts. In step 2, the plasma head is moved to the center of the wafer holder. Then, in step 3, the plasma head is scanned to the central third of the wafer. In step 4, the plasma head is moved to the right side of the wafer holder, and finally, in step 5, the plasma head is scanned to the right third of the wafer. Throughout this procedure, the entire wafer is uniformly treated with atmospheric pressure hydrogen and argon plasma. Many other scanning paths may be devised, as will be apparent to those skilled in the art.

[0062] This concludes the description, including preferred embodiments of the present invention. The above description, including preferred embodiments of the present invention, is presented for illustrative and explanatory purposes only. It is not intended to be exhaustive or to limit the present invention to the exact forms disclosed. Many modifications and variations are possible within the scope of the teachings above. Additional variations of the present invention may be devised without departing from the concept of the invention described in the following claims.

Claims

1. A method of interconnection and coupling, The steps include: applying a plasma containing active hydrogen to at least a first metal contact supported on a first substrate via an atmospheric pressure plasma applicator to remove oxidation and create a newly deoxidized metal surface on the first metal contact; The steps include: coupling the first metal contact to a second metal contact supported on a second substrate by applying mechanical force, heat, or both to the first metal contact in contact with the second metal contact; and The steps include: performing the application of the plasma to create an inert gas environment in which the coupling of the first metal contact to the second metal contact is performed; The method wherein a metal interconnection is formed between the first metal contact and the second metal contact by coupling the first metal contact to the second metal contact.

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

3. The method according to claim 1, wherein the inert gas environment is generated by mounting the atmospheric pressure plasma applicator, the first substrate supporting the first metal contact, and the second substrate supporting the second metal contact inside a volume to be purged by an inert gas flow, and the air inside the volume is first displaced by the inert gas flow.

4. The method according to claim 2, wherein the inert gas stream is selected from the group including argon and nitrogen.

5. The method according to claim 1, wherein the application of the plasma is carried out by the atmospheric pressure plasma applicator passing through the entire first substrate supporting the first metal contact.

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

7. The method according to claim 1, wherein the contact of the first metal contact for bonding with the second metal contact occurs at a temperature of 20 to 250°C.

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

9. The first metal contact is the metal bump, the distance between the centers of the metal bumps is 5 to 100 microns. The method according to claim 1, comprising a two-dimensional array.

10. The method according to claim 1, wherein the first metal contact is kept in the inert gas environment during the application of the plasma and during coupling to the second metal contact.

11. The method according to claim 1, wherein the plasma containing the active hydrogen is also applied to the second metal contact supported on the second substrate via the atmospheric pressure plasma applicator, and the second substrate is kept in the inert gas environment.

12. The method according to claim 1, wherein the first metal contact and the second metal contact include a flip-chip assembly.

13. The method according to claim 1, wherein the first metal contact and the second metal contact each include a two-dimensional array of metal bumps having a center-to-center distance between the metal bumps of 5 to 100 microns, and coupling the first metal contact to the second metal contact includes aligning and coupling the metal bumps of the first metal contact with the metal bumps of the second metal contact.

14. The method according to claim 13, wherein the two-dimensional array of the first metal contacts includes an integrated circuit.

15. The method according to claim 13, wherein the two-dimensional array of the second metal contacts includes underbump metallization including a metal pad.

16. The method according to claim 1, wherein the atmospheric pressure plasma applicator includes an atmospheric pressure plasma head mounted on a cover plate having slotted passages for the plasma to pass to a first metal contact supported by the first substrate.

17. The method according to claim 16, wherein the cover plate includes at least one purge hole for an inert gas to pass through therein to form the inert gas environment.

18. The method according to claim 16, wherein the cover plate includes at least one sampling hole through which it passes.

19. The method according to claim 16, wherein the application of the plasma is performed by scanning the slotted passage across the first metal contact on the first substrate.

20. The method according to claim 16, wherein the cover plate seals the volume to which the plasma is applied to the first contact.

21. The method according to claim 20, wherein the volume includes a height of 10 millimeters or less from the slotted passage.

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