Sintering method, fusion welding method, bump formation method and sensor

Abstract: A sintering method and fusion welding method of the present invention are characterized in that energy is radiated to the black component of starting materials so as to convert it from the state of an energy absorber to the state of an energy reflector (the state where metallic luster is exhibited). The methods can be utilized effectively for forming the electrode of a sensor or the bump of an electronic component. When applied to the production of the sensor and the electrode, the methods can produce these products by a simple production process and with extremely high producibility.

Description: The invention relates to a method of sintering fine particles of at least one metal or of a mixture of fine particles of at least one metal and at least one non-metal to a sintered portion by radiating energy thereto, particularly laser energy. Various metallic and non-metallic films are formed during a fabrication process of electronic components. These films are formed mainly by vacuum deposition, spattering, CVD, and the like, and plating and screen printing are also employed depending on fabrication conditions. In the fabrication process of electronic components, there often occur the cases where the temperature of a substrate on which a film is to be formed cannot be raised but the film which is from several microns to a dozen of microns thick must be worked or etched in a predetermined shape. In such cases, vacuum deposition and spattering are not employed because a long period of time is necessary for forming the film, and an extended period of time is also required for etching unnecessary portions if the film thickness increases and accuracy drops. Particularly, vacuum deposition is not employed because the substrate is heated by radiation from a vacuum deposition source. Screen printing is not employed, either, because the substrate must be heated as a whole in order to bake a printed paste. Therefore, plating is employed under the conditions such as described above. Though plating can form a film without heating the substrate, it is not free from the problems in that the substrate must be dipped into a plating solution, a long period of time is necessary for plating, process steps such as an electrode formation step, a photoresist step, and the like become complicated as a whole, the film thickness is likely to be non-uniform and the yield is low. Although these problems result in the increase in the cost of production, plating is employed at present under the above-mentioned condition. High cost of production is unavoidable in the case of plating because its process steps are complicated and its yield is low. Particularly because plating is a wet process, it renders various disadvantages to other process steps. According to JP-A-57 160 975 particles of metals, of metalloids, of semiconductor materials, of glass, of resins, including organic solvents, or mixtures of these substances are sintered with heating by irradiation with a laser optic. In an example, a mixture of vanadium oxide (VO₂) having a mean particle size of 2 µm and glass powder in a weight ratio of 2:1 is moulded with the addition of a binder and dried. It is irradiated by a ruby laser having a wavelength of 0.694 µm. The body is sintered sufficiently by 5 J/cm² of irradiation energy density. Sintering takes place rapidly and is completed without causing large changes of physical and chemical properties of the material. The sintering can be carried out locally to the desired depth and regardless of the nature of atmosphere. This prior art method cannot be applied to the fabrication of electronic components, for example. As a reference relating to laser radiation technique which is used by the present invention US-A 612 208 is to be mentioned. EP 0 168 605 describes a process of applying solder to form a permanent mechanical and electrical bond between two conductors on a printed circuit board by reflow soldering. During this process, individual solder pads are formed at the desired locations by use of molten solder which is then allowed to solidify. The desired electrical conductors are then placed in mechanical contact with their proper pads and the entire board is raised to the desired temperature, for instance by laser beam irradiation. With this process it is important to know the exact moment when a given solid turns to liquid or vice versa. This transition point can be detected, as prior to melting the surface reflexions will remain unchanged, which means that a detector signal will be constant. At the moment when the solid-to-liquid transition occurs at the surface, a discontinuity will be noted in the detector signal, which may be an increase or a decrease in the reflected light intensity which is due to a glossy reflexion or to a change in surface contour. A crystalline surface of a solid solder reflects rays spread throughout a small angle, some of them bypassing a detector. In the molten case, the reflexion is more mirror-like, so that the detector receives more reflected power. It is the object of the present invention to form a film or a linear or protrusive film by a dry and simple process without raising a substrate temperature, or to fusion-weld the film. This object is achieved by the method of the generic kind in that as at least part of the particles a black-color component in form of ultrafine particles of metal and/or ceramics is used the particle size of which is 0.01 µm to 1 µm or below and the reflection factor of which is below 50% of the reflection factor in bulk, the ultrafine particles of the black-color component having the state of an energy absorber before sintering are converted to the state of an energy reflector after sintering, and in that the radiation quantity of said energy applied for sintering is selected in such a manner as to stop or check the progress of heating of the sintered portion with completion of the sintering. It is preferred that the surface of the black-color component in form of the ultrafine particles is fused and welded by said radiated energy whereby the black component is varnished by fusion so as to exhibit metallic luster and is converted from light absorber to light reflector. It is convenient that the black-color component or the mixture including the black-color component is disposed on a base, particularly a portion fused before, and fuse welded therewith by said radiated energy. The energy radiation may be controlled by a sensor for the energy absorption factor or the energy reflection factor or both of them of the sintering and fusing metal. The material to be sintered or fused can be applied on predetermined portions of a substrate as a paste containing the black-color component, preferably by printing in a predetermined pattern. It is advantageous that a film of a material having a high reflection factor to the radiated energy is formed in advance on the surface of the substrate. It is also possible that a film for improving wettability between the portion obtained by said energy radiation and the substrate is formed in advance on the surface of the substrate. It can be convenient that at least two kinds of metallic particles are coated separately on the substrate or in-advance-prepared substrate to form a multi-layered metallic ultrafine particle film. The method according to the invention is preferrably used for connecting a lead wire to a substrate in form of an IC-chip by forming a bump. The radiation source of energy is preferrably a laser beam or an optical beam but other electromagnetic waves such as solar rays converged by a lens, an electron beam and so forth can be used in the present invention. Preferred is the monochromatic light of a YAG laser beam. When the metallic powder is pulverized and milled to ultra-fine particles, the light absorption factor increases and sintering by laser beam radiation becomes easy. Therefore, the laser radiation time becomes shorter remarkably and since the vacuum deposition film having a high reflection factor is formed at portions other than the sintered portion, heating of the substrate by laser radiation can be more checked. Since the metallic ultra-fine particle film turns to a high optical reflector after sintering, it is not heated unnecessarily and prevents heating of the substrate. In the following, embodiments of the invention are described with the aid of drawings. Figs. 1, 7, 13, 19 and 22 are sectional views of electronic components on which gold bump is formed by laser beam radiation by the sintering method in accordance with one embodiment of the present invention; Fig. 2 is a sectional view of a substrate as the material of the embodiment shown in Fig. 1; Fig. 3 is a sectional view of the substrate of Fig. 2 on which a gold ultra-fine particle film is coated; Fig. 4 is a sectional view of an IC chip package obtained by the embodiment of Fig. 1; Fig. 5 is a perspective view of the package shown in Fig. 4; Fig. 6 is a sectional view of a substrate as the material of the embodiment shown in Fig. 7; Fig. 8 is a partial perspective view of an electronic component on which gold bump is formed by laser beam radiation by the sintering method in accordance with another embodiment of the present invention; Fig. 9 is a perspective view of a substrate on which micro-wiring is disposed by the sintering method in accordance with an embodiment of the present invention; Fig. 10 is a perspective view of the substrate of Fig. 9 during the radiation of the laser beam to the substrate; Fig. 11 is a perspective view of the substrate with the wiring obtained by the step shown in Fig. 10; Fig. 12 is a characteristic diagram showing the reflection factor of a ultra-fine particle film in comparison with that of a bulk; Fig. 14 is a sectional view of an object of bond before it is fusion-welded by the fusion-welding method in accordance with another embodiment of the present invention; Fig. 15 is a sectional view of the object of bond showing the fusion-welding step by radiating a laser beam to the object of bond; Fig. 16 is a perspective view of a sensor in accordance with still another embodiment of the present invention; Fig. 17 is a sectional view of the embodiment shown in Fig. 16; Fig. 18 is a sectional view of a substrate as a material of still another embodiment shown in Fig. 19; Fig. 20 is a sectional view after the bump is formed on the substrate shown in Fig. 18; Fig. 21 is a sectional view of a substrate as the material of the embodiment shown in Fig. 22; Fig. 23 is a sectional view after the bump is formed on the substrate shown in Fig. 21; Fig. 24 is a photograph showing the result of surface observation of the gold print film which is printed and dried on the IC surface before laser radiation; Fig. 25 is a photograph showing the result of surface observation of the gold print film after laser radiation; Fig. 26 is a photograph showing the result of observation of the section of the gold print film printed and dried on the IC surface before laser radiation; and Fig. 27 is a photograph showing the result of observation of the section of the gold print film after laser radiation. (Embodiment 1) Fig. 1 shows a sintering method of gold ultra-fine powder by laser beam radiation in accordance with one embodiment of the present invention. In the drawing, reference numeral 1 represents a silicon substrate; 2 is an aluminum wiring; 3 is an insulating protective film; 4 is a gold vacuum deposition film; 5 is a gold ultra-fine powder film; 6 is a laser beam; and 7 is a laser oscillator. Fig. 2 is a sectional view of the substrate before the gold ultra-fine powder film 5 is formed and Fig. 3 is a sectional view of the substrate after the gold ultra-fine powder film 5 is formed in Fig. 2. Gold is originally a high light reflector and reflects about 95% of YAG laser (wavelength 1.06 µm) and about 98% of CO₂ laser (wavelength 10.6 µm). When turned to ultra-fine particles, however, gold becomes black and a high light absorber having an extremely high absorption factor. Metals in general become high light absorbers when they are turned to ultra-fine particles. Part of the present invention utilizes the characteristics of metals described above and sinters ultra-fine powder by laser radiation. First of all, a gold (Au) thin film is formed on an IC chip as shown in Fig. 2. There is no limitation to the film formation method; hence, spattering, vacuum deposition, CVD, or the like, can be employed. Next, a paste prepared by Au ultra-fine powder is printed as shown in Fig. 3, heated in the atmosphere of an inert gas, in the N₂ atmosphere or in the air to about 400°C and is then dried. The Au ultra-fine powder thus dried is then subjected to laser radiation for an extremely short period of at most a few seconds and is sintered as shown in Fig. 1. In this manner, a gold bump having a size of about 100 µm by 100 µm and a height of about 20 µm is formed. There is no limitation to the laser, in particular, and YAG laser, CO₂ laser, glass laser, or the like, can be employed. In this instance, the Au vacuum deposition film formed on the IC chip reflects the laser beam at portions other than at the Au ultra-fine powder film and prevents the temperature rise of the IC chip. Since Aℓ has low wettability to Au, sufficient bonding strength cannot be obtained between the Aℓ wiring and the gold bump after sintering by laser radiation if Au ultra-fine powder is directly coated on the Aℓ wiring. If the Au vacuum deposition film is formed in advance, however, the Au bump and the Aℓ wiring can be bonded sufficiently. Therefore, the vacuum deposition film may be formed by the material providing the bonding strength between the Au bump and the Aℓ wiring besides the Au vacuum deposition film, such as Pd, for example. As described above, it is possible in accordance with the present invention to form the Au bump while the IC chip is kept at a low temperature because the laser radiation portion is extremely small, because the laser radiation time is extremely short, because the Au vacuum deposition film on the IC chip reflects the laser beam at portions other than the Au ultra-fine powder film and because the metallic ultra-fine powder film becomes a high light reflector after sintering and its energy absorption after sintering drops. In accordance with this embodiment, the Au bump can be formed on the IC chip with extremely high reproducibility by a simple dry step. Fig. 4 is a sectional view of the TAB package obtained by the embodiment described above and Fig. 5 is its plan view. In the drawings, reference numeral 8 represents the IC chip; 9 is a polyimide film; 10 is a copper lead; and 11 is the Au bump. The TAB packaging method is an indispensable technique for various thin office automation (OA) equipment, pocket calculators and IC cards of the latest type for which limited thinness is of utmost importance because the TAB packaing method can reduce the thickness and size of IC devices and can increase the number of pins. In accordance with the prior art technique, a thin copper film is first bonded onto a polyimide film and copper leads are then formed by etching. On the other hand, an Au pad (hereinafter called the "bump") having a size of about 100 µm by 100 µm and a thickness of from about 20 to about 30 µm is formed by plating on the aluminum wiring of the CI chip which is to be connected to the copper lead. The Au bump and the copper lead are bonded together by bonding and the IC chip is mounted onto the 35 mm-thick polyimide film. Since the IC chip cannot be heated to a temperature higher than 450°C, the Au bump must be formed by plating which does not raise the substrate temperature. As described already, however, the plating method requires generally troublesome and long process steps such as (1) formation step of plating electrode, (2) photoresist step, (3) plating step, (4) removing step of photoresist and (5) etching step of electrode. Moreover, nonuniformity of the height of the Au bumps is likely to cause defective connection with the copper lead and the production yield drops consequently. The problems of the prior art technique described above can be solved by adopting the embodiment of the invention shown in Figs. 1 to 3. (Embodiment 2) The formation step of the Au bump on the copper lead in accordance with another embodiment of the present invention will be described with reference to Figs. 6 to 8. In Figs. 6 and 7, reference numeral 10 represents the copper lead; 9 is the polyimide film; 12 is an Sn vacuum deposition film; 5 is an Au ultra-fine powder film; 13 is a copper block; 6 is a laser beam; and 7 is a laser oscillator. The Au bump must be formed at a low temperature because the polyimide film cannot withstand a temperature higher than about 450°C in the same way as the IC chip. First of all, the Sn vacuum deposition film is formed on the copper lead as shown in Fig. 6. There is no limitation, in particular, to the formation method of the Sn vacuum deposition film. Therefore, the film may be formed by vacuum deposition, spattering or plating. Next, as shown in Fig. 7, a paste prepared from the Au ultra-fine powder is printed, heated to about 400°C in an inert gas atmosphere or an N₂ atmosphere and then dried. Next, as shown in Fig. 7, the copper lead is clamped by the copper blocks from above and below and the Au ultra-fine powder film is sintered by the laser beam in an inert gas atmosphere such as of N₂ or Ar for a short period not exceeding a few seconds at most, thereby forming the Au bump. In this case, the copper block absorbs the heat due to the laser beam radiation and prevents heating of the polyimide film. The Sn vacuum deposition film causes the eutectic reaction with the Au bump and generates sufficient bonding strength. Laser beam radiation must be carried out in an inert gas atmosphere of N₂ or Ar in order to prevent oxidation of the copper lead and the Sn vacuum deposition film. Fig. 8 shows an example of a production apparatus. In Fig. 8, reference numeral 10 represents the copper lead on which the Sn vacuum deposition film is formed; 9 is the polyimide film; 5 is the Au ultra-fine powder film; 13 is the copper block; 14 is the copper block which has the shape of a square cylinder; 7 is the laser oscillator; 15 is a pipe; and 6 is the laser beam. The copper lead to which the laser must be radiated is clamped between the copper blocks from above and below, and N₂ or Ar is blown inside the copper block having the form of the square cylinder through the pipe so that only the periphery of the laser radiation portion is encompassed by the N₂ or Ar atmosphere. In accordance with this method, the Au bump can be formed on the copper lead even in the air. As described above, it is possible in accordance with this embodiment to form the Au bump with high producibility while the polyimide film is kept at a low temperature because the laser radiation portion is extremely small, because the laser radiation time is short, because the copper blocks absorb the heat and because the reflection factor of the Au ultra-fine powder film increases but its absorption factor drops after sintering. (Embodiment 3) Still another embodiment of the present invention will be described with reference to Figs. 9 to 11. When the electrodes are to be connected mutually after completion of the IC chip, it is not efficient to form the Aℓ wiring through the ordinary steps of photoresist, Aℓ vacuum deposition and etching. Since the IC chip has already been completed, the substrate temperature cannot be raised and screen printing which is otherwise highly efficient cannot be employed. Figs. 9 to 11 show schematically the formation step of the Au wiring on the IC chip in accordance with this embodiment. In these drawings, reference numeral 8 represents the IC chip; 16 and 17 are the electrodes which are to be connected by the Au wiring; 18 is an insulating protective film; 5 is the Au ultra-fine powder film; 6 is the laser beam; and 7 is the laser oscillator. First of all, the Au ultra-fine powder film is coated between the electrodes to be connected, as shown in Fig. 10. In this case, only the Au ultra-fine powder film may be coated or a paste using the Au ultra-fine powder may be coated. After the Au ultra-fine powder film is formed, either the laser beam or the IC chip is moved and the laser beam is radiated in such a manner as to connect the electrodes and to sinter only the wiring portion that connects the electrodes. After laser radiation, the unsintered portions of the Au ultra-fine particle film are removed by water or an organic solvent, as shown in Fig. 11. As described above, it is possible in accordance with this embodiment to form efficiently the Au wiring having an arbitrary shape while the substrate temperature is kept at a low level because the laser radiation time is short and because the Au ultra-fine powder film becomes a high light reflector after sintering and does not absorb excessive heat. When the Au wiring is formed, it is possible to form it efficiently by printing only the wiring portion and sintering it by laser radiation after drying in the same way as the Au bump formation method described above. In the case of the Au thin film formed by coating an Au ultra-fine particle film on the silicon substrate on which an Au vacuum deposition film is formed and then radiating the laser beam to sinter the film in the disc-like shape having a diameter of about 1 mm, the photograph of the appearance of the film shows that the gold disc-like portion consists of the Au thin film sintered by laser radiation and the surrounding black portion consists of unsintered Au ultra-fine particle film. The laser used in this case is the YAG laser and the radiation time is about one second. Bonding strength with the substrate after sintering is high and the film does not peel even after a tape test. Though the Au bump is shown formed on the Aℓ wiring in this embodiment, the conductive metals other than Aℓ, such as Cu, Sn, Ag, Au, and the like, can also be used. The bump material may be those metals which are black in the form of ultra-fine particles but change reflective after sintering, such as Ag, Cu, Aℓ, and the like, besides Au. In accordance with the wiring method of the embodiment shown in Figs. 9 to 11, it is possible to form a multi-layered wiring by repeating similar wiring on insulating layers. Furthermore, in Fig. 10, the metal film can be formed by sintering a wider area by scanning a laser beam or a chip. Though this embodiment uses the laser beam, other heat sources (electron beam, infrared rays, plasma, optical beam, etc.) whose energy charge quantity changes before and after sintering of the ultra-fine particles may also be used. (Verification 1) Fig. 12 shows the relation between the reflection factor of light and the diameter of the ultra-fine particles when the gold ultra-fine particles are coated in the film form. Symbol  represents the reflection factor to the ray of light having a wavelength of 0.9 to 1.1 µm, and symbol ○ represents the reflection factor to the rays of light having a wavelength of 9 to 11 µm. The gold ultra-fine particle film having a mean particle diameter of about 100 Å absorbs almost all the rays of light corresponding to the YAG laser wavelength (1.06 µm). It is also a high absorber for the rays of light corresponding to the CO₂ laser wavelength (10.6 µm). For comparison, the diagram shows also the reflection factor of gold in bulk (mirror surface) to the rays of light having the same wavelength. Gold in bulk exhibits an extremely high reflection factor to the rays of light of either wavelength. This embodiment sinters the film by laser radiation by utilizing the characteristics of the reflection factor described above and the characteristics of low temperature sinterability of the metallic ultra-fine particles. (Embodiment 4) Fig. 13 shows a modified embodiment of Fig. 1. In the drawing, reference numeral 19 represents a reflectivity meter (sensor). Originally, gold is a high reflector of light. As shown in Fig. 12, it reflects about 95% of the YAG laser (wavelength: 1.06 µm) and about 98% of the CO₂ laser (wavelength: 10.6 µm). When converted to the ultra-fine particles, however, gold becomes a high absorber of light. Generally, metals become high absorbers of light when they are converted to the ultra-fine particles. The difference of this embodiment from the embodiment shown in Fig. 1 lies in that the progress of sintering is grasped appropriately by a sensor by utilizing the characteristics described above. Namely, a laser beam is radiated to a dried Au ultra-fine powder film for an extremely short period of up to a few seconds as shown in Fig. 13 so as to sinter the film within an extremely short period and a gold bump which has a shape of about 100 µm by about 100 µm and which is about 20 µm high is formed. The reflection factor of the gold ultra-fine particle film is measured by the reflectivity meter at the time of laser radiation. The progress of sintering of the ultra-fine particle film is estimated from the reflection factor (reflectivity) thus measured and the output and radiation time of the laser beam are controlled. The subsequent steps are substantially the same as those of the first embodiment. (Embodiment 5) Still another embodiment of the present invention will be described with reference to Figs. 14 and 15. As shown in this embodiment, the present invention can be applied to micro-bonding while keeping the bond portion at a low temperature besides the formation of the bump, the wiring and the thick film. An application example of the present invention to micro-bonding will be described with reference to Figs. 14 and 15. In these drawings, reference numerals 20 and 21 represent articles as the objects of bonding; 22 is an Au vacuum deposition film; 5 is an Au ultra-fine particle film; 6 is a laser beam; 7 is a laser oscillator; and 19 is a measuring element of a reflectivity meter. It is generally extremely difficult to bond articles as the objects of bonding when they are extremely small in size and cannot be heated depending on other conditions. This embodiment is particularly effective in such a case. First of all, the Au vacuum deposition film is formed on each article in order to protect the article and to improve wettability with the Au ultra-fine particle film. Next, as shown in Fig. 14, the Au ultra-fine particle film is printed onto the bond portions. After the film is dried, a laser beam is radiated for an extremely short period of up to a few seconds to the Au ultra-fine particle film which is printed on the bond portion so as to sinter the Au ultra-fine particle film and to bond the bond portions, as shown in Fig. 15. Bonding can be made while the substrate is kept at a low temperature because the laser radiation time is extremely short and the laser beam is reflected by the Au vacuum deposition film at portions other than Au ultra-fine particle film. (The above represents the application of the invention to bonding). Though the foregoing embodiments represent the case where the paste consisting of the Au ultra-fine particles is used, it is not necessary that all the Au particles in the paste are ultra-fine particles. In other words, it is possible to use a paste consisting of a mixture of gold small particles and gold ultra-fine particles, a mixture of ceramic powder and gold ultra-fine particles, and so forth ... (use of mixed powder). (Embodiment 6) Next, an application example of the present invention to a sensor will be described with reference to Figs. 16 and 17. In these drawings, reference numeral 23 represents a ceramic substrate; 24 is a detection portion, that is, an electrode; and 25 is a protective film. This embodiment shows an example of a heat ray flow meter for detecting a gas flow rate or for measuring a temperature. In sensors such as a thermometer and a gas flow meter, a detection portion is formed by (1) fixing a thin metallic wire, (2) printing an electrically conductive paste, (3) plating a wiring and (4) spattering or the like. Resistance change of the detection portion is utilized for measuring the temperature and the change of the resistance value, due to the temperature change which results from a gas stream, of the detection portion heated to a predetermined temperature, is utilized for measuring the gas flow rate. In the present invention, ultra-fine particles of metals such as platinum, nickel, copper, gold, and the like, are printed or coated on a ceramic substrate such as Aℓ₂O₃, SiC, AℓN (as shown in the drawing) and are then subjected to radiation by laser beam, optical beam or other optical energy, whereby sintering proceeds by the energy charged into the blank ultra-fine particles and the particles change to a metal. As a result, the reflection factor becomes great, the application of energy is self-controlled and sintering is complete. The wiring must be kept elongated in order to improve sensitivity, but a long and continuous detection portion can be obtained by swinging the optical energy or the sample as shown in the drawing. It is advisable to use a material (SiC, AℓN, Aℓ₂O₃, quartz, diamond, etc.) having a large electrical resistance and high thermal conductivity as the protective film of the substrate and the detection portion. Incidentally, the use of SiC, AℓN, Aℓ₂O₃, or the like, is effective as the protective film. (Embodiment 7) A bump formation method in accordance with still another embodiment of the present invention will be described with reference to figs. 18 to 20. In these drawings, reference numeral 1 represents an IC chip (silicon substrate); 26 is a bump starting material, which is a gold paste consisting of gold small particles in this embodiment; 2 is an aluminum wiring; 3 is an insulating film; 4 is a gold vacuum deposition film; 6 is a laser beam; and 7 is a laser oscillator. Incidentally, this embodiment uses the YAG laser. Figs. 18 to 20 are longitudinal sectional views showing the bump formation steps. First of all, a gold thin film which is a high reflector of the YAG laser is formed on the entire portion of the IC. The film may be formed by any method without limitation, such as vacuum deposition or spattering. Next, a gold paste is printed in the bump form by screen printing on the aluminum wiring to be connected to a lead wire. In this embodiment, the printed film has a size of 100 µm by 100 µm and a height of 10 to 20 µm (see Fig. 18). After printing, heating is made within a tolerable range of the IC so as to remove an organic solvent inside the paste. This embodiment uses the gold paste whose mean particle diameter is about 1.5 µm. Alternatively, the printed film is kept at about 380°C for about 10 minutes in the atmosphere after printing so as to remove the organic solvent in the gold paste. After the organic solvent is removed, the YAG laser beam is radiated to the dried gold print film as shown in Fig. 19 and sintered so as to form the gold bump. This embodiment uses the laser beam having a beam diameter of from 1 to 5 mm and the energy density is within the range of 0.5 to 1.5 J/mm². The laser radiation time is from 0.1 to 2 ms. After the printed film is sintered, the gold thin film 4 formed on the entire surface of the IC at portions other than on the bump is removed. The apparatus shown in Fig. 8 can be used for forming the bump on the lead wire in accordance with this embodiment. In such a case, the gold paste is first of all printed in the bump form at the tip of the lead wire 10 by screen printing. After printing is made, heating is made at a temperature within a tolerable range of the polyimide film 9 so as to remove the organic solvent in the paste. The printed film is kept at about 380°C for 20 minutes in a nitrogen atmosphere in this embodiment. Next, the lead wire is clamped from above and below between the copper blocks 13 and 14. An inert gas is blown from the pipe 9 into the portion encompassed by the copper blocks in order to prevent the oxidization of the copper lead by the laser beam radiation and to establish the inert gas atmosphere. In this inert atmosphere, the dired gold film is sintered by radiating the YAG laser beam thereto to form the gold bump. In this instance, the copper blocks 13 and 14 absorb the heat of the copper lead which is heated by the laser radiation and prevent heating of the polyimide film 9 which is not highly resistant to heat. In this embodiment, the radiation time of the laser beam is from 0.1 to 2 ms and the energy density is from 0.5 to 1.5 J/mm². The atmosphere is of the nitrogen atmosphere. (Embodiment 8) When the gold bump and the copper lead wire formed on the IC are bonded together, it is effective to form a metallic film which improves their bonding on the gold bump and on the copper lead. Hereinafter, still another embodiment of the present invention, wherein the metallic film for improving bonding strength is formed on the bump, will be described with reference to Figs. 21, 22 and 23. In these drawings, reference numeral 1 represents the IC chip; 26 is the gold paste; 27 is the gold bump; 2 is the aluminum wiring; 3 is the insulating film; 4 is the gold thin film; 6 is the laser beam; 7 is the laser oscillator; and 28 is the thin film for improving bonding between the lead wire and the bump, which is a tin thin film in this embodiment. First of all, a tin paste consisting of tin ultra-fine particles is printed on the gold bump. After printing, the printed paste is heated within a tolerable temperature range of IC inside an inert gas so as to remove an organic solvent in the tin paste. The dried and printed tin film is instantaneously sintered by radiating the YAG laser beam thereto to form the tin thin film on the gold bump. In this embodiment, the laser radiation time is from 0.1 to 2 ms and the energy density is from 0.1 to 1 J/mm². Since the heating time is extremely short in this embodiment, the alloying reaction takes place on only the interface between the tin thin film and the gold bump and about 1 µm-thick tin thin film can be formed on the gold bump with an extremely high level of bonding strength. In accordance with ordinary sintering methods which use a heating furnace, the tin thin film and the gold bump cause always the eutectic reaction so that the tin thin film cannot be formed. In accordance with this embodiment, heating can be made locally and instantaneously by use of the laser beam and the printed film can be sintered without heating other portions than the printed film. In addition, the high reflector film of the laser beam, which has in advance been formed before the formation of the printed film, reflects the laser beam radiated to the protions other than the printed film, prevents heating of these portions and protects the IC. Since the printed film consisting of the metallic particles becomes the high reflector of the laser beam after sintering, the excessive laser beam radiated after sintering is reflected and any unnecessary heating dones not take place. Since the bump can be formed with extremely high producibility in accordance with this embodiment, a drastic reduction of production cost can be accomplished in the TAB packaging method. From some dozens to some hundreds of bumps exist in the IC for TAB packaging which is a few millimeters square, but only a few seconds of time is necessary to sinter all the bumps of this IC in accordance with the method of this embodiment by sintering one some dozens of bumps by use of a laser beam which is a few millimeters in diameter, because the laser beam radiation time necessary for sintering in this embodiment is only a few milliseconds. Furthermore, since the bump that has one been sintered is a high reflector of the laser beam, no adverse influences are exterted even if the laser beam radiation is carried out superposedly, and this is extremely advantageous. (Verification 2) Figs. 24 to 27 are photographs showing how the gold ultra-fine particles constituting the gold printed film have changed due to the laser radiation. Fig. 24 is a surface observation photograph of the gold printed film before laser radiation which is printed and dried on the IC surface, and Fig. 26 is its sectional observation photograph. Fig. 25 is a surface observation photograph of the gold printed film after laser radiation which is printed and dried on the IC surface, and Fig. 27 is sectional observation photograph. Reference numeral 29 in Figs. 26 and 27 represents a resin used for fixing the substrate for the convenience's sake. As can be seen from Figs. 24 and 26, the gold printed film consists of the laminate of the gold ultra-fine particles having a mean particle diameter of about 1.5 µm before the laser beam radiation. After the laser radiation, however, the gold ultra-fine particles are fused with one another, as can be seen from Figs. 25 and 27, and form a compact thick film. As can also be seen from Fig. 27, fusing occurs up to the lowermost particles (or the particles at the bottom) in the longitudinal direction. Incidentally, the YAG laser is used as the laser, the radiation time is 1 ms and the power density is about 0.8 J/mm².

Claims: A method of sintering fine particles of at least one metal or of a mixture of fine particles of at least one metal and at least one non-metal to a sintered portion by radiating energy thereto, particularly laser energy, characterized in that as at least part of the particles a black-color component in form of ultrafine particles of metal and/or ceramics is used the particle size of which is 0.01 µm to 1 µm or below and the reflection factor of which is below 50% of the reflection factor in bulk, in that the ultrafine particles of the black-color component having the state of an energy absorber before sintering are converted to the state of an energy reflector after sintering, and in that the radiation quantity of said energy applied for sintering is selected in such a manner as to stop or check the progress of heating of the sintered portion with completion of the sintering. A method according to claim 1, characterized in that the surface of the black-color component in form of the ultrafine particles is fused and welded by said radiated energy whereby the black component is varnished by fusion so as to exhibit metallic luster and is converted from light absorber to light reflector. A method according to claim 2, characterized in that the black-color component or the mixture including the black-color component is disposed on a base before, particularly a portion to be fused, and welded therewith by said radiated energy. A method according to claims 1 to 3, characterized in that said energy radiation is controlled by a sensor for the energy absorption factor or the energy reflection factor or both of them of the sintering and fusing metal. A method according to one of the claims 1 to 4, characterized in that the material to be sintered or fused is applied on predetermined portions of a substrate as a paste containing the black-color component, preferably by printing in a predetermined pattern. A method according to claim 5, characterized in that a film of a material having a high reflection factor to the radiated energy is formed in advance on the surface of the substrate. A method according to claim 5 or 6, characterized in that a film for improving wettability between the portion obtained by said energy radiation and the substrate is formed in advance on the surface of the substrate. A method according to one of the claims 5 to 7, characterized in that at least two kinds of metallic particles are coated separately on the substrate or in-advance-prepared substrate to form a multi-layered metallic ultrafine particle film. Use of the method of one of the preceeding claims for connecting a lead wire to a substrate in form of an IC-chip by forming a bump.

Inventors: ARAYA TAKESHI, HANEDA MITSUAKI, HIOKI SUSUMU, OKADA RYOJI, ARAYA, TAKESHI, HANEDA, MITSUAKI, HIOKI, SUSUMU, OKADA, RYOJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: B22F3
IPC codes: B23K26, B22F7, B22F3, H01L21, B23K35, C04B35