APPARATUS FOR BONDING SEMICONDUCTOR WAFERS

Abstract: An apparatus for bonding semiconductor wafers com­ prises first and second tables (20a, 20b) having/first and second elastic bases (24a, 24b) each of which has one surface on which a semiconductor wafer having a mirror surface is mounted with the mirror surface fac­ ing up and the other surface, and a plurality of through holes (26a, 26b) extending from one to the other sur­ faces thereof, a suction mechanism for reducing pressure in the through holes and chucking the semiconductor wafers to said bases and a projecting mechanism, pro­ vided to face the other surface of at least said second base, for urging the other surface to elastically deform said base corresponding thereto, thereby projecting a central portion of said base to project the mirror sur­ face of the semiconductor wafer. The projecting mecha­ nism includes projecting member (34) having a conical end surface which is capable of being brought into contact with the other surface of the second elastic base.

Description: The present invention relates to a method of bond­ ing mirror surface finished semiconductor wafers and an apparatus therefor. In a typical technique used in the manufacture of semiconductor devices, a semiconductor layer of a spe­ cific conductivity type is formed on a semiconductor substrate by a CVD method and a functional element is formed in this region. In this CVD method, it is neces­ sary to prevent an impurity from diffusing or mixing in from the substrate into the semiconductor layer. Recently, a demand for an element with a high di­ electric withstand voltage is high. Dielectric with­ stand voltage can be improved by thickening the semicon­ ductor layer. However, it takes extremely long time to do this by the CVD method, and it is more difficult to prevent a diffusing of impurity. In the invention described in Japanese Patent Publication No. 49-26455, semiconductor wafers are bond­ ed such that semiconductor layers oppose each other, so as to preventimpurity mixing and to reduce manufactur­ ing time as compared with the case wherein a semiconduc­ tor layer is formed using the CVD method. The method is performed as follows. Mirror surface finished semi­ conductor wafers are placed to oppose each other in an atmosphere of a gas mixture consisting of oxygen and nitrogen, and phosphorus is diffused into each mirror surface at a temperature of l,200°C for 30 minutes using phosphorus oxychloride as an impurity. The wafers are then heated at a temperature of l,l50°C for an hour. After placing the wafers in a vacuum, the wafers are bonded at a pressure of l50 kg/cm² and a temperature of l,300°C. However, when a bonding temperature is higher than that during diffusion as described above, the diffusion impurity is redistributed during bonding to change a surface concentration of an impurity layer. In addi­ tion, wafer alignment in a vacuum is difficult to per­ form and results in high cost. In order to solve the above problem, a method of bonding wafers in air at room temperature is disclosed by the present inventor in "Nikkei Electronics", January 27, l986, p. l08 to ll0. In this method, after the mir­ ror surfaces of wafers are washed with water and excess water is removed, the wafers are bonded together in a clean atmosphere utilizing residual water on the mirror surfaces. According to this method, impurity redistri­ bution is prevented and wafer alignment is easy. However, in this method, air bubbles are left be­ tween wafers to form voids in their interfaces. It is an object of the present invention to provide a method of bonding wafers without forming voids even when wafers are bonded in air at room temperature, and a mechanism therefor. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. l is a perspective view schematically showing a wafer bonding system which uses a bonding apparatus according to an embodiment of the present invention; Fig. 2 is a partially broken side view showing a bonding apparatus according to the embodiment of the present invention with wafers mounted and chucked; Fig. 3 is a partially broken side view showing the apparatus with wafers opposing each other and the center of a mirror surface of an upper wafer projecting; Figs. 4A to 4C are sectional views showing a part of the apparatus in each step which illustrates a method of bonding wafers; and Fig. 5 is a sectional view showing a part of a modified mechanism. An embodiment of the present invention will now be described with reference to the accompanying drawings. Fig. l shows a wafer bonding system which uses a wafer bonding apparatus of the present invention com­ prising first and second supports 2 and 4. Rectangular enclosure 8 with an open front and a transparent top and clean unit l0 positioned therebehind are provided on pedestal 6. A filter (not shown) which cleans air supplied horizontally from a blower of the clean unit to the enclosure is provided between unit l0 and enclosure 8. With this mechanism, the interior of enclosure 8 is kept clean. In enclosure 8 on pedestal 6, first and second supports 2 and 4 are installed adjacent and opposite each other and panel l2 and TV monitor l4 are spaced apart from supports 2 and 4. Monitor camera l6 is positioned at a distance of, e.g., 500 mm from and above the top surface of enclosure 8. Operations of supports 2 and 4 are monitored by camera l6 and displayed on TV monitor l4. Operations of supports 2 and 4 can be controlled by observing TV monitor l4 and operating panel l2. As shown in Fig. 2, first and second supports 2 and 4 have first and second chuck tables 20a and 20b, respectively. Tables 20a and 20b comprise disk-like first and second elastic bases 24a and 24b. Base 24a and 24b have flat surfaces on which semiconductor wafers 22a, 22b are mounted with their mirror surfaces facing up. Bases 24a and 24b may be made of an elastic materi­ al such as rubber, or a material which is not elastic itself but imparts elasticity upon being formed into a thin member. Bases 24a and 24b have a plurality of through holes 26a and 26b, respectively. These holes are preferably formed near the periphery of the base such that the center of the base is conveniently pro­ jected. Tables 20a and 20b have support members 30a and 30b which support bases 24a and 24b on their upper surfaces, respectively. Members 30a and 30b are cylindrical with their upper portions enlarged horizon­ tally. Cavities 28a and 28b whose upper surfaces are closed by elastic bases 24a and 24b are formed in the upper portions of support members 30a and 30b. Paths 32a and 32b are respectively formed in support members 30a and 30b so that their upper ends communicate with cavities 28a and 28b and their lower ends are open at the lower surfaces of support members 30a and 30b. Projecting member 34 which faces the other surface of base 24b and projects at the center thereof is vertical­ ly movably housed in cavity 28b of table 20b of support 4. More specifically, an upper surface of projecting member 34 is conical. Columnar sliding portion 37 pro­ jects from the lower surface of projecting portion 34 and is slidable with respect to the inner upper wall of path 32b of support member 30b. Path 36 is formed in projecting member 34 so that its lower end is open at the lower surface of sliding portion 37 and its upper end at the conical surface. Path 36 normally communi­ cates cavity 28b with path 32b. Lower ends of paths 32a and 32b of support members 30a and 30b are respectively communicated with vacuum pumps 33a and 33b for evacuat­ ing paths 32a and 32b. Wafers 22a and 22b respectively mounted on bases 24a and 24b are drawn upon operation of vacuum pumps through paths 32a and 32b, cavities 28a and 28b, and holes 26a and 26b, and are chucked to surfaces of bases 24a and 24b. Support member 30b is slidably fitted in a vertical through hole in bracket 42 mounted on one end of rotat­ ing arm 40. In the state shown in Fig. 2, support member 30b is held such that its lower surface is in contact with the upper surface of bracket 42. First fixing member 54a is mounted on a lower portion of support portion 30b projecting from the lower end of bracket 42, and fixes at its one end first terminal 52a constituting a contact sensor for wafers. Second termi­ nal 52b cooperates with terminal 52a to constitute the sensor. Terminal 52b is coaxial and at a predetermined distance from terminal 52a, and is fixed by second fix­ ing member 54b projecting horizontally on the lower portion of bracket 42 of arm 40. The contact sensor generates an electrical signal when its terminals are not in contact with each other. The contact sensor is electrically connected to a motor so as to close a drive circuit of a Z pulse motor which is described later, and to a drive circuit of a vacuum pump for evacuating a pedestal so as to stop the operation of the pump by an electrical signal. Coil spring 55 is arranged between fixing members 54a and 54b so as to bias first fixing member 54a downwardly. Arm 40 is mounted through shaft 46 on arm support column 44 whose lower portion is fixed to pedestal 6 so as to pivot about the shaft in a vertical plane. Shaft 46 is rotated by motor 48 via a known transmission mechanism in column 44. First support 2 comprises block 3 which supports table 20a to be rotatable and vertically movable, and X, Y, Z, and ϑ pulse motors 60, 62, 64, and 66. Motors 60 and 62 can move X-Y stage 68 horizontally, i.e., in X and Y directions by desired amounts. Block 3 is mounted on X-Y stage 68 to move therewith. Z and ϑ moving mechanism is installed in block 3. Bolt 76 is connected through joint member 74 to rotating shaft 72 of motor 64 which is fixed to block 3. Bolt 76 is screwed into a female threaded through hole formed in lift plate 78. The lift plate is connected to support member 30a to be free from rotation of support member 30a and interlocked with vertical movement thereof. In order to provide this connection, a connecting mechanism comprising a recess which is formed at an outer periphery of support member 30b and engageable with a projection provided on plate 78 can be adopted. First gear 82 is coaxially mounted on rotating shaft 80 of motor 66 which is fixed to block 3. Gear 82 is meshed with second gear 84 which is coaxially mounted on the lower portion of support member 30a. Gears 82 and 84 are arranged such that gear 82 is meshed with gear 84 and is free from vertical movement thereof. A method of bonding semiconductor wafers using a bonding apparatus having the above construction will now be described with reference to Figs. 3 and 4A to 4C. Wafers 22a and 22b having mirror surface finished upper surfaces are respectively chucked on first and second elastic bases 22a and 22b by evacuating table 20a and 20b. Next, as shown in Fig. 3, motor 48 is driven to pivot arm 40 counterclockwise through l80°. As a result, second support member 30b moves downwardly against the biasing force of spring 55 by its own weight and is held where first terminal 52a is in contact with second terminal 52b. In addition, projecting member 34 moves downwardly by its own weight and urges base 24b such that the center portion thereof projects downward­ ly. It will be obvious that vertical alignment of sup­ port member 30b and a projection action of the wafer can be achieved automatically by pivoting arm 40 through l80°. In this state, support member 30a is horizontally shifted and pivoted by driving and controlling X, Y, and ϑ motors 60, 62, and 66 to match orientation flats of both wafers. Next, Z motor 64 is driven to move support member 30a upwardly to bring wafer 22a into contact with the central portion of wafer 22b, as shown in Fig. 4B. By this contact, the second wafer or the second support member is slightly shifted upwardly, terminals 52a and 52b are disconnected, and an electric signal is generat­ ed to stop motor 64 and the vacuum pumps. As a result, wafer 22b whose central portion projects downwardly is released from suction by base 24b. The periphery of wafer 22b moves downwardly to regain its flatness as a whole, and the entire surface thereof is brought into contact with wafer 22a. In this case, a bonding region of wafers 22a and 22b extends from the central portion to the periphery to prevent air bubbles from being trapped in the interface. The present invention is not limited to the above embodiment, and various changes and modifications may be made without departing from the spirit and scope of the present invention. For instance, a projecting member may be arranged so as to project lower wafers, or the central portions of two wafers may be projected. In this case, a means for moving the projecting members is required. In the above embodiment, a semiconductor wafer is projected and then moved to be brought into contact with another semiconductor wafer at their central portions. However, semiconductor wafers may be brought into contact with each other by projecting the central portion(s) of the wafer(s). In the above embodiment, the projecting member and the support member are provided independently of each other. However, they may be formed as an inte­ gral structure, as will be described with reference to Fig. 5. Cylindrical support member l00 is formed such that it has lower portion l00a, which extends through bracket 42 and is slidable in the axial direction there­ of, and upper portion l00b, which is projected from the upper surface of bracket 42 and has a diameter larger than that of lower portion l00a. The upper surface of upper portion l00b is formed to be conical, thus form­ ing a projecting member. Straight path 36 is provided in upper portion l00b such that it is open at the circumference of upper portion l00b. This path communi­ cates with path 32b provided in lower portion l00a. Cylindrical support base 24b, made of an elastic materi­ al such as rubber, is attached to the upper portion of support member l00 such that the interior of support base 24b defines a space sealed by upper portion l00b. Support base 24b is provided with through holes 26a at a position shifted from the center thereof. With the support member having the above construc­ tion, if the air in the support base 24b through paths 32b and 36 is sucked, the upper wall of support base 24b is pulled to support member l00 and deformed into a conical shape that conforms with the shape of the upper portion of support member l00. At this time, wafer 22b is deformed together with elastic member 24b in such a manner that the peripheral portions of wafer 22b are pulled toward the projecting member more greatly than the central portion. Accordingly, the central por­ tion of wafer 22b is projected, as in the embodiment described above.

Claims: 1. A method of bonding semiconductor wafers com­ prising the steps of projecting a central portion of a mirror surface of at least one wafer (22b) of two semiconductor wafers (22a, 22b) having mirror surfaces, opposing said two wafers each other and bringing the central portions of the mirror surfaces of the two wafers into contact with each other, and enlarging a contact region from the central portions to peripheries of the mirror surfaces of the wafers. 2. A method according to claim l, characterized in that said projecting step comprises the step of flatten­ ing the mirror surface of one wafer and projecting the central portion of the mirror surface of the other wafer toward said one wafer. 3. A method according to claim 2, characterized in that said projecting step comprises the step of conical­ ly projecting the mirror surface of the other wafer. 4. A method according to claim l, 2 or 3, charac­ terized in that said method comprises the step of oppos­ ing said wafers each other and aligning them before said projecting step. 5. An apparatus for bonding semiconductor wafers comprising: first and second tables (20a, 20b) having first and second elastic bases (24a, 24b) each of which has one surface on which a semiconductor wafer (22a, 22b) having a mirror surface is mounted with the mirror surface fac­ ing up and the other surface, and a plurality of through holes (26a, 26b) extending from one to the other sur­ faces thereof; a suction mechanism (33a, 33b) for reducing pres­ sure in said through holes and chucking the semiconduc­ tor wafers to said bases; a projecting mechanism (34), provided to face the other surface of at least said second base, for urging the other surface to elastically deform said base corre­ sponding thereto, thereby projecting a central portion of said base to project the mirror surface of the semi­ conductor wafer; first and second support mechanisms (30a, 30b) for supporting said first and second tables and said pro­ jecting mechanism; and drive means (33a, 33b) for moving said first and second support mechanisms relative to each other and pressing the mirror surfaces of the semiconductor wafers against each other with the central portion of the mirror surface of at least one semiconductor wafer. 6. An apparatus according to claim 5, character­ ized in that said first and second elastic bases (24a, 24b) are supported at one ends of the first and second support members, the support members having shafts and paths (32a, 32b) which communicate with said through holes (26a, 26b) of said bases, and said suction mecha­ nism evacuates said paths of said first and second sup­ port members to reduce pressure in said through holes. 7. An apparatus according to claim 5 or 6, charac­ terized in that said first support mechanism has a block (3) which supports said first table (20a) pivotally about its axis and movably along the axis. 8. An apparatus according to claim 7, character­ ized in that said drive means comprises a rotating mechanism (60, 80, 82, 84) for rotating said first table with respect to the block and a moving mechanism (64, 72, 76, 78) for axially moving said first table with respect to said block. 9. An apparatus according to claim 7 or 8, charac­ terized in that said apparatus comprises an X-Y table (68) having said block thereon. l0. An apparatus according to claim 6, characterized in that said second support mechanism comprises an arm (40) for axially and movably supporting said second table (20b) and a support column (44) for pivotally supporting said arm, and said drive means comprises an arm pivoting mechanism (48) for pivoting said arm to allow the semiconductor wafer on said second elastic base to face the semiconductor wafer on said first elastic base. 11. An apparatus according to claim l0, character­ ized in that said projecting mechanism comprises a pro­ jecting member (34) provided axially and movably in the path of said second table and having at a center of one end surface thereof a projection capable of being brought into contact with the other surface of said second elastic base. 12. An apparatus according to claim ll, character­ ized in that one end surface of said projecting member (34) is conical. 13. An apparatus according to any one of claims 5 to l2, which comprises a sensor (52a, 52b) which senses the contact between the central portions of said semiconductor wafers and thereupon releases evacuation in said through holes of the second elastic base.

Inventors: YOSHIKAWA, KIYOSHI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01L21
IPC codes: H01L21