MOLECULAR BEAM EPITAXY APPARATUS

Abstract: In a molecular beam epitaxy apparatus in accordance with the present invention, transfer means for transferring substrates (11), which have been transferred from an introduction chamber (4), to a growth chamber (2) and for transferring the substrates (11) after the growth of a film, which have been transferred from the growth chamber (2), to a discharge chamber (5) consists of a rotary disc (16) which supports thereon a plurality of substrates (11) and transfers them to the growth chamber (2) and then to the discharge chamber (5), and all of the introduction chamber (4), the growth chamber (2) and the discharge chamber (5) are disposed at predetermined positions, respectively, so that the molecular beam crystal growth can be effected in a clean room which is separated from a maintenance room.

Description: The invention relates to a molecular beam epitaxy apparatus for forming a film on a substrate by molecular beam crystal growth, comprising a growth chamber provided with a substrate holder for effecting molecular beam crystal growth, an introduction chamber for introducing substrates, a discharge chamber for discharging substrates after growth, gate valves between these chambers and transfer means for the substrates. Such an apparatus, as known from EP-A-0 067 705 comprises a vacuum lock chamber for introducing a substrate in form of a wafer and a further vacuum lock chamber for discharging the coated substrate in form of the wafer. Both vacuum lock chambers are connected with an implant chamber in which the wafer is treated by a ion beam. In the implant chamber a pivotable plate arrangement is provided for taking up the wafer introduced by means of the force of gravity and for releasing the ion beam treated wafer to the discharge chamber under the effect of the force of gravity. For keeping the vacuum within the vacuum lock chambers and the implant chamber, gate valves are provided at the inlet side and outlet side of the vacuum lock chambers. As this prior art apparatus allows introducing, treating and discharging of only one substrate, the apparatus is not suitable for mass production. JP-A-59 86 143 describes an automatic wafer exchange device having an intermittently rotated disk, a conveying unit that floats and conveys a magazin from a wafer extraction means to a wafer housing means, intermediate stations, where the dummy wafer between the disk and the conveying unit can be extracted and housed by a freely opening and closing arm piece as well as free conveying arm devices. Even when the number of the wafers that should be mounted on the disk is insufficient, the dummy wafer can automatically be mounted. When an ion is injected and processed, a wafer cooling unit at the rear of an opening of the disk that is not blocked by the wafer is irradiated with the ions. Thus, the inner part of a vacuum chamber can be prevented from being damaged by dirt. Further, the generation of dirt is prevented by the move of the magazin. This allows to improve the cleanliness of the wafers and a smooth and quick replacement of the wafers. In "Advances in Cassette to Cassette Sputtercoating Systems" by F.T. Turner and D.J. Hara, Solid State Technology, Vol.26 No.7 (July 1983), pages 115-123, a wafer handling mechanism is described having a wafer loading mechanism, onto which an operator places a cassette load of wafers so as to engage a drive pin that moves the cassettes. A wafer flat alignment mechanism is first elevated. This alignes all the flats in a cassette-load of wafers at one time. The cassette is then indexed sideways under the control of a microprocessor until the first wafer is located above a wafer elevator blade. This blade has a Vee groove in its upper face that engages with the edge of the wafer when the blade is elevated. On activation the blade rises through the cassette, lifting the wafer vertically to where it is positioned in front of the load door. The backside vacuum pick-up is then activated to hold the wafer to the door, thus allowing the blade to retract below the cassette. There are actuators that clear the edges of the wafer and that are retracted to allow clips to grip the wafer by its edge. The wafer is released from the door by deactivation of the vacuum pick-up that is a load lock which is evacuated and through which the wafer is taken to a vacuum process. With this device wafers are handled only by their edges or backsides and they remain oriented vertically through all process steps. There is no wafer support pallet to be carried with the wafer. It is the object of the invention to provide a molecular beam epitaxy apparatus of the generic kind which is compact in size and suitable for mass production. This object is obtained with the molecular beam epitaxy apparatus of the generic kind by a transfer chamber connected with the growth chamber, the introduction chamber and the discharge chamber through the respective gate valve, by a rotary disk in said transfer chamber for supporting and transferring substrates and having a start of rotation amidst the introduction chamber and the discharge chamber, by a position of the introduction chamber within the range of rotation of the disk from the start of rotation to a point of 3/8 rotation, by a position of the growth chamber within the range of rotation of the disk from the point of 3/8 rotation to a point of 5/8 rotation, by a position of the discharge chamber within the range of rotation of the disk from the point of 5/8 rotation to the start of rotation, by a first swivel arm for transferring a substrate from the introduction chamber onto the disk in a first position, to which a first substrate push-up means is associated for transferring the substrate from the first swivel arm onto the disk, by a second swivel arm for transferring a substrate from a third position, to which a third push-up means is associated for transferring the substrate from the disk onto the second swivel arm, into the discharge chamber, and by a retractile substrate transfer mechanism fixed to the transfer chamber for transferring a substrate from a second position, to which a second push-up means is associated for transferring the substrate between the disk and the retractile substrate transfer mechanism, onto the substrate holder in the growth chamber and from the substrate holder to the disk. Preferably the introduction chamber is positioned within the range from the start of rotation to the point of 2/8 rotation, and said discharge chamber is positioned within the range from the point of 6/8 rotation to the start of rotation. Advantageously a cassette for storing a plurality of said substrates is stored in said introduction chamber. Conveniently the number of the substrates supported by said substrate support means is equal to, or greater than, the number of said substrates stored by said cassette. The gate valves can be disposed on the same plane and the transferring direction of the substrates can be a horizontal direction. The molecular beam epitaxy apparatus according to the invention can be installed easily at a position devided by a partition wall which devides a clean room from a maintenance room and is suitable as an apparatus for mass production and is compact in size. The inventon is further explained by means of drawings in which Fig. 1(a) is a plan view of a molecular beam epitaxy apparatus in accordance with one embodiment of the invention; Fig. 1(b) is an explanatory view useful for explaining the rotation angle of a rotary disc in the molecular beam epitaxy apparatus of the invention; Figs. 2 to 4 are detailed explanatory views useful for explaining in detail substrate transfer means in the molecular beam epitaxy apparatus of the invention; and Fig. 5 is an explanatory view useful for explaining the crystal growth process in the molecular beam epitaxy apparatus of the present invention. The apparatus shown in Fig. 1a is mounted on a base 1, and a molecular beam crystal growth chamber (hereinafter called the "growth chamber") 2, a transfer chamber 3, an introduction chamber 4 and a discharge chamber 5 are arranged on the base as shown in the drawing. Though the transfer chamber 3 in this embodiment is a square pole, it may be hexagonal prism so that the growth chamber 2, the introduction chamber 4 and the discharge chamber 5 can be disposed on every other sides, or the introduction chamber 4 and the discharge chamber 5 may be disposed reversely. Fig. 1(b) is a detailed view which is useful for explaining in detail the relation of position between the introduction chamber 4, the growth chamber 2 and the discharge chamber 5. The introduction chamber 4 is disposed within the range of from the start of rotation (0°) to a point of 3/8 rotation (135°) and preferably to a point of 2/8 rotation (90°), the growth chamber is disposed within the range of from the point of 3/8 rotation to a point of 5/8 rotation (225°) and the introduction chamber is disposed within the range of from the point of 5/8 rotation, preferably from a point of 6/8 rotation (270°), to the start of rotation. A vacuum exhaust system of each chamber is disposed substantially inside the base 1 (though the system is not shown in the drawing). The vacuum exhaust system comprises an appropriate combination of an ion pump, a turbo molecular pump, a cryopump, a titanium sublimation pump, an oil-sealed rotary pump, and the like. The growth chamber 2 and the transfer chamber 3 are separated by a gate valve 6, the transfer chamber 3 and the introduction chamber 4 are separated by a gate valve 7 and the transfer chamber 3 and the discharge chamber 5 are separated by a gate valve 8. These chambers are closed and kept vacuum independently of one another other than when the substrates 11 are transferred to other chambers. Incidentally, the vacuum pressured to be attained for each chamber is about 10⁻⁹Pa for the growth chamber 2, about 10⁻⁸Pa for the transfer chamber 3 and about 10⁻⁶Pa for the introduction and discharge chambers 4 and 5. A substrate holder 9 and a molecular beam source 10 are disposed inside the growth chamber 2. Six to ten molecular beam sources 10 are fitted as one set, though the number varies according to the application. The substrate holder 9 is constructed such that it can revolve round the axis of the main body as the center (represented by an arrow C), can hold the substrates 11 and can rotate round its own axis (represented by an arrow D) so as to rotate the substrates 11. The position of revolution can be changed in accordance with the applications such as reception of the substrates, the molecular beam crystal growth and analysis. Its rotation is used for making uniform the film thickness of the molecular beam crystal growth. Each molecular beam source 10 is equipped with a shutter 12 so that the molecular beam crystal growth can be controlled by opening and closing the shutter 12. The growth chamber 2 is equipped with shrouds 13 and 14 into which liquid nitrogen can be charged and which are free from contamination by excessive molecules evaporated from the molecular beam source 10. Substrate support means 16 is disposed in the transfer chamber 3 in such a manner as to be capable of rotation with a bearing 15 as the center. The substrate support means 16 can support the substrates 11 the number of which is close to, or greater than, the number of substrates that the cassette 17 can carry once. Accordingly, the substrates 11 that are charged into the introduction chamber 4 from the atmosphere are all transferred to the substrate support means 16 of the transfer chamber 3 immediately after the introduction chamber 4 is evacuated and then wait for the molecular beam crystal growth. As described already, since the pressure of vacuum is lower in the transfer chamber 3 than in the introduction chamber 4, the effect of cleaning the substrates can be improved by separate heating means or the like. Substrate push-up means 20 shown in Figs. 2 and 4 are pivotally disposed at three positions of the transfer chamber 3 represented by symbols M1 to M3, and a retractile substrate transfer mechanism 18 is disposed with a pin 21, which is fixed to the vessel of the transfer chamber 3, as the support point. The direction of retraction of the substrate transfer mechanism 18 is restricted by guide means or the like (not shown in the drawing) disposed separately. Swivel arms 23 and 24 are rotatably disposed at those positions which associate with the positions represented by symbols M1 and M3. Fig. 2 is an explanatory view showing the transfer method of the substrates 11 from the introduction chamber 4 to the transfer chamber 3. The cassette 17 set to the introduction chamber 4 can be moved up and down intermittently in accordance with the pitch of disposition of the substrates 11 by an elevation mechanism through bellows 33. Guide rails (not shown in the drawing) or the like prevent the oscillation of the cassette 17. Fig. 3 shows a driving mechanism of the swivel arm 24, and the swivel arm 23 has the same construction but in an opposite direction. The swivel arm 24 has a substrate loading position 24a, and a shaft 27 is fixed to the other end of the swivel arm 24. The shaft 27 is supported by the bearings 25 and 26 and a lever 28 is fixed to the other end of the shaft 27. The lever 28 is supported by a pin 29 so as to be rotatable with the driving mechanism 30, which is communicated with the atmosphere through bellows 31 and is driven straight by an actuater (not shown in the drawing). Here, a series of operations from the charge of the substrates 11 to discharge from the apparatus after completion of the molecular beam crystal growth will be described. Though the actuater and a sensor are omitted, the molecular beam epitaxy apparatus of the present invention can be operated fully automatically by the combination of the actuator using a pressurized fluid or electric power as its driving source and the sensor. A predetermined number of substrates 11 that are set to the cassette in the atmosphere are set to a predetermined position for each cassette inside the introduction chamber 4 by opening the door 4a of the introduction chamber 4 whose pressure is returned to the atomspheric pressure, and then the door 4a is closed. Since the door 4a is frequently opened and closed, an elastomer such as fluororubber is used as seal means of the door 4a, for example. At this time the gate valves 6, 7, 8 are all kept closed and each chamber other than the introduction chamber 4 is kept vacuum. Next, the introduction chamber 4 is evacuated and after the pressure is sensed to be below a predetermined pressure, the gate valve 7 is opened and the swivel arm 23 is rotated to the state shown in Fig. 1. Then, the cassette 17 is lowered to place the lowermost substrate 11 on the swivel arm 23. Fig. 2 shows the state where the lowermost substrate 11 is placed on the swivel arm 23. Thereafter the swivel arm 23 is returned to a position represented by the symbol M1. At this time, the substrate 11 exists at the position 11b. Next, the push-up mechanism 20 is operated to cut off the connection with the swivel arm 23 and after the swivel arm 23 rotates and retreats to the position symmetric with the swivel arm 24, the substrate 11 is delivered to the substrate support means 16 while the push-up mechanism is being lowered. Then, the substrate support means 16 is rotated by one pitch. This operation is repeated in the number of times corresponding to the number of substrates, whereby all the substrates 11 of the cassette 17 are transferred to the transfer chamber 3. After the gate valve 7 is closed, the introduction chamber 4 enters the waiting state for the introduction of the next substrate. The substrate support means 16 is indexed at the position of each substrate and rotates and stops so that the substrate 11 which is to be subjected to the molecular beam crystal growth comes to the position of M2. Here, the push-up mechanism 20 shown in Fig. 4 pushes up the substrate 11 to the position 11a, extends to the position where the substrate support portion 22 of the transfer mechanism 18 comes to the position M2, once stops there, and transfers the substrate 11 to 22 by lowering the push-up mechanism 20. Then, after the gate valve 6 is opened, the transfer mechanism 18 is extended inside the growing chamber 2 to the position of delivery with respect to the rotary manipulator 9. At this time, the rotary manipulator 9 rotates (revolves) by 90° from the state shown in Fig. 1 so that the substrate fitting surface faces downward and faces the substrate 11 that is carried by the transfer mechanism 18. Next, the substrate 11 is pushed up to a suitable position by the push-up mechanism and is meshed with the rotary manipulator 9, and then the push-up mechanism is lowered so that the transfer mechanism 18 is returned to the original position and the gate valve 6 is closed. In this manner, the preparation for the molecular beam crystal growth is complete. In Fig. 1, the rotary manipulator 9 which receives the substrate 11 rotates by about 90° and effects the molecular beam crystal growth under the state shown in Fig. 1. After the molecular beam crystal growth is complete, the substrate 11 is returned from the growth chamber 2 to the position M2 by opposite means to the means for carrying the substrate into the rotary manipulator. When the substrate 11 is immediately taken out, the substrate support means 16 is rotated so that the substrate 11 comes to the position M3, and then the take-out operation may be carried out. However, it is more reasonable from the respect of keeping vacuum to sequentially stock the substrates 11 after the molecular beam crystal growth in the substrate support means 16 and are then taken out altogether when processing of one cassette of substrates is complete. The sequence for taking out the substrates is substantially reverse to the take-in operation which is briefly as follows. Namely, the substrate 11 at the position M3 is lifted by the push-up mechanism 20 at the position M3, then the swivel arm 24 is rotated so as to receive the substrate 11 from the push-up mechanism 20, the gate valve 8 is opened and the swivel arm 24 is further rotated to deliver the substrates 11 to the discharge chamber 5. The difference of this operation from the operation in the introduction chamber 4 is that since the cassette 17 pushes up and receives the substrate 11 from below the swivel arm 24, the cassette 17 receives the substrates 11 from its upper stage side. In accordance with the embodiment described above, the introduction chamber 4 and the discharge chamber 5 are disposed in such a fashion that they can be divided easily from the growth chamber 2 by the partition wall so that the so-called "through-the-wall" of the molecular beam epitaxy apparatus can be disposed easily. Even when baking treatment is applied to both the growth chamber and the transfer chamber in order to obtain ultra-high vacuum, the dust generated thereby can be confined in the maintenance room and the introduction and discharge of the substrates into and from the apparatus can be always carried out in the clean room. Moreover, since the transfer means of the substrates consists of the rotary disc, the apparatus can be made compact (or the floor area for installation can be reduced by about a half). Incidentally, the relation of position between the introduction chamber 4 and the discharge chamber 5 may be reverse to that of the embodiment described above. In such a case, producibility somewhat drops but the other effects remain the same as those of the embodiment described above. Furthermore, the ratio of the length (r) of the lever on the driving side to the length (R) of the arm on the transfer side is intentionally set to an extremely large ratio so that the moving quantity δ of the driving portion can be minimized with respect to the necessary transfer quantity Δ, and the discharge quantity can be reduced by the extension and contraction of the bellows. Fig. 5 is a diagram which is useful for explaining the process for the molecular beam crystal growth by a typical temperature T M of the molecular beam source and a substrate temperature T S . After temperature elevation of the molecular beam source is started, the substrate is received by the rotary manipulator and the substrate temperature is then raised (the sequence may be reverse). Here, a time t₁ represents a substrate delivery time. When the temperatures T M and T S reach the predetermined steady values, respectively, the molecular beam crystal growth is effected (t E1 ) and after it is complete, the temperature T M of the molecular beam source is somewhat lowered or kept as such to wait for the next molecular beam crystal growth. On the other hand, the substrate temperature is lowered to a level where no problem occurs for the transfer operation. (It is not necessary to reduce the temperature to its original temperature.) Then, the substrate is replaced by a next substrate (t₂). After the temperature of the second substrate is raised and after the predetermined temperature condition is established, the molecular beam crystal growth is effected (t E2 ). Since the processing period t f of this second substrate is repeated, the molecular beam epitaxy apparatus of the present invention becomes fully automatic including not only the transfer operation of course but also the molecular beam crystal growth process.

Claims: A molecular beam epitaxy apparatus for forming a film on a substrate (11) by molecular beam crystal growth, comprising a growth chamber (2) provided with a substrate holder (9) for effecting molecular beam crystal growth, an introduction chamber (4) for introducing substrates (11), a discharge chamber (5) for discharging substrates (11) after growth, gate valves (6, 7, 8) between these chambers (2, 4, 5) and transfer means for the substrates (11) characterized by a transfer chamber (3) connected with the growth chamber (2), the introduction chamber (4) and the discharge chamber (5) through the respective gate valve (6, 7, 8), by a rotary disk (16) in said transfer chamber (3) for supporting and transferring substrates (11) and having a start of rotation amidst the introduction chamber (4) and the discharge chamber (5), by a position of the introduction chamber (4) within the range of rotation of the disk (16) from the start of rotation to a point of 3/8 rotation, by a position of the growth chamber (2) within the range of rotation of the disk (16) from the point of 3/8 rotation to a point of 5/8 rotation, by a position of the discharge chamber (5) within the range of rotation of the disk (16) from the point of 5/8 rotation to the start of rotation, by a first swivel arm (23) for transferring a substrate (11) from the introduction chamber (4) onto the disk (16) in a first position (M1) to which a first substrate push-up means (20) is associated for transferring the substrate (11) from the first swivel arm (23) onto the disk (16), by a second swivel arm (24) for transferring a substrate (11) from a third position (M3) to which a third push-up means (20) is associated for transferring the substrate (11) from the disk (16) onto the second swivel arm (24), into the discharge chamber (5), and by a retractile substrate transfer mechanism (18) fixed to the transfer chamber (3) for transferring a substrate (11) from a second position (M2), to which a second push-up means (20) is associated for transferring the substrate (11) between the disk (16) and the retractile substrate transfer mechanism (18), onto the substrate holder (9) in the growth chamber (2) and from the substrate holder (9) to the disk (16). A molecular beam epitaxy apparatus according to claim 1, characterized in that the introduction chamber (4) is positioned within the range from the start of rotation to the point of 2/8 rotation and said discharge chamber (5) is positioned within the range from the point of 6/8 rotation to the start of rotation. A molecular beam epitaxy apparatus according to claim 1 or 2, characterized in that a cassette (17) for storing a plurality of said substrates (11) is stored in said introduction chamber (4). A molecular beam epitaxy apparatus according to claim 3, characterized in that the number of said substrates (11) supported by said substrate support means (16) is equal to, or greater than, the number of said substrates (11) stored by said cassette (17). A molecular beam epitaxy apparatus according to anyone of the claims 1 to 4, wherein the gate valves (6, 7, 8) are disposed on the same plane, and the transferring direction of said substrate (11) is a horizontal direction.

Inventors: ICHIHASHI, KAZUAKI, KAMOHARA, HIDEAKI, KANAI, NORIO, TAMURA, NAOYUKI
Applicants: HITACHI, LTD.
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, C30B35, C30B23