Molecular beam epitaxy apparatus

Abstract: A molecular beam epitaxy apparatus according to the present invention comprises a growth chamber (2) evacuatable to a ultra high vacuum, and a holder support (3) arranged in the growth chamber for supporting a substrate holder (H). The growth chamber has a mounting portion (2b) which is provided with a vaporizer (5a) for a group V element as well as with other vaporizers (5b, 5c) for additional materials. The vaporizers respectively have emission openings directed toward the center of the holder supported by the holder support. The emission openings of the additional material vaporizers are respectively closable by closure devices (7), whereas the emission opening of the group V element is always held open.

Description: MOLECULAR BEAM EPITAXY APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to molecular beam epitaxy apparatus according to the preamble of claim 1. 2. Description of the Prior Art: Molecular beam epitaxy (often abbreviated to "MBE") has attracted attentions as a method for expitaxially growing a thin layer on a monocrystal substrate in the production of compound semiconductors, particularly group III-V compound semiconductors. In MBE, which is one of vacuum deposition methods, group III elements such as Ga, Al, In and group V elements such as As, P are emitted in the form of molecular or atomic beams (As₂ or As₄ in the case of arsenic) under a ultra high vacuum of e.g. about 1.3 x 10⁻⁸ Pa (10⁻¹¹ Torr), and deposited on a monocrystal substrate of e.g. GaAs, InP to form an epitaxial layer or layers of e.g. GaAs, AlGaAs, InP, InGaAsP. The MBE is known to have various advantages. Several of these advantages are as follows: (1) Due to the use of a ultra high vacuum, it is possible to always keep clean the growth front of each substrate by expelling gaseous impurities, consequently improving the product quality. (2) Because of high vacuum, it is possible to deposit a uniform layer or film over a large area. (3) It is possible to precisely control the film thickness in angstroms because the crystal growth rate can be made very low and accurately adjusted. (4) It is possible to readily obtain a thin film of multi-component mixed crystals by simply increasing molecular beam sources. (5) Molecular beams used for crystal formation can be also used to detect the surface or growth conditions during crystal growth, so that useful information can be immediately fed back for controlling the crystal growth. For further understanding of the MBE technique, reference is now made to Figures 14 and 15 of the accompanying drawings which show a typical conventional MBE apparatus. As shown in Figure 14, the prior art MBE apparatus comprises a growth chamber 102 connected to a high vacuum pump 101 for evacuating the chamber interior to a ultra high vacuum. Centrally within the chamber 102 is a holder support 103 which is controllably rotated about a central vertical axis L. The support 103 receives a substrate holder H' loaded with a plurality of substrates B' (see Figure 15). Rotation of the holder support is required to ensure uniform crystal deposition on every substrate B'. The support 103 is provided with a heater (not shown) to heat the substrate to a temperature suitable for crystal growth. The growth chamber 102 is provided at its bottom with a plurality (only two shown) of molecular beam sources or vaporizers 105 disposed in an annular arrangement around the central vertical axis L of the growth chamber for generating molecular beams of different materials. Each source 105 is substantially equally spaced from the substrate holder H', and has a longitudinal axis A directed to the center of the holder. The source 105 includes a crucible 104 for receiving a suitable material which is heated by a heater (not shown) for vaporization. The source 105 further includes a shroud or cool trap 106 to which is supplied liquid nitrogen for preventing the source from being thermally influenced by the other sources. The molecular beam emission is controlled by opening and closing a shutter 107 arranged at the emission opening of the vaporizer 105. For depositing a GaAs layer on GaAs monocrystal substrates for example with the above apparatus, two of the vaporizers 105 for Ga and As respectively are heated while the substrate holder H' with the substrates B' is also heated to a suitable temperature, and thereafter the relevant shutters 107 are opened for a predetermined period. An additional element Al may be simultaneously vaporized to form a Ga x Al 1-x As layer in which the value of x is determined by the ratio in vaporized amount between Ga and Al. Further, simultaneous vaporization of Si or Sn provides a n-type crystal layer, whereas simultaneous inclusion of Be or Mg provides a p-type crystal layer. As shown in Figure 15, the MBE apparatus further comprises a preparation chamber 110 connected to the growth chamber 102 via a first gate valve 108. The preparation chamber 110 is provided with a second gate valve 109 for hermetically separating the preparation chamber from the atmosphere. For feeding a new set of substrates, the first gate valve 108 is held closed while the second gate valve 109 is opened, and the set of substrates B' retained by the substrate holder H' is supplied from outside into the preparation chamber 110 through the open second gate valve 109. Subsequently, the second gate valve is closed, and the preparation chamber is evacuated to a high vacuum, whereupon the first gate valve 108 is opened to supply the substrate set into the growth chamber. In this way, the time required for re-evacuating the growth chamber 102 to a ultra high vacuum is greatly reduced, thereby increasing the productivity of the apparatus. In the absence of the preparation chamber, obviously, vacuum leakage upon feeding a new set of substrates makes it necessary to re-evacuate the growth chamber from the very start, consequently requiring a long time. It is for this reason that the MBE has been formerly evaluated only as an experimental or research tool. The arrangement shown in Figure 15 opens up (gives at least a hint to) the way for application of the MBE on commercial scale. However, such an arrangement still has the following disadvantage. According to the arrangement of Figure 15, all the vaporizers 105 are closable by the respective shutters 107. Thus, the respective vaporizers 105 must be spaced from each other at least by a distance enough to allow opening movement of the respective shutters 107. Otherwise, each shutter, when opened, interferes with an adjacent shutter or the emission opening of an adjacent vaporizer. On the other hand, each vaporizer 105 needs to have a large capacity or volume in order to prolong the operating time of the epitaxy apparatus per each supply batch of the relevant material without recharging thereof. Particularly, the vaporizer for a group V element needs to have a large capacity because the consumption of this material is highest. However, enlargement of the vaporizer inevitably results in corresponding enlargement of the relevant shutter, so that a larger spacing between the thus enlarged vaporizer and an adjacent vaporizer is necessary to allow opening movement of the enlarged shutter. Thus, with this prior art arrangement wherein all the vaporizers are respectively provided with shutters, there is a great limitation in enlarging the capacity of the respective vaporizers, particularly the group V element vaporizer, because the mounting area or space within the growth chamber is limited. Further, there is also a great limitation in increasing the number of vaporizers to be mounted within the limited mounting area or space of the growth chamber. A molecular beam epitaxy apparatus according to the preamble of claim 1 is already known from FR-A-2358921. With this apparatus the group V element vaporizer is non-closable and is during operation being held open, the opening of the As vaporizer projecting toward the substrate holder. However, this pre-known apparatus cannot avoid sufficiently a malfunction and needs time-consuming and difficult maintenance. It is, therefore, an object of the present invention to provide an MBE apparatus which reduces the likelihood of malfunction and provides easy maintenance. According to the invention this object is obtained by an MBE apparatus according to the preamble of claim 1, comprising characterizing features thereof. Further advantageous embodiments are claimed in dependent claims 2 to 7. Technical feasibility of the present invention will be specifically described hereinafter. Other objects, features and advantages of the present invention will be fully understood from the following detailed description given with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Figure 1 is a plan view showing the whole arrangement of an MBE apparatus embodying the invention; Figure 2 is a sectional view taken on lines II-II in Figure 1; Figure 3 is a sectional view taken on lines III-III in Figure 1 to show a substrate transfer assembly; Figure 4 is a plan view showing a substrate holder used in the MBE apparatus of Figure 1; Figure 5 is a sectional view taken on lines V-V in Figure 4; Figure 6 is a plan view showing a first transfer tray incorporated in the MBE apparatus of Figure 1; Figure 7 is a plan view showing a second transfer tray incorporated in the MBE apparatus of Figure 1; Figure 8 is a sectional view taken on lines VIII-VIII in Figure 7; Figure 9 is a sectional view taken on lines IX-IX in Figure 1; Figure 10 is a sectional view taken on lines X-X in Figure 1; Figure 11 is a perspective view showing a post of the substrate transfer assembly illustrated in Figure 3; Figure 12 is a schematic perspective view showing the flow of substrates together with the corresponding holder within the MBE apparatus of Figure 1; Figures 13a to 13c are views showing a preferred shutter assembly as seen in three different directions; Figure 14 is a schematic view in vertical section showing the growth chamber of a prior art MBE apparatus; and Figure 15 is a plan view showing the prior art MBE apparatus with a means of supplying substrates. DETAILED DESCRIPTION Referring to Figure 1 of the accompanying drawings, a molecular beam epitaxy apparatus according to the invention mainly includes a growth chamber 2, a substrate preparation chamber 10 connected to the growth chamber by means of a first gate valve 8, and a substrate loading chamber 11 connected to the preparation chamber by means of a second gate valve 9. Crystal growth occurs within the growth chamber 2 simultaneously with respect to a plurality of substrates or wafers B held by a substrate holder H. The growth chamber is evacuated to a ultra high vacuum about (1.3 x 10⁻⁸ Pa for example) by a high ability evacuation pump (not shown) connected thereto. It should be appreciated that the epitaxy apparatus according to the invention actually utilizes a plurality of substrate holders H each receiving a set of substrates B although only one of such holders is placed in the growth chamber 2 for one operating cycle. According to this example, the growth chamber 2 comprises an upright vessel which has a cylindrical upper portion 2a and a conical or frustoconical bottom portion 2b, as illustrated in vertical section in Figure 2. The growth chamber further has a ceiling 12 centrally penetrated by a support shaft 19 which is movable up and down by means of an actuator 18 mounted on the ceiling 12 outside the growth chamber. Within the growth chamber 2 is arranged a support frame 3 which is mounted on the support shaft 19. The support frame has, at its lower end, a pair of horizontal support arms 13 which are parallel to each other as spaced diametrically of the growth chamber (see also Figure 1). The substrate holder H is placed on the support arms 13. The support frame further has an upper boss portion 14 which is rotatably fitted around the support shaft 9 by means of bearings 14a. The boss portion 14 carries a gear 15 in mesh with a drive pinion 17 which is rotated by an external motor 16 mounted on the ceiling 12. Thus, the support frame 3 together with the substrate holder H is rotated about the support shaft 19 upon actuation of the motor 16 to ensure uniform crystal growth on every substrate B. On the other hand, the support frame 3 is movable vertically with the support shaft 19 upon activation of the actuator 18. Such vertical movement of the support frame 3 is necessary to enable transfer of the substrate holder B to and from the support frame 3, as hereinafter described. The support shaft 19 is provided at its lower end with a holder cover 19a in which is disposed a heater 20 for heating, from above, the substrates B to a desired temperature. On the other hand, an annular cooling shroud 21, to which is supplied liquid nitrogen, is arranged within the growth chamber 2 along the cylindrical portion 2a thereof. The cooling shroud 21 serves to provide a cooled inner surface on which vaporized impurities deposit. The frustoconical bottom portion 2b of the growth chamber 2 is provided with a plurality of material vaporizers or molecular beam sources 5a, 5b, 5c which have respective axes a, b, c directed to the center of the substrate holder H. According to the illustrated example, the vaporizer 5a for a group V element (arsenic As for example) is disposed directly under the substrate holder H, whereas the remaining or side vaporizers b, c are disposed in an annular arrangement around the central vaporizer 5a. In Figure 2, only two vaporizers 5b, 5c are shown besides the central vaporizer 5a to avoid complication of illustration. In reality, however, any number (five to seven for example) of such vaporizers may be arranged around the central vaporizer 5a depending on a desired composition of the crystal to be formed on each wafer B. For the convenience of subsequent description, all vaporizers 5a, 5b, 5c (and etc.) are equally referred to by reference numeral 5 unless distinction among them is necessary. Each vaporizer 5 includes a crucible 4 for a suitable crystal material. The crucible is provided with a heater (not shown) for heating the crystal material to a specified temperature. The crucible is surrounded by a cylindrical cooling shroud 6 to which is supplied liquid nitrogen to make the vaporizer thermally independent of the other vaporizers and to prevent contamination by vapor impurities. The crystal material in each crucible 4, when heated to the specified temperature under a ultra high vacuum, is emitted in the form of molecular or atomic beams to be deposited on each substrate B for crystal growth. Such crystal formation is initiated and interrupted by operating a shutter assembly 7 provided at each side vaporizer 5b, 5c. The shutter assembly 7 includes a rotary shaft 7a projecting into the growth chamber 2, and a shutter plate 7b mounted on the projecting end of the rotary shaft. A more specific configuration of the shutter assembly will be described hereinafter. It suffices at this stage to mention that the shutter plate 7b is displaceable to close and open the outlet opening of the corresponding vaporizer upon rotation of the rotary shaft 7a. The central vaporizer 5a is not provided with a shutter assembly. However, the absence of the shutter assembly with respect to the central vaporizer poses no problem in controlling the crystal growth, and is in fact preferred for the reasons to be described later. As shown in Figures 4 and 5, each substrate holder H, which is made of molybdenum or a molybdenum alloy, is in the form of a disc having a plurality (seven in Figure 4) of stepped circular retaining holes 22. Each substrate B is fitted, from above, into a corresponding one of the retaining holes 22. Each retaining hole 22 is formed with a plurality (three in Figure 4) of equiangularly spaced radial cutouts 22a for the purpose to be described later. A set (seven according to the illustrated embodiment) of substrates B is placed onto a first transfer tray 23 within the loading chamber 11 (see Figure 1), and then transferred onto a corresponding substrate holder H supported on a second transfer tray 24 within the preparation chamber 10. The set of substrates B retained by the substrate holder H is transferred into the growth chamber 2 by the advancing movement of the second transfer tray 24. The preparation chamber 10, in which the second transfer tray 24 is movable, is cylindrical and extends horizontally from the growth chamber 2 substantially at the level of the support arms 13 of the support frame 3 (see Figure 2). The loading chamber 11, in which the first transfer tray 23 is movable, is also cylindrical and extends horizontally from an intermediate portion of the preparation chamber 10 perpendicularly thereto (see also Figure 3). As illustrated in Figure 6, the first transfer tray 23 is formed with a plurality (seven according to this embodiment) of stepped circular retaining holes 26 in corresponding relation to those of each substrate holder H. These retaining holes 26 are connected to each other axially of the loading chamber 11 by straight passages 27 of a predetermined width opening toward the preparation chamber 10. The purpose of these passages 27 will be explained later. Each retaining hole 26 is provided with a plurality (three in Figure 6) of equiangularly spaced cutouts 26a in corresponding relation to those of the substrate holder H. The first transfer tray 23 is movable axially of the loading chamber 11 between a retreated position shown in solid lines in Figure 1 and an advanced transfer position shown in phantom lines. The substrate transfer position is located where the axis of the loading chamber 11 intersects that of the preparation chamber 10. Such movement of the first tray 23 is imparted by an actuator (not shown) having an output rod 25 connected to the tray 23. As shown in Figure 3, the loading chamber 11 is provided with an access opening 29 located immediately above the first transfer tray 23 which has been brought to its retreated position. The access opening 29 is hermetically closable, and used to supply a set of new substrates B and to take out a set of processed substrates. The loading chamber is further provided with an evacuation opening 30 connected to a vacuum pump for evacuating the loading chamber. The second transfer tray 24 has a non-circular configuration defined by a pair of diametrically opposite arcs 24a and a pair of diametrically opposite chords 24b, as illustrated in Figure 7. The pair of chords 24b are parallel to the axis of the preparation chamber 10. The width of the second tray, namely the distance between the chords 24b, is smaller than the distance between the support arms 13 of the support frame 3 (see Figure 1), so that the second tray, when advanced, can be positioned between the support arms 13. The second tray 24 is formed with an upwardly opening recess 34 which is defined by a pair of stepped vertical wall surfaces 34a extending adjacent and along the profile arcs 24a, as illustrated in Figures 7 and 8. The recess 34 is used to fittingly receive each substrate holder H from above. The second tray 24 is further formed with a plurality (seven in Figure 7) of circular passage holes 33 in corresponding relation to the retaining holes 22 of the substrate holder H. Each passage hole 33 is formed with a plurality (three in Figure 7) of radial cutouts 33a in corresponding relation to those of the substrate holder. It should be note that two of the passage holes 33 are interrupted by the profile chords 24b of the tray. The second transfer tray 24 is movable axially of the preparation chamber 10 between an advanced position shown in phantom lines in Figure 1 and a retreated transfer position shown in solid lines. Such movement of the second tray 24 is performed by a sliding feed assembly 31 which includes a slide rod or bar 32 connected to the second tray. Specifically, as better shown in Figure 9, the slide rod 32 is slidably guided by a longitudinal guide G, and carries a rack 31a in mesh with a pinion 31b mounted to an output shaft 31c of an external drive motor 31d (Figure 1). Thus, the slide rod 32 is moved back and forth by actuating the drive motor 31d. Indicated at 41 is an evacuation opening connected to an external vacuum source for evacuating the preparation chamber 10 to a high vacuum. Transfer of each substrate set B between the first transfer tray 23 and the corresponding substrate holder H is conducted by a substrate transfer assembly 28 shown in Figures 3, 11 and 12. The substrate transfer assembly 28 is located at a position where the respective axes of the preparation chamber 10 and the loading chamber 11 intersect, namely where the first tray 23 in its advanced position overlaps the second tray 24 in its retreated position. As appreciated from Figure 3, the first tray 23 is located slightly above the second tray 24. The substrate transfer assembly 28 comprises a circular base plate 36 fixed to the upper end of a vertically extending support rod 35 which is moved up and down by an external actuator (not shown). The base plate 36 carries a plurality (seven in Figure 12) of upstanding posts 28a in corresponding relation to the retaining holes 22 of each substrate holder H (Figure 4). As better shown in Figure 11, each post 28a is provided with a plurality of radially extending pawls 28a in corresponding relation to the radial cutouts 22a of the substrate holder H. These pawls are used to engage a corresponding substrate B from below at three peripheral portions thereof, thereby retaining it. Obviously, the cutouts 22a (Figure 4), 26a (Figure 6), 33a (Figure 7) are utilized to allow passage of the pawls 28a of the substrate transfer assembly 28 when the first tray 23, the second tray 24 and the substrate holder H are vertically overlapped, as shown in Figure 3. Normally, the base plate 36 of the transfer assembly 28 assumes a lowered position in which the pawls 28b are located below the second tray 24, as indicated by phantom lines in Figure 3. Due to the provision of the cutouts, however, the base plate 36 may be brought to a raised position in which the pawls are located above the first tray 23, as indicated in solid lines in Figure 3. Each substrate set B can be transferred from the first tray 23 to a corresponding substrate holder H on the second tray 24 in the following manner. The first tray 23 with a set of newly loaded substrates B is brought to its advanced position (phantom line position in Figure 1), whereas the second tray 24 with an empty substrate holder H is brought to its retreated position (solid line position in Figure 1). At this time, the base plate 36 of the substrate transfer assembly 28 assumes its lowered position (phantom line position in Figure 3). Subsequently, the base plate 36 is raised, so that the substrates B are lifted off the first tray 23 by the correspondingly raised pawls 28b, as indicated by solid lines in Figure 3. In this condition, the first tray 23 is moved to its retreated position (solid line position in Figure 1). Such retreating movement of the first tray 23 is possible because the substrate retaining holes 26 are interconnected by the straight passages 27 (Figure 6) to clear the posts 28a in the axial direction of the loading chamber 11. Finally, the base plate 36 of the transfer assembly 28 is brought to its lowered position. Upon passage of the pawls 28a through the cutouts 22a of the substrate holder H, the substrates B held by the pawls are fitted into and retained by the respective retaining holes 22 (Figure 4) of the holder. A set of processed substrates B may be transferred from the corresponding substrate holder H to the first tray 23 for discharge by reversing the above operations. Specifically, the second tray 24 with the loaded substrate holder H supported thereon is brought to its retreated position, and the pawls 28a are raised to bring the processed substrates B off the holder to the highest position. Then, the first tray 23 is advanced, and the pawls 28a are lowered to place the substrates into the respective retaining holes 26 (Figure 6) of the first tray 23. The substrate holder H having loaded with the set of newly loaded substrates B is fed into the growth chamber 2 by the advancing movement of the second tray 24. According to the embodiment illustrated in Figure 1, the holder H with the set of new substrates B is subjected to pretreatment in a preliminary heating unit 37 before being supplied into the growth chamber 2. As shown in Figures 10 and 12, the preliminary heating unit 37 comprises a stocker 39 having a plurality of vertically aligned shelves 38. Each shelf 38 includes a pair of shelf plates 38a which are spaced transversely of the moving path of the second transfer tray 24 (see Figure 12). The spacing between the pair of shelf plates is such as to enable engagement, from below, with the corresponding holder H while allowing free passage of the second tray 24 therebetween. The stocker 39 is supported on a vertical rod 37a which is movable up and down by means of an external actuator (not shown). Represented by reference numeral 40 is a heater to preliminarily heat each substrate B for degassing. For storing each substrate holder H in the heating unit 37, the stocker 39 is adjusted in vertical position so that a selected shelf 38 is located slightly below the moving path of the second tray 24. Then, the second tray 24 with the substrate holder H is advanced to the stocker position, and the vertical rod 37 is raised. As a result, the holder is passed onto the selected shelf. For transferring each stocked substrate holder H onto the second tray 24 from a particular shelf 38, the stocker 39 is adjusted in vertical position so that the shelf is positioned slightly above the path of the second tray. Subsequently, the tray 24 is advanced to the stocker to be positioned below that particular shelf but above the next lower shelf, whereupon the stocker 29 is lowered to pass the holder onto the second tray. The substrate holder H thus received from the stocker 39 can be transferred onto the support frame 3 (Figures 1, 2 and 12) within the growth chamber 2 in the following manner. The support frame 3 is adjusted in vertical and rotational position so that its support arms 13 become parallel to the moving path of the second tray 24 as located slightly thereunder. Subsequently, the second tray with the loaded substrate holder H is advanced maximally into the growth chamber 2. In this condition, the second tray is arranged between the support arms 13. Finally, the support frame 3 is raised without rotation, whereby the holder H is lifted off the second tray which is thereafter retreated into the preparation chamber 10. After crystal growth is completed within the growth chamber 2, the holder H with the processed substrates B is transferred from the support frame 3 to the second tray 24 by reversing the above operations. Overall operation of the molecular beam epitaxy apparatus thus described will now be described more systematically with reference to Figures 1 and 12 in particular. It is now assumed that the growth chamber 2 has been previously evacuated to a ultra high vacuum and ready for crystal growth, while the preparation chamber 10 has been put under a high vacuum. In this condition, the first and second gate valves 8, 9 are hermetically closed to prevent vacuum leakage. Within the preparation chamber 10, the second tray 24 supporting an empty substrate holder H (taken from the preliminary heating unit 37) assumes its retreated position to wait for substrate supply. Within the loading chamber 11, the first tray 23 also assumes its retreated position as ready for substrate supply. Starting from the above condition, the access opening 29 (see Figure 3) of the loading chamber 11 is opened to supply a fresh set of substrates B to the respective retaining holes 26 (see Figure 6) of the first tray 23. The access opening is then closed, and the loading chamber is evacuated to a vacuum level corresponding to that of the preparation chamber 10. The second gate valve 9 is opened, and the first tray 23 is advanced into the preparation chamber 10 to be positioned immediately above the second tray 24. The substrate transfer assembly 28 is operated to transfer the substrates B into the respective retaining holes 22 (Figure 4) of the holder H, as already described. The first tray 23 is withdrawn into the loading chamber 11 to assume its retreated position, and the second gate valve 9 is again closed to enable loading of another fresh set of substrates. Within the preparation chamber 10, the second tray 24 now supporting the loaded holder H advances to the preliminary heating unit 37 to place the holder onto a selected shelf 38 of the stocker 39 (Figure 10), as described hereinbefore. The second tray 24 repeats reciprocating movement between the heating unit 37 and the retreated position until all holders in the stocker 39 are fully loaded. The holders H together with the substrates B are subjected to heating for degassing under high vacuum. Upon completion of the above preparatory steps, the first gate valve 8 is opened. Thereafter, the second tray 24 having received a loaded holder H from the heating unit 37 is advanced further into the growth chamber 2 to place it onto the support frame 3, as described hereinbefore. The second tray 24 is then retracted into the preparation chamber 10, and the first gate valve 8 is again closed. Within the growth chamber 2, the holder H together with the retained substrates B is further degassed under heating (by the heater 20) and ultra high vacuum. Each substrate then undergoes crystal growth by the molecular beam epitaxy. During such crystal formation, the support frame 3 is rotated about a vertical axis to ensure uniform crystal growth with respect to every substrate. Upon completion of required crystal growth, the first gate valve 8 is again opened to bring the growth chamber 2 into communication with the preparation chamber 10 which has been evacuated to a high vacuum. The support frame 3 is adjusted in vertical and rotational position for substrate discharge, and the second tray 24 is maximally advanced to receive, from the support frame 3, the holder H with the set of processed substrates B. During this cycle, the second gate valve 9 is closed. The second tray 24 is retracted to assume its maximally retreated position, and the first valve 8 is closed. By this time, the loading chamber 11 has been evacuated to a high vacuum. Then, the second gate valve 9 is opened, and the first tray 23 is advanced to receive the set of processed substrates B from the holder H with the aid of the transfer assembly 28, as hereinbefore described. The first tray 23 with the processed substrates B is retreated, and the second gate valve 9 is again closed. The access opening 29 (Figure 3) is then opened to take out the processed substrates. If required, the first tray 23 may receive a new set of substrates immediately after taking out the processed substrates. Within the preparation chamber 10, the second tray 24 supporting the emptied holder H may be advanced immediately to return the empty holder into the heating unit 37. Alternatively, the second tray with the empty holder may stay retreated until the holder is loaded with a new set of substrates, and thereafter advanced to store the loaded holder in the heating unit 37. This alternative enables continuous running of the epitaxy apparatus. It should be appreciated that the above manner of operation is only one example. The epitaxy apparatus of the invention can be operated in different ways. For example, each substrate holder H may be fed into the growth chamber 2 without pretreatment at the heating unit 37 immediately after loaded with a set of unprocessed substrates B. In either manner of operation, each substrate holder H moves only between the growth chamber 2 under ultra high vacuum and the preparation chamber 10 under high vacuum. In other words, the holder H never comes into direct contact with the atmosphere and/or the contaminating surroundings during repeated operation of the epitaxy apparatus, thereby greatly reducing the time required for removing contaminants by degassing within the growth chamber 2 under heating and ultra high vacuum. Thanks to this, the productivity of the molecular beam epitaxy apparatus according to the invention is improved enough to allow practical use on a commercial scale. It is virtually inevitable in any epitaxy apparatus that substrates themselves come into contact with the atmosphere. The point of the invention, therefore, resides in reducing the chance of contaminant entry into the growth chamber as much as possible. Returning to Figure 2, the central vaporizer 5a, unlike the side vaporizers 5b, 5c, is not provided with a shutter assembly, as already described. To explain the reasons for this, reference is now made to further details of the molecular beam epitaxy. It is known in molecular beam epitaxy that the rate of deposition of certain crystal elements is determined by the amount of other elements already deposited on the growth front of a substrate. Now, the concept of "coefficient of deposition" is used, which is defined as the ratio in number of deposited particles (molecules) to incident particles. When depositing a group V element, arsenic As (As₂ or As₄ molecule) for example, onto the growth front of a GaAs substrate, the coefficient of deposition of As is almost 1 (one) in the presence of deposited Ga or Al on the growth front but reduces almost to O (zero) in the absence of Ga or Al. It is thus possible to indirectly control the deposition of the group V elements by controlling the deposition of the other elements. It is also known that the group V elements emit molecular beams at a relatively low temperature of about 300°C as opposed to the other elements. Thus, it is easier to initiate and interrupt the molecular beam emission of the group V elements by temperature control. For these reasons, the central vaporizer 5a for a group V element (arsenic As in particular) is not provided with a shutter assembly. The molecular beam emission from the central vaporizer is controlled by adjusting the temperature thereof, while the deposition of As on the growth face of each substrate B is controlled by controlling the shutter assembly 7 of each side vaporizer 5b, 5c. In fact, the consumption of a group V element is highest in molecular beam epitaxy. Therefore, a large capacity vaporizer for such an element is required to provide continued or repeated crystal growth per single supply batch of that material. If this vaporizer is provided with a shutter plate large enough to cover the emission opening of the vaporizer, other vaporizers must be located relatively away from the V element vaporizer to allow the opening movement of the large shutter plate. Due to the limited mounting space of a growth chamber, however, there is also a limitation on the size of the shutter plate, that is, on the size of the vaporizer. Thus, with a conventional MBE apparatus in which all vaporizers are provided with shutter assemblies, the group V element vaporizer cannot have a sufficient capacity, thereby requiring frequent recharging of the material. Apparently, non-provision of a shutter assembly with respect to the central vaporizer 5a not only simplifies the structure of the growth chamber 2, but also is advantageous in increasing the capacity (volume) of the crucible 4 for receiving As. In this way, it is possible to run the apparatus for a prolonged period with a single operation of material supply while reducing the likelihood of running failures and the frequency of maintenance. According to the particular arrangement shown in Figure 2, the V element vaporizer 5a is located at the center of the frustoconical bottom portion 2b of the growth chamber 2, whereas the remaining vaporizers are disposed around the central vaporizer. This arrangement also contributes to increasing the capacity of the central crucible 4 while enabling to arrange a maximum number of other vaporizers around the central vaporizer. Thanks to the absence of a shutter assembly, the central vaporizer 5a may have its emission opening projecting toward the substrates B. This arrangement is advantageous in reducing stray molecular beams (undeposited material), consequently preventing material waste. On the other hand, it is feared that the non-closable emission opening of the central vaporizer 5a may allow impurity fallings to mix with the material (V element) therein. However, as described above, the V element vaporizer 5a works at a relatively low temperature, so that the mixed fallings will not vaporize at the working temperature of the central vaporizer. As illustrated in Figure 2, the rotary shaft 7a of each shutter assembly 7 has an axis Y which approaches the longitudinal axis b (or c) of the corresponding vaporizer 5b (or 5c). The shutter plate 7b is connected to the rotary shaft 7a by a bent arm 7c in a manner such that the shutter plate 7b becomes substantially parallel to the emission opening of the vaporizer when the shutter plate assumes a closing position. According to this arrangement, the shutter plate 7b is located nearest to the wall surface of the growth chamber 2 in the closing position, but becomes gradually farther from the wall surface upon opening movement, as indicated by phantom lines. On the other hand, if the pivotal axis Y of the shutter plate 7b is parallel to the longitudinal axis of the vaporizer, the shutter plate 7b approaches the wall surface of the growth chamber 2 upon opening movement because the bottom portion 2b of the growth chamber is conical. Thus, the shutter plate 7b, when assuming its closing position, must be located away from the emission opening of the corresponding vaporizer in order to allow opening movement of the shutter plate. As a result, the shutter plate 7b in the closing position may fail to effectively cut off molecular beams, thereby hindering accurate control of crystal growth on one hand and permitting leakage of vaporized material on the other. Further, if the shutter plate 7b in the closing position is located unacceptably adjacent the emission opening of the vaporizer, it will come into contact with the chamber wall surface to produce unwanted impurities due to contact shocks which may give adverse influences on crystal growth. The non-parallel orientation of the pivotal axis Y relative to the corresponding vaporizer axis is therefore advantageous in that the shutter plate 7b in its closing position may be located maximally close to the emission opening of the vaporizer without the problem of impurity production, consequently enabling accurate control of crystal growth. This arrangement is further advantageous in preventing vapor leakage when the shutter plate assumes its closing position. As shown in Figures 13a to 13c, the bent arm 7c of the shutter assembly preferably extends tangentially to the generally circular shutter plate 7b and is connected the trailing edge thereof (as seen in the direction of the shutter opening movement). It should be appreciated here that Figure 13b is a view as seen in the direction of an arrow M in Figure 13a which is a plan view of the shutter plate, whereas Figure 13c is a view as seen in the direction of an arrow N in Figure 13a. According to the preferred arrangement of Figures 13a to 13c, all portions of the shutter plate 7b displace away from the wall surface of the growth chamber 2 (or the conical bottom portion 2b thereof) immediately upon start of the shutter opening movement, as best illustrated in Figure 13c. Therefore, the shutter plate 7b may be located even nearer the emission opening of the corresponding vaporizer. Referring again to Figure 2, the rotary shaft 7a of each shutter assembly 7 should preferably includes two axially separable portions 7a', 7a'' connected by a transversing bolt (not shown). The rotary shaft is enclosed in a tubular housing 16 which also has two axially separable portions respectively provided with annular flanges 17a, 17b for connection by bolting. For removing the shutter plate 7b for cleaning for example, the flanges 17a, 17b of the tubular housing 16 are axially separated slightly. At this time, the two shaft portions 7a', 7a'' are still connected and pulled downward together. Subsequently, a suitable tool such as a driver is introduced into the tubular housing 16 through a small gap between the separated flanges 17a, 17b, and the bolt (not shown) connecting the two shaft portions 7a', 7a'' is removed. Finally, the shutter plate 7b together with the upper shaft portion 7a'' are taken out through a hermetically closable view port (not shown) of the growth chamber 2 or through the upper opening of the growth chamber 2 (after removal of the ceiling plate 12). Obviously, remounting of the shutter plate 7b may be conducted in the reverse order. Conventionally, all operations required for removing and remounting of a shutter plate must be conducted by inserting the operator's hands through a narrow view port of a growth chamber, which is really time-taking and troublesome. According to the preferred shutter assembly 7 shown in Figure 2, on the other hand, most operations for shutter removal and remounting can be conducted from outside at the position of the annular flanges 17a, 17b.

Claims: A molecular beam epitaxy apparatus comprising: a growth chamber (2) evacuatable to a ultra high vacuum, said growth chamber having a mounting portion (2b) which is provided with a vaporizer (5a) for a group V element as well as with other vaporizers (5b, 5c) for additional materials; and a holder support means (3) arranged in said growth chamber for supporting a substrate holder (H); said vaporizers (5a, 5b, 5c) respectively having emission openings directed toward the center of the holder (H) supported by said holder support means (3); the emission openings of said other vaporizers (5b, 5c) being closable respectively by closure means (7) in the form of a shutter assembly which includes a shutter plate (7b) pivotable about a rotary shaft (7a); the emission opening of said group V element vaporizer (5a) being not provided with any closure means to be always held open; characterized in that said rotary shaft (7a) has two axially separable portions (7a', 7a") accommodated in a housing (16) which also has two axially separable portions. The apparatus as defined in claim 1, wherein said mounting portion (2a) is a frustoconical bottom portion of said growth chamber (2). The apparatus as defined in claim 1, wherein said group V element vaporizer (5a) is located in direct opposition to said holder support means (3), said other vaporizers (5b, 5c) being disposed around said group V element vaporizer (5a) in an annular arrangement. The apparatus as defined in claim 2, wherein said group V element vaporizer (5a) is located directly below said holder support means (3), said other vaporizers (5b, 5c) being disposed around said group V element vaporizer (5a) in an annular arrangement. The apparatus as defined in claim 1, wherein the emission opening of said group V element vaporizer (5a) projects toward said holder support means (3). The apparatus as defined in claim 1, wherein each of said other vaporizers (5b, 5c) has a longitudinal axis (b, c) directed toward the center of the holder (H) supported by said holder support means (3), each of said closure means (7) comprising a shutter assembly which includes a shutter plate (7b) pivotable about a rotational axis (Y) approaching the longitudinal axis (b, c) of a corresponding one of said other vaporizers (5b, 5c) toward said holder support means (3). The apparatus as defined in claim 6, wherein said rotational axis (Y) of said shutter assembly (7) is provided by a rotary shaft (7a) which is connected to said shutter plate (7b) by an arm (7c), said arm (7c) extending substantially tangentially to said shutter plate (7b).

Inventors: ISHIDA YUHJI, MUSHIAGE MASATO, TANAKA HARUO, ISHIDA, YUHJI, MUSHIAGE, MASATO, TANAKA, HARUO, Ishida, Yuhji, c/o Rohm Co., Ltd., Mushiage, Masato, c/o Rohm Co., Ltd., Tanaka, Haruo, c/o Rohm Co., Ltd.
Applicants: ROHM CO LTD, ROHM CO., LTD.
Application year: 1989
Main classification: C30B23
IPC codes: C30B23, H01L21