Heat Treatment Equipment

Abstract

To provide a thermal treatment device capable of adjusting an exposure time of a flash light while being corresponded to a heavy-current.SOLUTION: In a discharge circuit of a flash lamp FL, a capacitor 93 and three coils 94a, 94b, and 94c are serially connected, and three switches 99a, 99b, and 99c which are parallely connected one by one to all of the three coils 94a, 94b, and 94c are provided. By switching the three switches 99a, 99b, and 99c to appropriately functioning the three coils 94a, 94b, and 94c as an inductor, a waveform of a current flowing in the flash lamp FL can be changed without an IGBT, and an exposure time of the flash light can be adjusted. Also, since the IGBT is not used, a heavy-current can be flown to the discharge circuit of the flash lamp FL.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present invention relates to a heat treatment apparatus for heating a thin precision electronic substrate such as a semiconductor wafer (hereinafter simply referred to as "substrate") by irradiating the substrate with flash light. [Background technology]

[0002] Flash lamp annealing (FLA), which heats semiconductor wafers in an extremely short time, is attracting attention in the semiconductor device manufacturing process.Flash lamp annealing is a heat treatment technology that uses a xenon flash lamp (hereinafter, simply referred to as "flash lamp" means a xenon flash lamp) to irradiate the surface of a semiconductor wafer with flash light, thereby raising the temperature of only the surface of the semiconductor wafer in an extremely short time (a few milliseconds or less).

[0003] The spectral distribution of radiation from a xenon flash lamp is in the ultraviolet to near-infrared range, with a shorter wavelength than conventional halogen lamps and a wavelength that roughly matches the fundamental absorption band of silicon semiconductor wafers. Therefore, when a semiconductor wafer is irradiated with flash light from a xenon flash lamp, little light is transmitted, making it possible to rapidly heat the semiconductor wafer. It has also been found that if the flash light is irradiated for an extremely short period of time, less than a few milliseconds, it is possible to selectively heat only the area near the surface of the semiconductor wafer.

[0004] Flash lamp annealing is used in processes that require heating for an extremely short period of time, such as activating impurities implanted in a semiconductor wafer. By irradiating the surface of a semiconductor wafer into which impurities have been implanted by ion implantation with a flash light from a flash lamp, the surface of the semiconductor wafer can be heated to the activation temperature in an extremely short period of time, allowing only the impurities to be activated without diffusing them deeply.

[0005] Patent Document 1 discloses that the surface temperature of a semiconductor wafer is raised to a target processing temperature by irradiating a flash of light from a flash lamp onto the semiconductor wafer, which has been heated to a predetermined preheating temperature. The reason why the semiconductor wafer is heated to the preheating temperature before the flash light irradiation is that it is difficult to raise the surface temperature of the semiconductor wafer to the target temperature by flash light irradiation alone.

[0006] Patent Document 1 also discloses that an IGBT (insulated gate bipolar transistor) is incorporated into the discharge circuit of a flash lamp, and the IGBT controls the waveform of the current flowing through the flash lamp, thereby appropriately adjusting the flash light irradiation time from the flash lamp. By adjusting the flash light irradiation time with the IGBT, it becomes possible to meet the needs of various processes. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-177496 Summary of the Invention [Problem to be solved by the invention]

[0008] In recent years, advances in semiconductor device miniaturization and material changes have led to a demand for annealing processes with lower thermal budgets. Low-thermal budget annealing is a thermal process that can raise the surface temperature of a semiconductor wafer to a target temperature with a small total amount of heat. To achieve this, a semiconductor wafer preheated to a lower temperature than conventional methods must be exposed to a shorter flash of light (less than 1 millisecond) to achieve a high jump temperature of 600°C or higher. The jump temperature is the actual temperature rise resulting from the flash and corresponds to the difference between the target temperature and the preheat temperature. Achieving a high jump temperature with a shorter flash exposure time requires a very large instantaneous current to flow through the flash lamp discharge circuit. For example, to achieve a jump temperature of 600°C or higher with a 0.1-millisecond flash, a large current of up to 5000 A must be passed through the flash lamp discharge circuit. Therefore, the circuit elements comprising the flash lamp discharge circuit must have a rated current of over 5000 A.

[0009] However, the upper limit of the rated current of currently available IGBTs is around 1500 A, and there are no IGBTs that can withstand currents exceeding 5000 A. Therefore, it is not possible to incorporate an IGBT into the discharge circuit of a flash lamp that achieves a flash light irradiation time of 0.1 milliseconds. As a result, the waveform of the current flowing through the flash lamp is a fixed wave determined by the capacitor and coil, making it impossible to adjust the flash light irradiation time.

[0010] The present invention has been made in view of the above-mentioned problems, and has an object to provide a heat treatment apparatus that can adjust the irradiation time of flash light while accommodating a large current. [Means for solving the problem]

[0011] In order to solve the above problem, the invention of claim 1 provides a heat treatment apparatus for heating a substrate by irradiating the substrate with flash light, the heat treatment apparatus comprising: a chamber for accommodating a substrate; a holder for holding the substrate in the chamber; a flash lamp for irradiating flash light onto the substrate held by the holder; a capacitor for discharging accumulated charge to the flash lamp; a plurality of coils connected in series with the flash lamp and the capacitor; and a plurality of switches connected in parallel one-to-one to all of the plurality of coils. a control unit that controls opening and closing of the plurality of switches; Equipped with The control unit extracts the open / close states of the plurality of switches corresponding to the irradiation time required for the flash lamp from a table showing the correlation between the irradiation time of the flash light and the opening / closing of the plurality of switches, and opens / closes the plurality of switches. It is characterized by:

[0014] Also, claims 2 The invention of The invention of claim 1 The heat treatment apparatus according to the above aspect is characterized in that it comprises a plurality of flash lamps that irradiate the substrate with light, and the plurality of switches are opened and closed so that the inductances of the plurality of flash lamps are different from each other.

[0015] Also, claims 3 The invention is 2 The heat treatment apparatus according to the present invention is characterized by further comprising a delay circuit that determines the light emission timing of each of the plurality of flash lamps. [Effects of the Invention]

[0016] Claims 1 to 1 3 According to the invention, the device is equipped with a plurality of coils connected in series with the flash lamp and capacitor, and a plurality of switches connected in parallel one-to-one to all of the plurality of coils, so that the inductance of the discharge circuit can be changed without using an IGBT, and the irradiation time of the flash light can be adjusted while accommodating large currents. Furthermore, since the control unit opens and closes a plurality of switches in accordance with the irradiation time required for the flash lamp, once the desired irradiation time is specified, the opening and closing of the plurality of switches is automatically determined. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a vertical cross-sectional view showing the configuration of a heat treatment apparatus according to the present invention. [Figure 2]FIG. 2 is a perspective view showing the overall appearance of the holding portion. [Figure 3] FIG. [Figure 4] FIG. 2 is a cross-sectional view of a susceptor. [Figure 5] FIG. [Figure 6] FIG. [Figure 7] FIG. 2 is a plan view showing the arrangement of a plurality of halogen lamps. [Figure 8] FIG. 2 is a diagram showing a discharge circuit of a flash lamp. [Figure 9] FIG. 4 is a diagram showing the waveform of a current flowing through a flash lamp. [Figure 10] FIG. 10 is a diagram showing an example of a table showing the correlation between the irradiation time of flash light and the opening and closing of a switch. [Figure 11] FIG. 10 is a diagram illustrating another example of a discharge circuit for a flash lamp. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0020] FIG. 1 is a vertical cross-sectional view showing the configuration of a heat treatment apparatus 1 according to the present invention. The heat treatment apparatus 1 in FIG. 1 is a flash lamp annealing apparatus that heats a disk-shaped semiconductor wafer W as a substrate by irradiating the semiconductor wafer W with flash light. The size of the semiconductor wafer W to be treated is not particularly limited, but may be, for example, φ300 mm or φ450 mm. Note that in FIG. 1 and the subsequent figures, the dimensions and number of various parts are exaggerated or simplified as necessary for ease of understanding.

[0021] Heat treatment apparatus 1 includes a chamber 6 that accommodates a semiconductor wafer W, a flash heating unit 5 that incorporates multiple flash lamps FL, and a halogen heating unit 4 that incorporates multiple halogen lamps HL. The flash heating unit 5 is provided above chamber 6, and the halogen heating unit 4 is provided below. Heat treatment apparatus 1 also includes, inside chamber 6, a holder 7 that holds the semiconductor wafer W in a horizontal position, and a transfer mechanism 10 that transfers the semiconductor wafer W between the holder 7 and the outside of the apparatus. Heat treatment apparatus 1 also includes a control unit 3 that controls the operating mechanisms provided in the halogen heating unit 4, flash heating unit 5, and chamber 6 to perform heat treatment on the semiconductor wafer W.

[0022] The chamber 6 is constructed by attaching quartz chamber windows to the top and bottom of a cylindrical chamber side portion 61. The chamber side portion 61 has a roughly cylindrical shape with openings at the top and bottom, with an upper chamber window 63 attached to and closing the upper opening, and a lower chamber window 64 attached to and closing the lower opening. The upper chamber window 63, which forms the ceiling of the chamber 6, is a disc-shaped member made of quartz and functions as a quartz window that transmits the flash light emitted from the flash heating unit 5 into the chamber 6. The lower chamber window 64, which forms the floor of the chamber 6, is also a disc-shaped member made of quartz and functions as a quartz window that transmits the light from the halogen heating unit 4 into the chamber 6.

[0023] Furthermore, a reflective ring 68 is attached to the upper part of the inner wall surface of the chamber side 61, and a reflective ring 69 is attached to the lower part. Both reflective rings 68, 69 are formed in an annular shape. The upper reflective ring 68 is attached by fitting it from the upper side of the chamber side 61. On the other hand, the lower reflective ring 69 is attached by fitting it from the lower side of the chamber side 61 and fastening it with screws (not shown). In other words, both reflective rings 68, 69 are detachably attached to the chamber side 61. The internal space of the chamber 6, i.e., the space surrounded by the upper chamber window 63, the lower chamber window 64, the chamber side 61, and the reflective rings 68, 69, is defined as a heat treatment space 65.

[0024] By attaching the reflecting rings 68, 69 to the chamber side portion 61, a recess 62 is formed on the inner wall surface of the chamber 6. That is, the recess 62 is formed by a central portion of the inner wall surface of the chamber side portion 61 where the reflecting rings 68, 69 are not attached, the lower end surface of the reflecting ring 68, and the upper end surface of the reflecting ring 69. The recess 62 is formed in an annular shape along the horizontal direction on the inner wall surface of the chamber 6, and surrounds the holder 7 that holds the semiconductor wafer W. The chamber side portion 61 and the reflecting rings 68, 69 are made of a metal material (e.g., stainless steel) that has excellent strength and heat resistance.

[0025] Furthermore, a transfer opening (furnace port) 66 is formed in the chamber side portion 61, through which a semiconductor wafer W is loaded into and unloaded from the chamber 6. The transfer opening 66 can be opened and closed by a gate valve 185. The transfer opening 66 is connected to the outer peripheral surface of the recessed portion 62. Therefore, when the gate valve 185 opens the transfer opening 66, the semiconductor wafer W can be loaded into and unloaded from the heat treatment space 65 through the transfer opening 66 and the recessed portion 62. Furthermore, when the gate valve 185 closes the transfer opening 66, the heat treatment space 65 in the chamber 6 becomes an airtight space.

[0026] Furthermore, a through-hole 61a is formed in the chamber side 61. A radiation thermometer 20 is attached to the portion of the outer wall surface of the chamber side 61 where the through-hole 61a is provided. The through-hole 61a is a cylindrical hole for guiding infrared light emitted from the underside of a semiconductor wafer W held on a susceptor 74 (described later) to the radiation thermometer 20. The through-hole 61a is provided at an angle with respect to the horizontal direction so that the axis of the through-hole 61a intersects with the main surface of the semiconductor wafer W held on the susceptor 74. Therefore, the radiation thermometer 20 is provided diagonally below the susceptor 74. A transparent window 21 made of barium fluoride material that transmits infrared light in a wavelength range that can be measured by the radiation thermometer 20 is attached to the end of the through-hole 61a facing the heat treatment space 65.

[0027] Furthermore, gas supply holes 81 are formed in the upper part of the inner wall of the chamber 6 to supply processing gas to the heat treatment space 65. The gas supply holes 81 are formed at a position above the recessed portion 62 and may be provided in the reflecting ring 68. The gas supply holes 81 are connected to a gas supply pipe 83 via a buffer space 82 formed in an annular shape inside the side wall of the chamber 6. The gas supply pipe 83 is connected to a processing gas supply source 85. A valve 84 is inserted in the gas supply pipe 83. When the valve 84 is opened, processing gas is supplied from the processing gas supply source 85 to the buffer space 82. The processing gas that has flowed into the buffer space 82 spreads within the buffer space 82, which has lower fluid resistance than the gas supply holes 81, and is supplied from the gas supply holes 81 into the heat treatment space 65. The processing gas may be, for example, an inert gas such as nitrogen (N2), argon (Ar), or helium (He), or a reactive gas such as hydrogen (H2), ammonia (NH3), or a mixed gas of these.

[0028] Meanwhile, a gas exhaust hole 86 is formed in the lower part of the inner wall of the chamber 6 to exhaust gas from the heat treatment space 65. The gas exhaust hole 86 is formed below the recess 62 and may be provided in the reflecting ring 69. The gas exhaust hole 86 is connected to a gas exhaust pipe 88 via a buffer space 87 formed in an annular shape inside the side wall of the chamber 6. The gas exhaust pipe 88 is connected to an exhaust unit 190. A valve 89 is inserted in the gas exhaust pipe 88. When the valve 89 is opened, the gas in the heat treatment space 65 is exhausted from the gas exhaust hole 86 through the buffer space 87 to the gas exhaust pipe 88. The gas supply hole 81 and the gas exhaust hole 86 may be provided in multiple numbers along the circumferential direction of the chamber 6, or may be slit-shaped. The process gas supply source 85 and the exhaust unit 190 may be mechanisms provided in the heat treatment apparatus 1 or may be utilities of a factory where the heat treatment apparatus 1 is installed.

[0029] 2 is a perspective view showing the overall appearance of the holder 7. The holder 7 is configured to include a base ring 71, a connecting portion 72, and a susceptor 74. The base ring 71, the connecting portion 72, and the susceptor 74 are all made of quartz. In other words, the entire holder 7 is made of quartz.

[0030] The base ring 71 is an arc-shaped quartz member with a portion missing from the annular shape. This missing portion is provided to prevent interference between the base ring 71 and a transfer arm 11 of the transfer mechanism 10, which will be described later. The base ring 71 is placed on the bottom surface of the recess 62, and is supported by the wall surface of the chamber 6 (see FIG. 1). A plurality of connecting portions 72 (four in this embodiment) are erected on the upper surface of the base ring 71 along the circumferential direction of the annular shape. The connecting portions 72 are also quartz members, and are fixed to the base ring 71 by welding.

[0031] The susceptor 74 is supported by four connecting portions 72 provided on the base ring 71. FIG. 3 is a plan view of the susceptor 74. FIG. 4 is a cross-sectional view of the susceptor 74. The susceptor 74 includes a holding plate 75, a guide ring 76, and a plurality of substrate support pins 77. The holding plate 75 is a substantially circular, flat member made of quartz. The diameter of the holding plate 75 is larger than the diameter of the semiconductor wafer W. That is, the holding plate 75 has a planar size larger than that of the semiconductor wafer W.

[0032] A guide ring 76 is installed on the periphery of the upper surface of the holding plate 75. The guide ring 76 is an annular member having an inner diameter larger than the diameter of the semiconductor wafer W. For example, if the diameter of the semiconductor wafer W is φ300 mm, the inner diameter of the guide ring 76 is φ320 mm. The inner periphery of the guide ring 76 has a tapered surface that widens upward from the holding plate 75. The guide ring 76 is made of quartz, the same as the holding plate 75. The guide ring 76 may be welded to the upper surface of the holding plate 75, or may be fixed to the holding plate 75 by a separately processed pin or the like. Alternatively, the holding plate 75 and the guide ring 76 may be processed as an integrated member.

[0033] The area of ​​the upper surface of the holding plate 75 that is inside the guide ring 76 is a flat holding surface 75a that holds the semiconductor wafer W. A plurality of substrate support pins 77 are provided on the holding surface 75a of the holding plate 75. In this embodiment, a total of 12 substrate support pins 77 are provided at 30° intervals along a circumference concentric with the outer circumferential circle of the holding surface 75a (the inner circumferential circle of the guide ring 76). The diameter of the circle on which the 12 substrate support pins 77 are arranged (the distance between opposing substrate support pins 77) is smaller than the diameter of the semiconductor wafer W. If the diameter of the semiconductor wafer W is 300 mm, the diameter is 270 mm to 280 mm (270 mm in this embodiment). Each substrate support pin 77 is made of quartz. The plurality of substrate support pins 77 may be provided on the upper surface of the holding plate 75 by welding, or may be machined integrally with the holding plate 75.

[0034] Returning to FIG. 2, four connecting portions 72 erected on the base ring 71 are fixed to the peripheral edge of the holding plate 75 of the susceptor 74 by welding. That is, the susceptor 74 and the base ring 71 are fixedly connected by the connecting portions 72. The base ring 71 of the holding portion 7 is supported on the wall surface of the chamber 6, and the holding portion 7 is thereby attached to the chamber 6. When the holding portion 7 is attached to the chamber 6, the holding plate 75 of the susceptor 74 is in a horizontal position (a position in which the normal line coincides with the vertical direction). That is, the holding surface 75a of the holding plate 75 is a horizontal plane.

[0035] The semiconductor wafer W carried into the chamber 6 is placed and held in a horizontal position on the susceptor 74 of the holder 7 attached to the chamber 6. At this time, the semiconductor wafer W is supported by twelve substrate support pins 77 erected on a holding plate 75 and held on the susceptor 74. More precisely, the upper ends of the twelve substrate support pins 77 contact the underside of the semiconductor wafer W to support the semiconductor wafer W. The heights of the twelve substrate support pins 77 (the distance from the upper ends of the substrate support pins 77 to the holding surface 75a of the holding plate 75) are uniform, so the twelve substrate support pins 77 can support the semiconductor wafer W in a horizontal position.

[0036] Furthermore, the semiconductor wafer W is supported by a plurality of substrate support pins 77 at a predetermined distance from the holding surface 75a of the holding plate 75. The thickness of the guide ring 76 is greater than the height of the substrate support pins 77. Therefore, the guide ring 76 prevents the semiconductor wafer W supported by the plurality of substrate support pins 77 from shifting in the horizontal direction.

[0037] 2 and 3, an opening 78 is formed in the holding plate 75 of the susceptor 74, penetrating vertically. The opening 78 is provided so that a radiation thermometer 20 can receive radiation (infrared light) emitted from the underside of the semiconductor wafer W. That is, the radiation thermometer 20 receives the light emitted from the underside of the semiconductor wafer W through the opening 78 and a transparent window 21 attached to the through-hole 61 a of the chamber side 61, thereby measuring the temperature of the semiconductor wafer W. Furthermore, the holding plate 75 of the susceptor 74 is formed with four through-holes 79 through which lift pins 12 of a transfer mechanism 10 (described later) pass to transfer the semiconductor wafer W.

[0038] FIG. 5 is a plan view of the transfer mechanism 10. FIG. 6 is a side view of the transfer mechanism 10. The transfer mechanism 10 includes two transfer arms 11. The transfer arms 11 are arc-shaped so as to fit the generally annular recess 62. Two lift pins 12 are provided on each of the transfer arms 11. The transfer arms 11 and the lift pins 12 are made of quartz. Each transfer arm 11 is rotatable by a horizontal movement mechanism 13. The horizontal movement mechanism 13 horizontally moves the pair of transfer arms 11 between a transfer operation position (position indicated by a solid line in FIG. 5) where the transfer arms 11 transfer the semiconductor wafer W to the holder 7 and a retracted position (position indicated by a two-dot chain line in FIG. 5) where the transfer arms 11 do not overlap the semiconductor wafer W held by the holder 7 in a plan view. The horizontal movement mechanism 13 may be one that rotates each transfer arm 11 using an individual motor, or one that uses a link mechanism to rotate a pair of transfer arms 11 in conjunction with one another using a single motor.

[0039] Furthermore, the pair of transfer arms 11 are raised and lowered together with the horizontal movement mechanism 13 by the lifting mechanism 14. When the lifting mechanism 14 raises the pair of transfer arms 11 to the transfer operation position, a total of four lift pins 12 pass through through holes 79 (see FIGS. 2 and 3 ) formed in the susceptor 74, and the upper ends of the lift pins 12 protrude from the upper surface of the susceptor 74. On the other hand, when the lifting mechanism 14 lowers the pair of transfer arms 11 to the transfer operation position to remove the lift pins 12 from the through holes 79, and the horizontal movement mechanism 13 moves the pair of transfer arms 11 so as to open, each transfer arm 11 moves to a retracted position. The retracted position of the pair of transfer arms 11 is directly above the base ring 71 of the holder 7. Because the base ring 71 is placed on the bottom surface of the recess 62, the retracted position of the transfer arms 11 is inside the recess 62. In addition, an exhaust mechanism (not shown) is also provided near the location where the drive part of the transfer mechanism 10 (horizontal movement mechanism 13 and lifting mechanism 14) is located, and is configured to exhaust the atmosphere around the drive part of the transfer mechanism 10 to the outside of the chamber 6.

[0040] Returning to FIG. 1 , the flash heating unit 5, which is provided above the chamber 6, is configured with a light source made up of multiple (30 in this embodiment) xenon flash lamps FL inside a housing 51, and a reflector 52 provided to cover the light source from above. A lamp light emission window 53 is attached to the bottom of the housing 51 of the flash heating unit 5. The lamp light emission window 53, which forms the floor of the flash heating unit 5, is a plate-shaped quartz window made of quartz. By installing the flash heating unit 5 above the chamber 6, the lamp light emission window 53 faces the upper chamber window 63. The flash lamps FL irradiate a heat treatment space 65 with flash light from above the chamber 6 through the lamp light emission window 53 and the upper chamber window 63.

[0041] The flash lamps FL are each a rod-shaped lamp having a long cylindrical shape, and are arranged in a plane so that their longitudinal directions are parallel to each other along the main surface of the semiconductor wafer W held by the holder 7 (i.e., along the horizontal direction). Therefore, the plane formed by the arrangement of the flash lamps FL is also a horizontal plane. The area in which the flash lamps FL are arranged is larger than the planar size of the semiconductor wafer W.

[0042] Fig. 8 is a diagram showing a discharge circuit for a flash lamp FL. A discharge circuit like the one shown in Fig. 8 is provided for each of the 30 flash lamps FL. As shown in the figure, a capacitor 93, a first coil 94a, a second coil 94b, a third coil 94c, and a flash lamp FL are connected in series. Note that when there is no need to distinguish between the first coil 94a, the second coil 94b, and the third coil 94c, they are collectively referred to as coil 94.

[0043] The flash lamp FL comprises a rod-shaped glass tube (discharge tube) 92 filled with xenon gas and having an anode and a cathode disposed at both ends, and a trigger electrode 91 attached to the outer surface of the glass tube 92. The trigger electrode 91 is connected to a trigger circuit 97 via a delay circuit 98. A high voltage can be applied to the trigger electrode 91 from the trigger circuit 97. The timing at which the trigger circuit 97 applies a voltage to the trigger electrode 91 is determined by the delay circuit 98 under the control of the control unit 3.

[0044] A predetermined voltage is applied to capacitor 93 by power supply unit 95, and an electric charge corresponding to the applied voltage (charging voltage) is stored in capacitor 93. Capacitor 93 then releases the stored electric charge to flash lamp FL. Because the xenon gas sealed in glass tube 92 of flash lamp FL is an electrical insulator, no electricity normally flows through glass tube 92, even if an electric charge is stored in capacitor 93. However, if trigger circuit 97 applies a high voltage to trigger electrode 91, causing insulation breakdown, a discharge occurs between the anode and cathode at either end of glass tube 92, causing a current to instantaneously flow through glass tube 92. This excitation of xenon atoms or molecules causes a flash of light to be emitted from flash lamp FL.

[0045] Three coils 94, namely, a first coil 94a, a second coil 94b, and a third coil 94c, are connected in series to the flash lamp FL and the capacitor 93. The coils 94 function as inductors to smooth out sudden current changes that occur in the discharge circuit when the flash lamp FL discharges, adjusting the waveform of the current flowing through the flash lamp FL and lengthening the emission time of the flash light. The first coil 94a, the second coil 94b, and the third coil 94c have different inductances. For example, the inductance of the first coil 94a is 25 μH, the inductance of the second coil 94b is 50 μH, and the inductance of the third coil 94c is 100 μH.

[0046] In this embodiment, a switch is connected in parallel to each of the three coils 94. As shown in Fig. 8, a first switch 99a is connected in parallel to the first coil 94a, a second switch 99b is connected in parallel to the second coil 94b, and a third switch 99c is connected in parallel to the third coil 94c. The opening and closing of the first switch 99a, the second switch 99b, and the third switch 99c are controlled by the control unit 3. Note that when there is no need to distinguish between the first switch 99a, the second switch 99b, and the third switch 99c, they are collectively referred to as switches 99.

[0047] When a switch 99 is closed, the corresponding coil 94 loses its function as an inductor. Conversely, when a switch 99 is open, the corresponding coil 94 functions as an inductor.

[0048] The inductance of the discharge circuit can be varied by changing the opening and closing pattern of the three switches 99. When the first switch 99a, the second switch 99b, and the third switch 99c are all closed, no coil 94 functions as an inductor, and the inductance of the discharge circuit is minimized. However, due to unavoidable parasitic inductance, the inductance of the discharge circuit does not become completely 0 μH even when all three switches 99 are closed. On the other hand, when the first switch 99a, the second switch 99b, and the third switch 99c are all open, all three coils 94 function as inductors, and the inductance of the discharge circuit is maximized.

[0049] FIG. 9 shows the waveform of the current flowing through the flash lamp FL. The solid line in the figure shows the waveform of the current flowing through the flash lamp FL when all three switches 99 are closed, and the dotted line shows the waveform of the current flowing through the flash lamp FL when all three switches 99 are open. When all three switches 99 are closed, the coil 94 functioning as an inductor is no longer present in the discharge circuit, the inductance of the discharge circuit is extremely small, and the current flowing through the flash lamp FL rises sharply and then drops sharply. As a result, as shown by the solid line in FIG. 9 , the waveform of the current flowing through the flash lamp FL becomes peaky, and the flash light emission time is approximately 0.1 milliseconds. However, even without the coil 94 functioning as a passive element, the discharge circuit still has parasitic inductance and wiring resistance, which determine the waveform of the current flowing through the flash lamp FL.

[0050] On the other hand, when all three switches 99 are open, all three coils 94 function as inductors, the inductance of the discharge circuit increases, and the current flowing through the flash lamp FL rises relatively slowly and then decreases slowly. As a result, as shown by the dotted line in Figure 9, the waveform of the current flowing through the flash lamp FL becomes broader than when all three switches 99 are closed.

[0051] When any one of the first switch 99a, second switch 99b, and third switch 99c is closed and the rest are open, the waveform of the current flowing through the flash lamp FL will be somewhere between the solid line and dotted line in Figure 9. The greater the inductance of the discharge circuit, which is determined by the open / close states of the three switches 99, the gentler the waveform of the current flowing through the flash lamp FL will be (the closer it is to the waveform shown by the dotted line). Note that, because a discharge circuit such as that shown in Figure 8 is provided for each of the 30 flash lamps FL, the current waveform and flash light emission time can be adjusted individually for each of the 30 flash lamps FL.

[0052] 1, reflector 52 is provided above the multiple flash lamps FL so as to cover them entirely. The basic function of reflector 52 is to reflect the flash light emitted from the multiple flash lamps FL toward heat treatment space 65. Reflector 52 is made of an aluminum alloy plate, and its surface (the surface facing the flash lamps FL) is roughened by blasting.

[0053] The halogen heating unit 4, which is provided below the chamber 6, has a plurality of halogen lamps HL (40 in this embodiment) built into the inside of the housing 41. The halogen heating unit 4 heats the semiconductor wafer W by irradiating light from the plurality of halogen lamps HL from below the chamber 6 through a lower chamber window 64 into a heat treatment space 65.

[0054] FIG. 7 is a plan view showing the arrangement of multiple halogen lamps HL. 40 halogen lamps HL are arranged in two rows, upper and lower. 20 halogen lamps HL are arranged in the upper row, which is closer to the holder 7, and 20 halogen lamps HL are also arranged in the lower row, which is farther from the holder 7 than the upper row. Each halogen lamp HL is a rod-shaped lamp having a long cylindrical shape. In both the upper and lower rows, the 20 halogen lamps HL are arranged so that their longitudinal directions are parallel to each other along the main surface of the semiconductor wafer W held by the holder 7 (i.e., along the horizontal direction). Therefore, the plane formed by the arrangement of the halogen lamps HL in both the upper and lower rows is a horizontal plane.

[0055] 7, the halogen lamps HL are arranged more densely in the region facing the periphery of the semiconductor wafer W held by the holder 7 on both the upper and lower tiers than in the region facing the center of the semiconductor wafer W. That is, on both the upper and lower tiers, the halogen lamps HL are arranged at a shorter pitch in the periphery of the lamp arrangement than in the center. This allows a greater amount of light to be irradiated onto the periphery of the semiconductor wafer W, which is prone to temperature drop during heating due to light irradiation from the halogen heating unit 4.

[0056] The lamp group consisting of the halogen lamps HL on the upper row and the lamp group consisting of the halogen lamps HL on the lower row are arranged so as to intersect in a grid pattern. That is, a total of 40 halogen lamps HL are arranged so that the longitudinal direction of the 20 halogen lamps HL arranged on the upper row and the longitudinal direction of the 20 halogen lamps HL arranged on the lower row are perpendicular to each other.

[0057] A halogen lamp HL is a filament-type light source that emits light by passing electricity through a filament placed inside a glass tube, causing it to incandescent. The glass tube is filled with an inert gas, such as nitrogen or argon, containing trace amounts of halogen elements (iodine, bromine, etc.). The introduction of halogen elements makes it possible to set the filament temperature at a high temperature while preventing filament breakage. Therefore, compared to standard incandescent light bulbs, halogen lamps HL have the characteristics of a longer lifespan and the ability to continuously emit strong light. In other words, halogen lamps HL are continuous lamps that emit light continuously for at least one second. Furthermore, because halogen lamps HL are rod-shaped, they have a long lifespan, and by arranging them horizontally, they achieve excellent radiation efficiency toward the semiconductor wafer W above.

[0058] Also, a reflector 43 is provided below the two-tiered halogen lamps HL inside the housing 41 of the halogen heating unit 4 (FIG. 1). The reflector 43 reflects the light emitted from the multiple halogen lamps HL toward the heat treatment space 65.

[0059] The control unit 3 controls the various operating mechanisms provided in the heat treatment device 1. The hardware configuration of the control unit 3 is similar to that of a general computer. That is, the control unit 3 includes a CPU, which is a circuit that performs various arithmetic processing, a ROM, which is a read-only memory that stores basic programs, a RAM, which is a readable and writable memory that stores various information, and a magnetic disk that stores control software, data, and the like. The CPU of the control unit 3 executes a predetermined processing program, thereby causing the heat treatment device 1 to perform processing. The control unit 3 also adjusts the waveform of the current flowing through the flash lamp FL and the irradiation time of the flash light by individually controlling the opening and closing of the first switch 99a, the second switch 99b, and the third switch 99c.

[0060] In addition to the above configuration, the heat treatment apparatus 1 is equipped with various cooling structures to prevent excessive temperature rise in the halogen heating unit 4, flash heating unit 5, and chamber 6 due to the thermal energy generated by the halogen lamps HL and flash lamps FL during heat treatment of the semiconductor wafer W. For example, a water-cooled pipe (not shown) is provided in the wall of the chamber 6. The halogen heating unit 4 and flash heating unit 5 also have an air-cooled structure that creates a gas flow inside to remove heat. Air is also supplied to the gap between the upper chamber window 63 and the lamp light emission window 53 to cool the flash heating unit 5 and upper chamber window 63.

[0061] Next, the processing operation in the heat treatment apparatus 1 will be described. The semiconductor wafer W to be processed is a silicon (Si) semiconductor substrate into which impurities have been implanted by ion implantation as a previous process. The impurities are activated by an annealing process performed by the heat treatment apparatus 1. The processing procedure for the semiconductor wafer W, which will be described below, progresses as the control unit 3 controls each operating mechanism of the heat treatment apparatus 1.

[0062] First, prior to processing of the semiconductor wafer W, the gas supply valve 84 is opened, and the exhaust valve 89 is also opened to start supplying and exhausting gas to and from the chamber 6. When the valve 84 is opened, nitrogen gas is supplied to the heat treatment space 65 through the gas supply hole 81. When the valve 89 is opened, the gas inside the chamber 6 is exhausted through the gas exhaust hole 86. As a result, the nitrogen gas supplied from the upper part of the heat treatment space 65 inside the chamber 6 flows downward and is exhausted from the lower part of the heat treatment space 65.

[0063] Next, gate valve 185 is opened to open transfer opening 66, and a semiconductor wafer W to be processed is carried into heat treatment space 65 in chamber 6 through transfer opening 66 by a transfer robot outside the apparatus. At this time, there is a risk that the atmosphere outside the apparatus may be drawn in as the semiconductor wafer W is carried in, but since nitrogen gas is continuously supplied to chamber 6, the nitrogen gas flows out from transfer opening 66, making it possible to minimize the drawing in of such external atmosphere.

[0064] The semiconductor wafer W carried in by the transfer robot advances to a position directly above the holder 7 and stops there. Then, the pair of transfer arms 11 of the transfer mechanism 10 move horizontally from the retracted position to the transfer operation position and rise, causing the lift pins 12 to pass through the through holes 79 and protrude from the upper surface of the holding plate 75 of the susceptor 74 to receive the semiconductor wafer W. At this time, the lift pins 12 rise to a position higher than the upper ends of the substrate support pins 77.

[0065] After the semiconductor wafer W is placed on the lift pins 12, the transfer robot exits the heat treatment space 65, and the transfer opening 66 is closed by the gate valve 185. Then, the pair of transfer arms 11 descend, transferring the semiconductor wafer W from the transfer mechanism 10 to the susceptor 74 of the holder 7, where it is held from below in a horizontal position. The semiconductor wafer W is supported by a plurality of substrate support pins 77 erected on the holding plate 75 and held on the susceptor 74. The semiconductor wafer W is held on the holder 7 with the surface on which the pattern has been formed and on which impurities have been implanted facing upward. A predetermined gap is formed between the back surface (the main surface opposite to the front surface) of the semiconductor wafer W supported by the plurality of substrate support pins 77 and the holding surface 75a of the holding plate 75. The pair of transfer arms 11, which have descended to below the susceptor 74, are retracted to a retracted position, i.e., inside the recess 62, by the horizontal movement mechanism 13.

[0066] After the semiconductor wafer W is held from below in a horizontal position by the susceptor 74 of the holder 7, which is made of quartz, the 40 halogen lamps HL of the halogen heating unit 4 are turned on all at once to begin preheating (assisted heating). The halogen light emitted from the halogen lamps HL passes through the lower chamber window 64 and the susceptor 74, both of which are made of quartz, and is irradiated onto the underside of the semiconductor wafer W. The semiconductor wafer W is preheated by being irradiated with light from the halogen lamps HL, and its temperature rises. Note that the transfer arm 11 of the transfer mechanism 10 is retracted inside the recess 62, so it does not interfere with heating by the halogen lamps HL.

[0067] The temperature of the semiconductor wafer W, which is heated by the light irradiation from the halogen lamps HL, is measured by a radiation thermometer 20. The measured temperature of the semiconductor wafer W is transmitted to the control unit 3. The control unit 3 controls the output of the halogen lamps HL while monitoring whether the temperature of the semiconductor wafer W, which is heated by the light irradiation from the halogen lamps HL, has reached a predetermined preheating temperature T1. That is, the control unit 3 feedback-controls the output of the halogen lamps HL based on the value measured by the radiation thermometer 20 so that the temperature of the semiconductor wafer W becomes the preheating temperature T1. The preheating temperature T1 in this embodiment is 200°C or higher and 400°C or lower, which is lower than the typical conventional preheating temperature (700°C to 800°C).

[0068] After the temperature of the semiconductor wafer W reaches the preheating temperature T1, the control unit 3 temporarily maintains the semiconductor wafer W at the preheating temperature T1. Specifically, when the temperature of the semiconductor wafer W measured by the radiation thermometer 20 reaches the preheating temperature T1, the control unit 3 adjusts the output of the halogen lamps HL to maintain the temperature of the semiconductor wafer W at approximately the preheating temperature T1.

[0069] By performing preheating using the halogen lamps HL in this manner, the temperature of the entire semiconductor wafer W is uniformly raised to the preheating temperature T1. During preheating using the halogen lamps HL, the temperature of the peripheral portion of the semiconductor wafer W, where heat dissipation is more likely, tends to be lower than that of the central portion, but the arrangement density of the halogen lamps HL in the halogen heating unit 4 is higher in the region facing the peripheral portion of the semiconductor wafer W than in the region facing the central portion. As a result, a greater amount of light is irradiated onto the peripheral portion of the semiconductor wafer W, where heat dissipation is more likely, and the in-plane temperature distribution of the semiconductor wafer W during the preheating stage can be made uniform.

[0070] When a predetermined time has elapsed since the temperature of the semiconductor wafer W reached the preheating temperature T1, the flash lamps FL of the flash heating unit 5 irradiate the surface of the semiconductor wafer W held on the susceptor 74 with flash light. At this time, part of the flash light emitted from the flash lamps FL heads directly into the chamber 6, and the other part is reflected by the reflector 52 before heading into the chamber 6, and the semiconductor wafer W is flash-heated by the irradiation of these flash lights.

[0071] Prior to irradiating the semiconductor wafer W with a flash of light, a table showing the correlation between the flash of light irradiation time and the opening and closing of the three switches 99 is created in advance. FIG. 10 is a diagram showing an example of a table showing the correlation between the flash of light irradiation time and the opening and closing of the three switches 99. The target flash of light irradiation time and the opening and closing of the first switch 99a, the second switch 99b, and the third switch 99c are correlated and registered in the table. Specifically, the inductance of the discharge circuit is determined by the opening and closing states of the three switches 99, and the flash of light irradiation time is calculated by a simulation based on that inductance. The opening and closing states of the three switches 99 and the calculated irradiation times are then correlated and registered to create a table like that shown in FIG. 10. The created table is stored in a storage unit (memory, magnetic disk, etc.) of the control unit 3.

[0072] The operator of the heat treatment apparatus 1 specifies the desired flash irradiation time through a user interface (e.g., a touch panel) of the control unit 3. In this embodiment, for example, the operator of the heat treatment apparatus 1 specifies 0.1 milliseconds as the desired flash irradiation time. The control unit 3 extracts the open / close states of the three switches 99 corresponding to the specified flash irradiation time from the table of FIG. 10. For example, if 0.1 milliseconds is specified as the flash irradiation time, the table of FIG. 10 determines that the first switch 99a, the second switch 99b, and the third switch 99c are all "closed." Note that instead of the operator specifying the flash irradiation time, the control unit 3 may determine the open / close states of the three switches 99 from the table of FIG. 10 based on the flash irradiation time described in the process recipe (which defines the processing procedure and processing conditions for the semiconductor wafer W). In other words, the control unit 3 may determine the open / close states of the three switches 99 based on the flash irradiation time required for the flash lamp FL.

[0073] Charge is accumulated in the capacitor 93. The first switch 99a, the second switch 99b, and the third switch 99c are individually opened and closed according to the opening and closing states of the three switches 99 determined by the control unit 3. A high voltage is applied from the trigger circuit 97 to the trigger electrode 91, causing the flash lamp FL to emit light. The flash light emitted from the flash lamp FL is irradiated onto the surface of the semiconductor wafer W held on the susceptor 74, thereby performing flash heating. In this embodiment, the flash lamp FL emits light with the first switch 99a, the second switch 99b, and the third switch 99c all closed. Therefore, the inductance of the discharge circuit is extremely small, causing the waveform of the current flowing through the flash lamp FL to become peaky (solid line in FIG. 9 ). The flash light emission time is approximately 0.1 milliseconds. When the charge accumulated in the capacitor 93 is discharged by the flash lamp FL in the extremely short time of 0.1 milliseconds, a large current of approximately 5000 A flows through the discharge circuit. As shown in FIG. 8, the discharge circuit of this embodiment does not incorporate an IGBT, so that a large current of about 5000 A can be passed through it.

[0074] A large current of approximately 5000 A flows over a period of 0.1 milliseconds, resulting in the emission of an extremely intense flash of light from the flash lamp FL. As a result, the surface of the semiconductor wafer W is instantaneously heated from the preheating temperature T1 to the target temperature T2. The target temperature T2 is between 800°C and 1200°C. In other words, by closing all three switches 99 and emitting a flash of light for a period of 0.1 milliseconds, the jump temperature (the temperature rise from the preheating temperature T1 to the target temperature T2) due to flash heating can be increased to 600°C or higher. The surface of the semiconductor wafer W is instantaneously heated to the target temperature T2, activating the impurities implanted into the surface. The surface temperature of the semiconductor wafer W then rapidly drops after reaching the target temperature T2. Because the surface of the semiconductor wafer W is instantaneously heated to the target temperature T2 and then immediately cooled rapidly, the diffusion of the implanted impurities is suppressed while the impurities are activated.

[0075] After the flash heating process is completed, the halogen lamps HL are extinguished after a predetermined time has elapsed. This causes the temperature of the semiconductor wafer W to rapidly decrease from the preheating temperature T1. The temperature of the semiconductor wafer W during this decrease is measured by the radiation thermometer 20, and the measurement results are transmitted to the control unit 3. The control unit 3 monitors, based on the measurement results from the radiation thermometer 20, whether the temperature of the semiconductor wafer W has decreased to a predetermined temperature. After the temperature of the semiconductor wafer W has decreased to or below the predetermined temperature, the pair of transfer arms 11 of the transfer mechanism 10 again move horizontally from the retracted position to the transfer operation position and rise, causing the lift pins 12 to protrude from the upper surface of the susceptor 74 and receive the heat-treated semiconductor wafer W from the susceptor 74. Next, the transfer opening 66, which had been closed by the gate valve 185, is opened, and the semiconductor wafer W placed on the lift pins 12 is removed from the chamber 6 by a transfer robot external to the apparatus, completing the heat treatment of the semiconductor wafer W.

[0076] In this embodiment, instead of incorporating an IGBT into the discharge circuit of the flash lamp FL, three coils 94 are connected in series, and three switches 99 are provided that are connected in parallel one-to-one to all of the three coils 94. By opening and closing the three switches 99 to appropriately cause the three coils 94 to function as inductors, it is possible to change the waveform of the current flowing through the flash lamp FL and adjust the flash irradiation time without using an IGBT. Furthermore, because an IGBT is not used, it is also possible to pass a large current through the discharge circuit of the flash lamp FL. In other words, this embodiment makes it possible to adjust the flash irradiation time while accommodating a large current.

[0077] In this embodiment, the control unit 3 opens and closes the three switches 99 according to the flash light irradiation time required of the flash lamps FL using a table such as that shown in Fig. 10. Therefore, the operator of the heat treatment apparatus 1 does not need to set the opening and closing of the three switches 99 one by one; the operator simply specifies the desired flash light irradiation time, and the three switches 99 are automatically opened and closed.

[0078] In particular, in this embodiment, by closing all three switches 99, the inductance of the discharge circuit can be minimized, allowing for flash light irradiation with an irradiation time of 0.1 milliseconds. By irradiating a high-intensity flash light with an irradiation time of 0.1 milliseconds, a high jump temperature can be achieved in a short time, allowing for a lower preheating temperature T1. As a result, the total amount of heat required for the surface temperature of the semiconductor wafer W to reach the target temperature T2 can be reduced, achieving heat treatment with a low thermal history and preventing the inactivation of activated impurities.

[0079] Although the above describes an embodiment of the present invention, various modifications can be made to the present invention without departing from the spirit and scope of the present invention. For example, in the above embodiment, three coils 94 are provided in the discharge circuit, but this is not limited to this. The number of coils 94 connected to the discharge circuit may be two, four, or more. In short, it is sufficient that multiple coils 94 are connected in series to the discharge circuit of the flash lamp FL, and multiple switches 99 are connected in parallel to all of the multiple coils 94 in a one-to-one relationship. The more coils 94 connected to the discharge circuit, the more variations in inductance are available, allowing for more precise adjustment of the waveform of the current flowing through the flash lamp FL and the irradiation time of the flash light, but the discharge circuit becomes larger.

[0080] Furthermore, a discharge circuit such as that shown in FIG. 8 is provided for each of the 30 flash lamps FL. Therefore, the combination of opening and closing the three switches 99 does not need to be the same for all 30 flash lamps FL; it may be different for each flash lamp FL. That is, the three switches 99 in each discharge circuit may be opened and closed so that the inductance for each of the 30 flash lamps FL is different. This allows each of the 30 flash lamps FL to have a different current waveform and flash light emission time. Alternatively, the 30 flash lamps FL may be divided into several lamp groups, and the three switches 99 may be opened and closed so that the inductance for each lamp group is different. In this way, for example, by reducing the inductance of the lamp group facing the center of the semiconductor wafer W and increasing the inductance of the lamp group facing the periphery, the emission time of the flash light irradiated to the center can be shortened and the emission time of the flash light irradiated to the periphery can be relatively long.

[0081] Furthermore, the timing at which the trigger circuit 97 applies a voltage to the trigger electrode 91 is determined by a delay circuit 98 (FIG. 8). That is, the delay circuit 98 determines the light emission timing of each of the 30 flash lamps FL. This allows the light emission timing of each of the 30 flash lamps FL to be different. By appropriately adjusting the light emission timing in addition to the waveform of the current flowing through each of the 30 flash lamps FL, a wide variety of flash light irradiation patterns can be achieved.

[0082] Alternatively, as shown in FIG. 11 , multiple capacitors 93 may be connected in parallel to the discharge circuit of the flash lamp FL, and multiple switches 99 may be provided, each connected in series to each of the multiple capacitors 93. In FIG. 11 , the same elements as in the above embodiment are denoted by the same reference numerals. In the discharge circuit shown in FIG. 11 , when a switch 99 is closed, the corresponding capacitor 93 functions as a capacitor. Conversely, when a switch 99 is open, the corresponding capacitor 93 does not function as a capacitor in the discharge circuit. By appropriately opening and closing the multiple switches 99, the capacitance of the discharge circuit can be varied. As a result, even if the charging voltage is the same, the waveform and intensity of the current flowing through the flash lamp FL can be changed, thereby changing the intensity and half-width of the flash light.

[0083] Furthermore, in the above embodiment, the flash heating unit 5 is provided with 30 flash lamps FL, but this is not limited to this and the number of flash lamps FL can be any number. Furthermore, the flash lamps FL are not limited to xenon flash lamps and may be krypton flash lamps. Furthermore, the number of halogen lamps HL provided in the halogen heating unit 4 is not limited to 40 and can be any number.

[0084] Furthermore, in the above embodiment, the semiconductor wafer W is preheated using a filament-type halogen lamp HL as a continuously lit lamp that emits light continuously for one second or more, but this is not limited to this, and preheating may also be performed using a discharge-type arc lamp (e.g., a xenon arc lamp) as a continuously lit lamp instead of the halogen lamp HL.

[0085] Furthermore, the substrate to be processed by the heat treatment apparatus 1 is not limited to a semiconductor wafer, but may be a glass substrate used in a flat panel display such as a liquid crystal display device, or a substrate for a solar cell. [Explanation of symbols]

[0086] 1. Heat treatment equipment 3. Control Unit 4 Halogen heating section 5 Flash heating section 6 chambers 7 Holding part 10 Transfer mechanism 20 Radiation thermometer 65 Heat Treatment Space 74 Susceptor 93 Capacitor 94 Coil 98 Delay Circuit 99 Switch FL flash lamp HL halogen lamp W Semiconductor wafer

Claims

1. A heat treatment apparatus for heating a substrate by irradiating the substrate with flash light, a chamber for housing the substrate; a holder that holds the substrate in the chamber; a flash lamp that irradiates the substrate held by the holder with a flash of light; a capacitor that discharges stored charge to the flash lamp; a plurality of coils connected in series with the flash lamp and the capacitor; a plurality of switches connected in parallel one-to-one with respect to all of the plurality of coils; a control unit that controls opening and closing of the plurality of switches; Equipped with The control unit extracts the opening and closing states of the plurality of switches corresponding to the irradiation time required for the flash lamp from a table showing the correlation between the irradiation time of flash light and the opening and closing of the plurality of switches, and opens and closes the plurality of switches.

2. In the heat treatment device according to claim 1, a plurality of flash lamps for irradiating the substrate with light; A heat treatment apparatus comprising: opening and closing the plurality of switches so that the inductances of the plurality of flash lamps are different from each other.

3. 3. The heat treatment apparatus according to claim 2, The heat treatment apparatus further comprises a delay circuit that determines the light emission timing of each of the plurality of flash lamps.

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