Control device for laser annealing device and laser annealing method

Abstract

This control device controls an annealing apparatus that performs laser annealing by allowing a laser beam to enter a surface of a semiconductor wafer and by moving a beam spot of the laser beam on the surface of the semiconductor wafer. This control device makes a sweep speed of the beam spot of the laser beam, faster than twice a value obtained by dividing a thermal diffusivity of the semiconductor wafer by a thickness of the semiconductor wafer. Accordingly, the temperature of a laser irradiation surface on the semiconductor wafer can be sufficiently increased and an increase in the temperature of a non-irradiation surface can be suppressed.

Description

[Technical Field]

[0001] The present invention relates to a control device for a laser annealing apparatus and a laser annealing method. [Background technology]

[0002] In order to activate dopants doped into semiconductor wafers such as silicon wafers, the semiconductor wafers must be heated (annealed). The manufacturing process for insulated gate bipolar transistors (IGBTs) includes a step in which circuit elements are formed on one side of the semiconductor wafer, and then impurities are doped onto the other side, followed by annealing. During annealing, a resin protective tape is attached to the circuit-formed surface. To prevent the protective tape from melting, it is desirable to suppress the temperature rise of the circuit-formed surface.

[0003] In order to sufficiently heat the surface opposite the circuit formation surface and suppress a temperature rise on the circuit formation surface, laser annealing is used in which laser light is irradiated onto the surface opposite the circuit formation surface (for example, Patent Document 1, etc.). A continuous wave (CW) laser or a pulsed laser such as a Q-switched laser or an excimer laser is used as a laser oscillator for annealing. Patent Document 1 discloses a laser annealing technology that uses a laser diode-pumped all-solid-state pulsed laser oscillator. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-114052 Summary of the Invention [Problem to be solved by the invention]

[0005] As semiconductor wafers become thinner, it becomes difficult to heat the surface irradiated with laser light (hereinafter referred to as the laser irradiated surface) to a sufficient temperature while suppressing the temperature rise of the circuit formation surface (hereinafter referred to as the non-irradiated surface).An object of the present invention is to provide a control device for a laser annealing device and a laser annealing method that can sufficiently increase the temperature of the laser irradiated surface of a semiconductor wafer and suppress the temperature rise of the non-irradiated surface. [Means for solving the problem]

[0006] According to one aspect of the present invention, A control device for controlling an annealing device that performs laser annealing by irradiating a laser beam onto a surface of a semiconductor wafer and moving a beam spot of the laser beam on the surface of the semiconductor wafer, comprising: The thermal diffusivity of the semiconductor wafer is less than twice the value divided by the thickness of the semiconductor wafer. To be faster, The sweep speed of the beam spot of the laser beam is and controlling the annealing device so that the sweep speed of the beam spot of the laser beam becomes the determined sweep speed. A control device is provided.

[0007] According to another aspect of the present invention, determining a laser power of a pulsed laser beam to be incident on the laser irradiated surface of the semiconductor wafer and a beam size on the laser irradiated surface of the semiconductor wafer from a first target value of a maximum temperature to be reached on the laser irradiated surface of the semiconductor wafer; determining a pulse repetition frequency and a sweep speed under the condition that, when the beam spot of the pulsed laser beam is swept over the laser irradiation surface of the semiconductor wafer, the beam spots of two successive shots partially overlap or contact each other; When determining the sweep rate, the sweep rate is further determined under the condition that the maximum temperature reached on the back surface of the semiconductor wafer opposite to the laser irradiated surface does not exceed a second target value; A laser annealing method is provided for laser annealing the semiconductor wafer with a determined laser power, beam size, pulse repetition frequency, and beam spot sweep speed. [Effects of the Invention]

[0008] By adjusting the sweep speed of the beam spot as described above, it becomes possible to sufficiently increase the temperature of the laser-irradiated surface of the semiconductor wafer and suppress the temperature rise of the non-irradiated surface. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic perspective view of a laser annealing apparatus according to an embodiment. [Figure 2] FIG. 2 is a schematic diagram of the laser annealing apparatus according to this embodiment. [Figure 3] FIG. 3 is a graph showing calculated values ​​of the change in surface temperature over time when one shot of a pulsed laser beam is incident on a silicon wafer. [Figure 4] FIG. 4 is a cross-sectional view of a semiconductor wafer on which a pulsed laser beam is incident. [Figure 5] 5A and 5B are graphs showing an example of the calculation results of the temperature distribution in the cross section of a semiconductor wafer. [Figure 6] FIG. 6 is a flowchart showing the procedure of the laser annealing method according to this embodiment. [Figure 7] 7A and 7B are schematic diagrams showing the movement history of the beam spot. DETAILED DESCRIPTION OF THE INVENTION

[0010] A laser annealing apparatus according to one embodiment will be described with reference to FIGS. 1 to 7B. 1 is a schematic perspective view of a laser annealing apparatus according to an embodiment. A laser light source 10 outputs a pulsed laser beam. The pulsed laser beam output from the laser light source 10 passes through a beam expander 11, a beam shaping optical element 12, folding mirrors 13 and 14, a beam scanner 15, and an fθ lens 16, and is incident on the laser irradiation surface of a semiconductor wafer 20 to be annealed.

[0011] The semiconductor wafer 20 is held by a wafer chuck 18 supported by a moving mechanism 17. The moving mechanism 17 moves the wafer chuck 18 in two directions in a horizontal plane. As the moving mechanism 17, for example, an XY stage is used.

[0012] The beam expander 11 adjusts the beam size (beam cross-sectional diameter) of the laser beam at the incident position on the beam shaping optical element 12. The beam shaping optical element 12 shapes the shape and intensity distribution of the beam spot on the beam irradiation surface of the semiconductor wafer 20. For example, a diffractive optical element is used as the beam shaping optical element 12. The beam scanner 15 includes a galvanometer mirror 15A and a motor 15B. The motor 15B rotates the galvanometer mirror 15A within a certain range in the tilt direction, thereby scanning the pulsed laser beam in a one-dimensional direction. The fθ lens 16 focuses the pulsed laser beam scanned by the beam scanner 15 onto the laser irradiation surface of the semiconductor wafer 20.

[0013] 2 is a schematic diagram of a laser annealing apparatus according to this embodiment. Explanation of the contents that overlap with the explanation of FIG. 1 will be omitted.

[0014] A fiber laser oscillator is used as the laser light source 10. An input optical fiber 32 is connected to one end of a gain fiber 31 doped with a laser active medium, and an output optical fiber 34 is connected to the other end. A high-reflectivity fiber Bragg grating 33 is formed in the input optical fiber 32, and a low-reflectivity fiber Bragg grating 35 is formed in the output optical fiber 34. The high-reflectivity fiber Bragg grating 33 and the low-reflectivity fiber Bragg grating 35 form an optical resonator.

[0015] Pumping light output from the laser diode 30 is introduced into the gain fiber 31 through the input optical fiber 32. The laser active medium doped in the gain fiber 31 is excited by the pumping light. When the laser active medium transitions to a low energy state, stimulated emission occurs, generating laser light. The laser light generated in the gain fiber 31 passes through the output optical fiber 34 and enters the wavelength conversion element 36. The laser beam wavelength-converted by the wavelength conversion element 36 passes through the beam expander 11, the beam shaping optical element 12, the folding mirrors 13 and 14, the beam scanner 15, and the fθ lens 16 and is incident on the semiconductor wafer 20. The gain fiber 31 outputs laser light in the infrared region, for example, and the wavelength conversion element 36 converts the infrared laser light into laser light in the green wavelength region.

[0016] The driver 37 drives the laser diode 30 based on a command from the control device 40. The command received from the control device 40 includes information specifying the repetition frequency of the laser pulses output from the laser diode 30. The driver 37 causes the laser diode 30 to output an excitation laser beam at the repetition frequency of the laser pulses commanded by the control device 40. As a result, a pulsed laser beam is output from the laser light source 10 at the commanded repetition frequency.

[0017] A moving mechanism 17 and a wafer chuck 18 are disposed in a chamber 50. A laser transmission window 51 is attached to the wall of the chamber 50 above a semiconductor wafer 20 held by the wafer chuck 18. The pulsed laser beam transmitted through the fθ lens 16 passes through the laser transmission window 51 and is incident on the laser irradiation surface of the semiconductor wafer 20. The laser annealing apparatus according to this embodiment performs activation annealing of dopants doped in the semiconductor wafer 20, for example. The semiconductor wafer 20 is, for example, a silicon wafer.

[0018] The control device 40 includes a console operated by a user. The user operates the console to input information specifying the pulse repetition frequency of the pulse laser beam. The control device 40 then provides the input information specifying the pulse repetition frequency to the driver 37.

[0019] The control device 40 further controls the beam scanner 15 and the movement mechanism 17 to move the beam spot on the laser irradiation surface of the semiconductor wafer 20. An xyz Cartesian coordinate system is defined in which the direction in which the beam spot moves when the pulse laser beam is scanned by the beam scanner 15 is the x direction, and the direction perpendicular to the x direction in the laser irradiation surface is the y direction. The movement of the beam spot in the x direction when the pulse laser beam is scanned by the beam scanner 15 is referred to as "sweeping." The control device 40 anneals the semiconductor wafer 20 by controlling the movement mechanism 17 to move the semiconductor wafer 20 in the y direction, while controlling the beam scanner 15 to sweep the beam spot in the x direction.

[0020] The maximum length over which the beam spot can be swept in the x direction depends on the deflection angle of the pulsed laser beam by the beam scanner 15 and the performance of the fθ lens 16. When the maximum sweep length is shorter than the dimensions of the semiconductor wafer 20, the process of sweeping the beam spot in the x direction while moving the semiconductor wafer 20 in the y direction can be repeated multiple times while moving the semiconductor wafer 20 in the x direction, thereby making it possible to anneal almost the entire area of ​​the semiconductor wafer 20.

[0021] Next, referring to FIG. 3, the change in surface temperature over time when a pulsed laser beam is incident on the semiconductor wafer 20 will be described.

[0022] For simplicity, the explanation will be given for the case where a laser pulse with a uniform power density P is incident on the semiconductor wafer 20. The surface temperature T of the laser-irradiated surface of the semiconductor wafer 20 can be expressed by the following equation.

number

[0023] If the pulse width of the pulsed laser beam is expressed as t0, the maximum temperature that can be reached on the laser irradiated surface is T a is expressed by the following formula:

number

[0024] Maximum temperature reached on the laser irradiated surface T a Once the target value of is determined, the power density P and pulse width t0 required to raise the temperature to the target value are determined.

[0025] 3 is a graph showing calculated values ​​of the change in surface temperature T over time when one shot of a pulsed laser beam is incident on a silicon wafer. The horizontal axis represents the elapsed time t from the rising edge of the laser pulse in the unit of "ns", the left vertical axis represents the surface temperature T of the semiconductor wafer 20 in the unit of "°C", and the right vertical axis represents the power density P of the pulsed laser beam in the unit of "MW / cm". 2 The dashed line in the graph indicates the change over time in the power density P of the pulsed laser beam, and the solid line indicates the change over time in the surface temperature T of the semiconductor wafer 20. The pulse width of the pulsed laser beam is t0, and the peak power density is 5 MW / cm 2 is.

[0026] During the laser pulse irradiation period (0≦t≦t0), the surface temperature T rises according to formula (1). The surface temperature T at the time when the time equivalent to the pulse width t0 has elapsed from the rising edge of the laser pulse (t=t0) is the maximum temperature T a After the laser pulse falls (t≧t0), the surface temperature T gradually decreases.

[0027] Next, the temperature rise of the non-irradiated surface of the semiconductor wafer 20 will be described with reference to FIGS. 4 to 5B. 4 is a cross-sectional view of a semiconductor wafer 20 onto which a pulsed laser beam is incident. The position of incidence of the laser beam is the heat source Pf. For simplicity, if we consider the temperature distribution directly below the heat source of an infinitely thick plate, the temperature rise ΔT at a position Pr on the non-irradiated surface directly below the heat source Pf is expressed by the following equation:

number

[0028] From equation (3), it can be seen that the slower the sweep speed v of the heat source Pf, the larger the temperature rise ΔT at point Pr on the non-irradiated surface. In particular, when the thickness h of the semiconductor wafer 20 is thin, the increase in the temperature rise ΔT becomes significant.

[0029] 5A and 5B are graphs showing an example of the calculation results of the temperature distribution within the cross section of a semiconductor wafer 20. Note that FIGS. 5A and 5B show the temperature distribution within the cross section of a wafer with a finite thickness and under adiabatic conditions for the non-irradiated surface. The horizontal axis represents the position of the heat source Pf in the sweep direction. The current position of the heat source Pf is set as the origin of the horizontal axis, and the direction of movement of the heat source is set as positive. The vertical axis represents the depth from the beam-irradiated surface in units of μm. FIGS. 5A and 5B show the temperature distribution when the sweep speed v of the heat source Pf is different. FIG. 5B shows the temperature distribution when the sweep speed v of the heat source Pf is faster than that of FIG. 5A. The curves in the graphs represent isotherms, and the numbers attached to each curve represent the temperature in units of °C.

[0030] It can be seen that when the sweep rate v is slow (FIG. 5A), the temperature gradient in the thickness direction is gentler than when it is fast (FIG. 5B). That is, when the sweep rate v is slow, the temperature rise ΔT on the non-irradiated surface is larger than when it is fast. In other words, by increasing the sweep rate v, the temperature rise ΔT on the non-irradiated surface can be reduced.

[0031] Next, the laser annealing method according to this embodiment will be described with reference to FIG. 6 is a flowchart showing the steps of the laser annealing method according to this embodiment. First, the laser power and the beam size on the beam irradiation surface are determined from a first target value of the maximum temperature to be reached on the laser-irradiated surface of the semiconductor wafer 20 (step S1). The power density P can be determined from the laser power and the beam size. The first target value of the maximum temperature to be reached can be determined based on the melting point of the semiconductor wafer 20. For example, if it is desired to melt the surface layer of the semiconductor wafer 20, the maximum temperature to be reached can be set to be equal to or higher than the melting point. If it is desired to perform annealing without melting, the maximum temperature to be reached can be set to be lower than the melting point.

[0032] The pulse repetition frequency and sweep speed v are determined under the condition that the beam spots of two consecutive shots partially overlap or touch each other and the maximum temperature of the non-irradiated surface is equal to or lower than a second target value (step S2). The second target value of the maximum temperature of the non-irradiated surface may be set to a temperature at which the protective tape attached to the non-irradiated surface is not damaged, for example.

[0033] Next, a method for determining the pulse repetition frequency f and the sweep rate v will be described with reference to FIGS. 7A and 7B.

[0034] 7A and 7B are schematic diagrams showing the movement history of the beam spot 25. FIG. 7A shows an example in which two successive shots of the beam spot 25 partially overlap, and FIG. 7B shows an example in which two successive shots of the beam spot 25 are in contact with each other. The dimension of the beam spot in the sweep direction is denoted as L, and the dimension of the area in the sweep direction where the two successive shots of the beam spot overlap is denoted as Lov. The dimension Lov of the overlap area is expressed by the following equation:

number

number

[0035] Laser annealing is performed with the laser power and beam size determined in step S1 shown in FIG. 6, and the pulse repetition frequency and beam spot sweep speed determined in step S2 (step S3).

[0036] Next, the excellent effects of the above embodiment will be described. From equation (3), it can be seen that when the sweep speed v of the beam spot is made faster than 2 k / h, the temperature rise ΔT of the non-irradiated surface is reduced to 1 / e (approximately 0.37 times) of the temperature rise ΔT when laser irradiation is performed with a sweep speed v of 0. In order to obtain a significant effect of reducing the temperature rise ΔT of the non-irradiated surface of the semiconductor wafer 20, it is preferable to make the sweep speed v faster than 2 k / h. In other words, it is preferable to make the sweep speed of the beam spot of the laser beam faster than twice the value obtained by dividing the thermal diffusivity of the semiconductor wafer 20 by the thickness of the semiconductor wafer 20.

[0037] In the above embodiment, the beam spot 25 is swept by scanning the pulsed laser beam with the beam scanner 15. Therefore, the sweep speed v can be made faster than when the beam spot is swept on the laser irradiation surface by operating the moving mechanism 17 to move the semiconductor wafer 20.

[0038] Next, a modification of the above embodiment will be described. To reduce the temperature rise ΔT on the non-irradiated surface, it is preferable to make the sweep speed v as fast as possible, as can be seen from equation (3). However, from equation (5), the upper limit of the sweep speed v is limited by the product of the pulse repetition frequency f of the pulse laser beam and the beam size L. To increase the upper limit of the sweep speed v, it is desirable to increase the pulse repetition frequency f and the beam size L.

[0039] If the beam size L is increased under the condition that the laser power is kept constant, the power density P on the laser-irradiated surface of the semiconductor wafer 20 will decrease. In order to maintain the maximum temperature Ta of the laser-irradiated surface under the condition that the power density P is reduced, the pulse width t0 must be increased. As the pulse width t0 increases, the amount of heat transferred in the thickness direction during the period when the laser pulse is incident increases. As a result, the temperature of the irradiated surface will increase. Therefore, the beam size L cannot be increased unconditionally.

[0040] Equation (5) shows that increasing the pulse repetition frequency f can increase the upper limit of the sweep rate v without increasing the beam size L. For example, to prevent an excessive rise in temperature on the non-irradiated surface of a semiconductor wafer with a thickness of 100 μm or less, the pulse repetition frequency f is preferably set to 15 kHz or higher, and more preferably to 100 kHz or higher.

[0041] When the pulse repetition frequency f is low, the effect of the temperature rise on the laser-irradiated surface due to the previous shot disappears, and the temperature drops to its original value before the next shot is incident on the semiconductor wafer 20. When the pulse repetition frequency f is increased, the next shot is performed while the thermal effect of the previous shot remains. In particular, when the pulse repetition frequency f is set to 15 kHz or higher, the next shot is incident while a temperature rise of 1% or more of the temperature rise ΔT on the laser-irradiated surface due to the previous shot remains. Furthermore, when the pulse repetition frequency f is set to 100 kHz or higher, the next shot is incident while a temperature rise of 5% or more of the temperature rise ΔT on the laser-irradiated surface due to the previous shot remains. This allows for more efficient heating of the beam-irradiated surface.

[0042] In the above embodiment, a fiber laser was used as the laser light source 10 (FIGS. 1 and 2), but other lasers such as a mode-locked laser may also be used. Also, in the above embodiment, a pulsed laser is used for laser annealing, but a continuous wave (CW) laser may also be used. A CW laser corresponds to a pulsed laser with an infinite pulse repetition frequency f.

[0043] Furthermore, in the above embodiment, a galvanometer scanner is used to sweep the beam spot at a sweep speed v, but if the semiconductor wafer 20 can be moved at a sufficient speed using an XY stage or the like, the path of the laser beam may be fixed and the beam spot may be swept on the laser-irradiated surface of the semiconductor wafer 20 by moving the semiconductor wafer 20.

[0044] The above-described embodiments are merely illustrative and the present invention is not limited to the above-described embodiments. For example, it will be obvious to those skilled in the art that various modifications, improvements, combinations, etc. are possible. [Explanation of symbols]

[0045] 10 Laser light source 11 Beam Expander 12 Beam shaping optical elements 13, 14 Folding mirror 15 Beam Scanner 15A Galvanometer Mirror 15B motor 16 fθ lens 17 Moving mechanism 18 Wafer chuck 20 Semiconductor wafers 25 beam spots 30 Laser Diode 31 Gain fiber 32 Input optical fiber 33 Fiber Bragg Grating 34 Output optical fiber 35 Fiber Bragg Grating 36 Wavelength conversion element 37 Drivers 40 Control device 50 Chambers 51 Laser transmission window

Claims

1. A control device for controlling an annealing device that performs laser annealing by irradiating a laser beam onto a surface of a semiconductor wafer and moving a beam spot of the laser beam on the surface of the semiconductor wafer, comprising: a control device that determines a sweep speed of the beam spot of the laser beam so that the sweep speed is faster than twice the value obtained by dividing the thermal diffusivity of the semiconductor wafer by the thickness of the semiconductor wafer, and controls the annealing device so that the sweep speed of the beam spot of the laser beam becomes the determined sweep speed.

2. The annealing device is a moving mechanism that holds the semiconductor wafer and moves it in a first direction; a beam scanner that scans the laser beam to move a beam spot on the surface of the semiconductor wafer in a second direction intersecting the first direction; and 2. The control device according to claim 1, wherein the sweep speed of the beam spot relative to the semiconductor wafer in the second direction is set to be faster than 2 k / h.

3. 3. The control device according to claim 2, wherein the laser beam is a pulsed laser beam, the pulse repetition frequency is 15 kHz or more, and the beam spot is moved in the second direction under the condition that the beam spots of two consecutive shots partially overlap or contact each other.

4. 4. The control device according to claim 3, wherein the pulse repetition frequency of the laser beam is 100 kHz or more.

5. determining a laser power of a pulsed laser beam to be incident on the laser irradiated surface of the semiconductor wafer and a beam size on the laser irradiated surface of the semiconductor wafer from a first target value of a maximum temperature to be reached on the laser irradiated surface of the semiconductor wafer; determining a pulse repetition frequency and a sweep speed under the condition that, when the beam spot of the pulsed laser beam is swept over the laser irradiation surface of the semiconductor wafer, the beam spots of two successive shots partially overlap or contact each other; When determining the sweep rate, the sweep rate is further determined under the condition that the maximum temperature reached on the back surface of the semiconductor wafer opposite to the laser irradiated surface does not exceed a second target value; A laser annealing method for performing laser annealing on the semiconductor wafer with the determined laser power, beam size, pulse repetition frequency, and beam spot sweep speed.

6. A laser annealing method for performing laser annealing by irradiating a laser beam onto a surface of a semiconductor wafer and moving a beam spot of the laser beam over the surface of the semiconductor wafer, comprising: determining a scanning speed of the beam spot of the laser beam so as to be faster than twice the value obtained by dividing the thermal diffusivity of the semiconductor wafer by the thickness of the semiconductor wafer; A laser annealing method comprising: moving the beam spot of the laser beam on the surface of the semiconductor wafer so that the sweep speed of the beam spot of the laser beam becomes a determined sweep speed.

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