Control device of laser annealing apparatus and laser annealing method

Abstract

A control device controls an annealing device which causes a laser beam to be incident on a surface of a semiconductor wafer and moves a beam spot of the laser beam on the surface of the semiconductor wafer to perform laser annealing. The control device sets a scanning speed of the beam spot of the laser beam to be twice as fast as a value obtained by dividing a thermal diffusivity of the semiconductor wafer by a thickness of the semiconductor wafer. Thus, the temperature of a laser irradiation surface of the semiconductor wafer can be sufficiently increased and the temperature increase of a non-irradiation surface can be suppressed.

Description

TECHNICAL FIELD

[0001] The present application relates to a control device of a laser annealing apparatus and a laser annealing method. BACKGROUND

[0002] In order to activate a dopant doped in a semiconductor wafer such as a silicon wafer, the semiconductor wafer needs to be heated (annealed). A manufacturing process of an insulated gate bipolar transistor (IGBT) or the like includes a process of annealing after doping an impurity on the other surface after forming a circuit element on one surface of a semiconductor wafer. When annealing is performed, a protective tape made of resin is attached to the circuit formation surface. In order to prevent the protective tape from melting, it is desired to suppress the temperature rise of the circuit formation surface.

[0003] In order to sufficiently heat the surface on the side opposite to the circuit formation surface and suppress the temperature rise of the circuit formation surface, laser annealing that irradiates laser light to the surface on the side opposite to the circuit formation surface is used (for example, Patent Literature 1 or the like). As a laser oscillator for annealing, a continuous wave (CW) laser or a pulse laser such as a Q-switched laser or an excimer laser is used. In Patent Literature 1, a laser annealing technique using a laser diode-pumped all-solid-state pulse laser oscillator is disclosed.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2011-114052 SUMMARY

[0007] PROBLEMS TO BE SOLVED BY THE INVENTION

[0008] If the thickness of the semiconductor wafer is thin, it is difficult to heat the surface on which laser light is irradiated (hereinafter, referred to as a laser irradiation surface) to a sufficient temperature and suppress the temperature rise of the circuit formation surface (hereinafter, referred to as a non-irradiation surface). An object of the present application is to provide a control device of a laser annealing apparatus and a laser annealing method capable of sufficiently increasing the temperature of the laser irradiation surface of a semiconductor wafer and suppressing the temperature rise of the non-irradiation surface.

[0009] MEANS FOR SOLVING THE PROBLEMS

[0010] According to one aspect of the present application, there is provided a control device that controls an annealing apparatus that causes a laser beam to be incident on a surface of a semiconductor wafer and moves a beam spot of the laser beam on the surface of the semiconductor wafer to perform laser annealing,

[0011] The control device sets a scanning speed of the beam spot of the laser beam to be faster than twice a value obtained by dividing a thermal diffusivity of the semiconductor wafer by a thickness of the semiconductor wafer.

[0012] According to another aspect of the present application, there is provided a laser annealing method including the steps of:

[0013] determining a laser power of a pulsed laser beam incident on a laser irradiation surface of the semiconductor wafer and a beam size on the laser irradiation surface of the semiconductor wafer according to a first target value of a maximum reached temperature of the laser irradiation surface of the semiconductor wafer;

[0014] determining a repetition frequency of the pulses and a scanning speed of the beam spot while scanning the beam spot of the pulsed laser beam on the laser irradiation surface of the semiconductor wafer, with the condition that two beam spots successively emitted are partially overlapped or in contact with each other;

[0015] Furthermore, in determining the scanning speed, the scanning speed is determined with the condition that a maximum reached temperature of a back surface on a side opposite to the laser irradiation surface of the semiconductor wafer is not more than a second target value; and

[0016] performing laser annealing of the semiconductor wafer at the determined laser power, beam size, repetition frequency of the pulses, and scanning speed of the beam spot.

[0017] Effects of Invention

[0018] By adjusting the scanning speed of the beam spot as above, it is possible to sufficiently increase the temperature of the laser irradiation surface of the semiconductor wafer and to suppress the temperature rise of the non-irradiation surface. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic perspective view of a laser annealing apparatus of an embodiment.

[0020] Figure 2 is a schematic view of a laser annealing apparatus of the present embodiment.

[0021] Figure 3 is a graph showing calculated values of temporal changes in surface temperature in a case where a pulsed laser beam of one pulse is incident on a silicon wafer.

[0022] Figure 4 is a sectional view of a semiconductor wafer to which a pulsed laser beam is incident.

[0023] Figure 5A and Figure 5B is a graph showing an example of a calculated result of temperature distribution in a cross section of a semiconductor wafer.

[0024] Figure 6 is a flowchart showing steps of a laser annealing method of the present embodiment.

[0025] Figure 7A and Figure 7Bis a schematic view of movement experienced by a beam spot of light. DETAILED DESCRIPTION

[0026] Reference Figures 1-7B A laser annealing apparatus of an embodiment is described.

[0027] Figure 1 is a schematic perspective view of a laser annealing apparatus of an embodiment. A laser light source 10 outputs a pulsed laser beam. The pulsed laser beam output from the laser light source 10 is incident on a laser irradiation surface of an annealing object (i.e., a semiconductor wafer 20) via a beam expander 11, a beam shaping optical element 12, turnaround mirrors 13, 14, a beam scanner 15, and an fθ lens 16.

[0028] The semiconductor wafer 20 is held by a wafer chuck 18, which is 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.

[0029] The beam expander 11 adjusts the beam size (diameter of the beam cross section) at the incident position on the beam shaping optical element 12 of the laser beam. The beam shaping optical element 12 shapes the shape and intensity distribution of the beam spot on the laser irradiation surface of the semiconductor wafer 20. As the beam shaping optical element 12, for example, a diffractive optical element is used. The beam scanner 15 includes a galvano mirror 15A and a motor 15B. The motor 15B rotates the galvano mirror 15A over a certain range in a swinging direction, thereby scanning the pulsed laser beam in one-dimensional direction. The fθ lens 16 converges the pulsed laser beam scanned by the beam scanner 15 onto the laser irradiation surface of the semiconductor wafer 20.

[0030] Figure 2 is a schematic view of a laser annealing apparatus of the present embodiment. The description of the contents repeated with Figure 1 is omitted.

[0031] As the laser light source 10, a fiber laser oscillator is used. An input-side optical fiber 32 is connected to one end of a gain optical fiber 31 doped with a laser active medium, and an output-side optical fiber 34 is connected to the other end. A high-reflectivity-type fiber Bragg grating 33 is formed on the input-side optical fiber 32, and a low-reflectivity-type fiber Bragg grating 35 is formed on the output-side optical fiber 34. The high-reflectivity-type fiber Bragg grating 33 and the low-reflectivity-type fiber Bragg grating 35 constitute an optical resonator.

[0032] The excitation light output from the laser diode 30 is introduced into the gain fiber 31 via the input-side optical fiber 32. The laser-active medium doped in the gain fiber 31 is excited by the excitation light. When the laser-active medium transitions to a lower energy state, stimulated emission occurs, thereby generating laser light. The laser light generated in the gain fiber 31 is incident on the wavelength conversion element 36 via the output-side optical fiber 34. The laser beam wavelength-converted by the wavelength conversion element 36 is incident on the semiconductor wafer 20 via the beam expander 11, the beam shaping optical element 12, the turning mirrors 13, 14, the beam scanner 15, and the fθ lens 16. The gain fiber 31 outputs, for example, laser light in the infrared region, and the wavelength conversion element 36 converts the laser light in the infrared region to laser light in the green wavelength region.

[0033] The driver 37 drives the laser diode 30 in accordance with an instruction from the control device 40. The instruction received from the control device 40 includes information specifying the repetition frequency of the laser pulse output from the laser diode 30. The driver 37 causes the laser diode 30 to output excitation laser light at the repetition frequency of the laser pulse instructed by the control device 40. As a result, the laser light source 10 outputs a pulsed laser beam at the instructed repetition frequency.

[0034] The moving mechanism 17 and the wafer chuck 18 are disposed in the chamber 50. A laser-transmitting window 51 is installed on the wall surface of the chamber 50 above the semiconductor wafer 20 held on the wafer chuck 18. The pulsed laser beam transmitted through the fθ lens 16 is transmitted through the laser-transmitting window 51 and is incident on the laser irradiation surface of the semiconductor wafer 20. The laser annealing apparatus of the present embodiment performs, for example, an activation anneal of a dopant doped in the semiconductor wafer 20. The semiconductor wafer 20 is, for example, a silicon wafer.

[0035] The control device 40 includes a console for a user to operate. The user inputs information specifying the repetition frequency of the pulse of the pulsed laser beam by operating the console. The control device 40 provides the driver 37 with the input information specifying the repetition frequency of the pulse.

[0036] Furthermore, the control device 40 controls the beam scanner 15 and the moving mechanism 17, thereby moving the beam spot on the laser irradiation surface of the semiconductor wafer 20. An xyz orthogonal coordinate system is defined such that a direction in which the beam spot is moved by scanning the pulsed laser beam with the beam scanner 15 is the x direction, and a direction orthogonal to the x direction in the laser irradiation surface is the y direction. The movement of the beam spot in the x direction caused by scanning the pulsed laser beam with the beam scanner 15 is referred to as "sweeping". The control device 40 controls the moving mechanism 17 to move the semiconductor wafer 20 in the y direction, and controls the beam scanner 15 to sweep the beam spot in the x direction, thereby performing an anneal of the semiconductor wafer 20.

[0037] The maximum length that can be swept along the beam point in the x-direction depends on the swing angle of the pulsed laser beam of the beam scanner 15 and the performance of the fθ lens 16. When the maximum sweep length is shorter than the size of the semiconductor wafer 20, almost the entire area of ​​the semiconductor wafer 20 can be annealed by moving the semiconductor wafer 20 along the x-direction and repeating the process of sweeping the beam point along the x-direction while moving the semiconductor wafer 20 along the y-direction multiple times.

[0038] Next, refer to Figure 3 The time-dependent change in surface temperature when a pulsed laser beam is incident on a semiconductor wafer 20 is explained.

[0039] For simplicity, the case where a laser pulse with uniform power density P is incident on the semiconductor wafer 20 will be described. The surface temperature T of the laser-irradiated surface of the semiconductor wafer 20 can be expressed by the following formula.

[0040] [Formula 1]

[0041]

[0042] Where t is the time elapsed since heating began, C is the specific heat capacity of the semiconductor wafer 20, ρ is the density of the semiconductor wafer 20, and λ is the thermal conductivity of the semiconductor wafer 20. For example, the unit of surface temperature T is "K", and the unit of power density P is "W / cm²". 2 The unit for time t is "seconds", the unit for specific heat capacity C is "J / g·K", and the unit for density ρ is "g / cm³". 3 The unit of thermal conductivity λ is W / cm·K.

[0043] If the pulse width of the pulsed laser beam is denoted as t0, then the highest temperature reached by the laser-irradiated surface is T. a It is expressed by the following formula.

[0044] [Formula 2]

[0045]

[0046] If the highest temperature reached by the laser irradiation surface is T a Once the target value is determined, the power density P and pulse width t0 required to raise the temperature to that target value can be determined.

[0047] Figure 3 This is a graph showing the calculated change in surface temperature T over time when a pulsed laser beam is incident on a silicon wafer. The horizontal axis represents the time t elapsed from the rise of the laser pulse in "ns", the left vertical axis represents the surface temperature T of the semiconductor wafer 20 in "°C", and the right vertical axis represents the temperature in "MW / cm²". 2"" represents the power density P of the pulsed laser beam. The dashed line in the graph represents the change of the power density P of the pulsed laser beam over time, and the solid line represents the change of the surface temperature T of the semiconductor wafer 20 over time. The pulse width of the pulsed laser beam is t0, and the peak power density is 5 MW / cm². 2 .

[0048] During the incident period of the laser pulse (0≤t≤t0), the surface temperature T rises according to equation (1). The surface temperature T at the moment when the time equivalent to the pulse width t0 has elapsed since the rise of the laser pulse (t=t0) is equal to the highest temperature T reached. a After the laser pulse begins to decrease (t≥t0), the surface temperature T gradually decreases.

[0049] Next, refer to Figures 4-5B The temperature rise of the non-irradiated surface of the semiconductor wafer 20 is explained.

[0050] Figure 4 This is a cross-sectional view of the semiconductor wafer 20 into which a pulsed laser beam is incident. The incident position of the laser beam is called the heat source Pf. If, for convenience, the temperature distribution directly below the heat source of an infinitely thick plate is considered, the temperature rise ΔT at the position Pr on the non-irradiated surface directly below the heat source Pf is expressed by the following formula.

[0051] [Formula 3]

[0052]

[0053] Here, Q represents the input heat from the heat source Pf to the semiconductor wafer 20, h represents the thickness of the semiconductor wafer 20, v represents the sweeping speed of the heat source Pf, and k represents the thermal diffusivity of the semiconductor wafer 20. For example, the unit of input heat Q is "W", the unit of thickness h of semiconductor wafer 20 is "cm", the unit of sweeping speed v is "cm / s", and the unit of thermal diffusivity k is "cm". 2 / s".

[0054] As shown in equation (3), the slower the sweeping speed v of the heat source Pf, the greater 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 temperature rise ΔT will increase significantly.

[0055] Figure 5A and Figure 5B This is a graph showing an example of the calculated temperature distribution within a cross-section of the semiconductor wafer 20. Additionally, Figure 5A and Figure 5BThe figure shows the temperature distribution within a cross-section of a wafer of finite thickness with its non-irradiated surface under adiabatic conditions. The horizontal axis represents the position of the heat source Pf in the sweeping 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 laser-irradiated surface in units of "μm". Figure 5A and Figure 5B The figure shows the temperature distribution when the sweeping velocities v of the heat sources Pf are different. Figure 5B The diagram shows that the sweeping velocity v of the heat source Pf is faster than... Figure 5A Temperature distribution over time. The curves in the graph represent isotherms, and the values ​​marked on each curve represent temperature in "°C".

[0056] As shown in the figure, when the sweeping speed v is slow ( Figure 5A When the temperature gradient in the thickness direction is faster than the sweeping velocity v, Figure 5B The temperature rise on the non-irradiated surface is more gradual. That is, when the sweeping speed v is slow, the temperature rise ΔT on the non-irradiated surface is greater than when the sweeping speed v is fast. In other words, by increasing the sweeping speed v, the temperature rise ΔT on the non-irradiated surface can be reduced.

[0057] Next, refer to Figure 6 The laser annealing method of this embodiment will be described.

[0058] Figure 6 This is a flowchart illustrating the steps of the laser annealing method of this embodiment. First, the laser power and the beam size on the laser irradiation surface are determined based on a first target value of the highest temperature reached on the laser irradiation surface of the semiconductor wafer 20 (step S1). The power density P can be determined based on the laser power and beam size. The first target value of the highest temperature reached can be determined based on the melting point of the semiconductor wafer 20. For example, if the surface layer of the semiconductor wafer 20 is to be melted, the highest temperature reached can be set above the melting point. If annealing is to be performed in a non-melting manner, the highest temperature reached can be set below the melting point.

[0059] The pulse repetition frequency and sweep speed v are determined based on the following conditions: the two continuously emitted beams partially overlap or contact each other, and the highest temperature reached on the non-irradiated surface is set as a second target value. The second target value of the highest temperature reached on the non-irradiated surface can be set, for example, to the temperature at which the protective tape pasted on the non-irradiated surface is not damaged.

[0060] Next, refer to Figure 7A and Figure 7B The method for determining the pulse repetition frequency f and sweep speed v is explained.

[0061] Figure 7A and Figure 7B This is a schematic diagram showing the movement of beam point 25. Figure 7AThe image shows an example of partial overlap between two consecutively emitted beam points 25. Figure 7B The diagram illustrates an example of two consecutively emitted beam points 25 in contact with each other. The dimension of the beam point in the sweeping direction is denoted as L, and the dimension of the overlapping region of the two consecutively emitted beam points in the sweeping direction is denoted as Lov. The dimension Lov of the overlapping region is expressed by the following formula.

[0062] [Formula 4]

[0063]

[0064] To prevent two consecutively emitted beams from separating, we can assume Lov ≥ 0. That is, the sweep velocity v can be determined to satisfy the following formula.

[0065] [Formula 5]

[0066] v≤fXL…(5)

[0067] by Figure 6 Laser annealing is performed using the laser power and beam size determined in step S1, and the pulse repetition frequency and beam sweep speed determined in step S2 (step S3).

[0068] Next, the superior effects of the above embodiments will be explained.

[0069] As shown in equation (3), if the sweeping speed v of the laser beam spot is faster than 2 k / h, the temperature rise ΔT of the non-irradiated surface will be reduced to 1 / e (approximately 0.37 times) of the temperature rise ΔT when laser irradiation is performed with the sweeping speed v set to 0. To achieve a significant reduction in the temperature rise ΔT of the non-irradiated surface of the semiconductor wafer 20, it is preferable to set the sweeping speed v to be faster than 2 k / h. In other words, it is preferable to set the sweeping speed of the laser beam spot to be faster than twice the value obtained by dividing the thermal diffusivity of the semiconductor wafer 20 by the thickness of the semiconductor wafer 20.

[0070] In the above embodiment, the beam spot 25 is swept by scanning the pulsed laser beam with the beam scanner 15. Therefore, by moving the semiconductor wafer 20 by actuating the moving mechanism 17, the sweeping speed v can be made faster than when the beam spot is swept across the laser irradiation surface.

[0071] Next, variations of the above embodiments will be described.

[0072] To reduce the temperature rise ΔT on the non-irradiated surface, it is preferable to increase the sweep speed v as much as possible, as shown in equation (3). However, as shown in equation (5), the upper limit of the sweep speed v is limited by the product of the pulse repetition frequency f of the pulsed 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 increase the beam size L.

[0073] If the beam size L is increased while maintaining a constant laser power, the power density P on the laser irradiation surface of the semiconductor wafer 20 will decrease. To maintain the maximum reach temperature T of the laser irradiation surface under the condition of decreased power density P, a specific method is needed. a Therefore, it is necessary to increase the pulse width t0. If the pulse width t0 increases, the heat transfer along the thickness direction during the laser pulse incidence will increase. As a result, the temperature of the non-irradiated surface will rise. Therefore, it is not possible to unconditionally increase the beam size L.

[0074] To increase the upper limit of the sweep speed v without increasing the beam size L, it can be seen from equation (5) that the pulse repetition frequency f can be increased. For example, in order to suppress the excessive temperature rise of the non-irradiated surface of a semiconductor wafer with a thickness of less than 100 μm, it is preferable to set the pulse repetition frequency f to 15 kHz or more, and more preferably to 100 kHz or more.

[0075] If the pulse repetition frequency f is low, the effect of the temperature rise of the laser irradiated surface from the previous emission will disappear, causing the temperature to drop back to its original temperature before the next emitted laser beam is incident on the semiconductor wafer 20. If the pulse repetition frequency f is increased, the next emission will occur while the thermal effects of the previous emission remain. Specifically, if the pulse repetition frequency f is set to 15 kHz or higher, the next emission will occur with a temperature rise of at least 1% of the temperature rise ΔT of the laser irradiated surface from the previous emission. Furthermore, if the pulse repetition frequency f is set to 100 kHz or higher, the next emission will occur with a temperature rise of at least 5% of the temperature rise ΔT of the laser irradiated surface from the previous emission. Therefore, the laser irradiated surface can be heated more effectively.

[0076] In the above embodiments, the laser source 10 ( Figure 1 and Figure 2 The laser annealing process utilizes a fiber laser, but mode-locked lasers can also be used. Furthermore, while pulsed lasers were used in the above embodiments, continuous-wave (CW) lasers can also be used. A CW laser is equivalent to setting the pulse repetition frequency f of a pulsed laser to infinity.

[0077] Furthermore, in the above embodiment, a Galvano scanner was used to sweep the beam point at a sweeping speed v. However, if an XY stage or the like is used to move the semiconductor wafer 20 at a sufficient speed, the path of the laser beam can be fixed while the semiconductor wafer 20 is moved, thereby sweeping the beam point across the laser irradiation surface of the semiconductor wafer 20.

[0078] The above embodiments are merely examples, and the present invention is not limited to the above embodiments. For example, the present invention can be modified, improved, and combined in various ways, which will be obvious to those skilled in the art.

[0079] Symbol Explanation

[0080] 10-Laser source, 11-Beam expander, 12-Beam shaping optical element, 13, 14-Reflecting mirror, 15-Beam scanner, 15A-Galvano lens, 15B-Motor, 16-fθ lens, 17-Moving mechanism, 18-Wafer chuck, 20-Semiconductor wafer, 25-Beam point, 30-Laser diode, 31-Gain fiber, 32-Input-side fiber, 33-Fiber Bragg grating, 34-Output-side fiber, 35-Fiber Bragg grating, 36-Wavelength conversion element, 37-Driver, 40-Control device, 50-Cavity, 51-Laser transmission window.

Claims

1. A control device for controlling an annealing apparatus, the annealing apparatus causing a laser beam to be incident on the surface of a semiconductor wafer and moving the beam point of the laser beam on the surface of the semiconductor wafer to perform laser annealing, characterized in that... The sweep speed of the laser beam spot is set to be faster than twice the value obtained by dividing the thermal diffusivity of the semiconductor wafer by the thickness of the semiconductor wafer.

2. The control device according to claim 1, characterized in that, The annealing apparatus has the following features: The moving mechanism holds the semiconductor wafer and moves it along the first direction; and A beam scanner, by scanning the laser beam, moves the beam point on the surface of the semiconductor wafer along a second direction intersecting the first direction. The control device sets the sweeping speed of the beam point relative to the semiconductor wafer along the second direction to be faster than 2k / h, where k is the thermal diffusivity of the semiconductor wafer and h is the thickness of the semiconductor wafer.

3. The control device according to claim 2, characterized in that, The laser beam is a pulsed laser beam with a pulse repetition frequency of 15kHz or higher. The control device uses the partial overlap or contact between two continuously emitted beam points as a condition to move the beam points along the second direction.

4. The control device according to claim 3, characterized in that, The repetition frequency of the laser beam pulses is above 100 kHz.

5. A laser annealing method, comprising the following steps: The laser power of the pulsed laser beam incident on the laser irradiation surface of the semiconductor wafer and the beam size on the laser irradiation surface of the semiconductor wafer are determined based on a first target value of the highest temperature reached on the laser irradiation surface of the semiconductor wafer. When the beam point of the pulsed laser beam sweeps across the laser irradiation surface of the semiconductor wafer, the repetition frequency and sweeping speed of the pulse are determined by taking the partial overlap or contact between two consecutively emitted beam points as conditions. Furthermore, when determining the sweeping speed, the condition that the highest temperature reached on the back side opposite to the laser irradiation surface of the semiconductor wafer does not exceed a second target value is used to determine the sweeping speed; and The semiconductor wafer is laser annealed using the determined laser power, beam size, pulse repetition frequency, and beam sweep speed.

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