Patent Description:
In order to activate a dopant doped in a flat plate-shaped annealed target such as a silicon wafer, it is necessary to heat (anneal) the annealed target. In the process of manufacturing an insulated gate bipolar transistor (IGBT) or the like, a circuit element is formed on one surface of a semiconductor wafer, and then an impurity is doped on the other surface to perform annealing. When annealing is performed, a protective tape made of resin is attached to the circuit forming surface. In order to prevent the protective tape from melting, it is desired to suppress the temperature of the circuit forming surface from rising.

In order to sufficiently heat the surface opposite to the circuit forming surface and suppress the temperature of the circuit forming surface from rising, laser annealing has been used to irradiate the surface opposite to the circuit forming surface with a laser light (see, for example, Patent Literature <NUM>, etc.). In the annealing technique described in Patent Literature <NUM>, after one laser pulse is incident, a laser pulse of the next cycle is incident on a portion in the cooling process after the temperature rise. As a result, the energy of the laser pulse can be effectively utilized, and therefore the amount of energy to be input to the semiconductor wafer is reduced. By reducing the amount of energy input, it is possible to suppress the temperature of the surface opposite to the laser irradiation surface from rising.

Document <CIT> discloses the preamble of claims <NUM> and <NUM>.

If the annealed target such as a semiconductor wafer is thin, the temperature of the surface opposite to the laser irradiation surface tends to rise. An object of the disclosure is to provide a control device of an annealing device, an annealing device, and an annealing method which are capable of further suppressing the temperature of the surface opposite to the laser irradiation surface from rising.

According to one aspect of the disclosure, an annealing device according to claim <NUM> is provided.

According to another aspect of the disclosure, an annealing method according to claim <NUM> is provided.

By sweeping the beam spot in the longitudinal direction of the beam spot, it is possible to suppress the temperature of the surface opposite to the laser irradiation surface of the annealed target from rising under the same condition of pulse energy density.

An annealing device and an annealing method according to an embodiment will be described with reference to <FIG>.

<FIG> is a schematic perspective view of the annealing device according to an embodiment. A laser light source <NUM> outputs a pulsed laser beam. The pulsed laser beam output from the laser light source <NUM> passes through a beam expander <NUM>, a beam shaping optical element <NUM>, reflective mirrors <NUM> and <NUM>, a beam scanner <NUM>, and an fθ lens <NUM> to be incident on the laser irradiation surface of an annealed target <NUM>. The annealed target <NUM> is, for example, a semiconductor wafer in which a dopant is ion-implanted.

The annealed target <NUM> is held by a chuck mechanism <NUM> supported by a movable stage <NUM>. The movable stage <NUM> moves the chuck mechanism <NUM> in two directions in a horizontal plane. The movement of the chuck mechanism <NUM> causes the annealed target <NUM> to move. An XY stage, for example, is used as the movable stage <NUM>.

The beam expander <NUM> adjusts the beam size (diameter of the beam cross section) at the incident position of the laser beam on the beam shaping optical element <NUM>. The beam shaping optical element <NUM> shapes the beam spot on the beam irradiation surface of the annealed target <NUM> into a long shape in one direction and makes the intensity distribution uniform. A diffractive optical element, for example, is used as the beam shaping optical element <NUM>. The beam scanner <NUM> includes a galvano mirror 15A and a motor 15B. The motor 15B rotates the galvano mirror 15A within a range in the tilting direction to scan the pulsed laser beam in one-dimensional direction. By this scanning, the beam spot moves in the longitudinal direction thereof on the surface of the annealed target <NUM>. The fθ lens <NUM> concentrates the pulsed laser beam scanned by the beam scanner <NUM> on the laser irradiation surface of the annealed target <NUM>.

<FIG> is a schematic view of a laser annealing device according to the present embodiment. The description of the contents overlapping with the description of <FIG> will be omitted.

A fiber laser oscillator is used as the laser light source <NUM>. An input side optical fiber <NUM> is connected to one end of a gain fiber <NUM> doped with a laser active medium, and an output side optical fiber <NUM> is connected to the other end. A high reflectance type fiber Bragg grating <NUM> is formed on the input side optical fiber <NUM>, and a low reflectance type fiber Bragg grating <NUM> is formed on the output side optical fiber <NUM>. An optical resonator is constituted by the high reflectance type fiber Bragg grating <NUM> and the low reflectance type fiber Bragg grating <NUM>.

An excitation light output from a laser diode <NUM> is introduced into the gain fiber <NUM> through the input side optical fiber <NUM>. The laser active medium doped in the gain fiber <NUM> is excited by the excitation light. Induced emission occurs when the laser active medium transitions to a low energy state, and a laser light is generated. The laser light generated by the gain fiber <NUM> passes through the output side optical fiber <NUM> and is incident on a wavelength conversion element <NUM>. The laser beam wavelength-converted by the wavelength conversion element <NUM> passes through the beam expander <NUM>, the beam shaping optical element <NUM>, the reflective mirrors <NUM> and <NUM>, the beam scanner <NUM>, and the fθ lens <NUM> to be incident on the annealed target <NUM>. The gain fiber <NUM> outputs, for example, a laser light in an infrared region, and the wavelength conversion element <NUM> converts the laser light in the infrared region into a laser light in a green wavelength region.

A driver <NUM> drives the laser diode <NUM> based on a command from a control device <NUM>. The command received from the control device <NUM> includes information that specifies the repetition frequency of the laser pulse output from the laser diode <NUM>. The driver <NUM> outputs the excitation laser light from the laser diode <NUM> at the repetition frequency of the laser pulse commanded by the control device <NUM>. As a result, the pulsed laser beam is output from the laser light source <NUM> at the commanded repetition frequency.

The movable stage <NUM> and the chuck mechanism <NUM> are arranged in a chamber <NUM>. A laser transmission window <NUM> is attached to a wall surface of the chamber <NUM> above the annealed target <NUM> held by the chuck mechanism <NUM>. The pulsed laser beam transmitted through the fθ lens <NUM> passes through the laser transmission window <NUM> and is incident on the laser irradiation surface of the annealed target <NUM>. The laser annealing device according to the present embodiment performs activation annealing on the dopant doped in the annealed target <NUM>, for example. The annealed target <NUM> is, for example, a silicon wafer.

The control device <NUM> includes a console to be operated by a user. The user operates the console to input information that specifies the repetition frequency of the pulse of the pulsed laser beam. The control device <NUM> gives the driver <NUM> the information that specifies the repetition frequency of the input pulse.

The control device <NUM> further controls the beam scanner <NUM> and the movable stage <NUM> to move the beam spot on the laser irradiation surface of the annealed target <NUM>. By scanning the pulsed laser beam with the beam scanner <NUM>, an xyz orthogonal coordinate system is defined in which the direction in which the beam spot moves is an x direction and the direction orthogonal to the x direction in the laser irradiation surface is a y direction. The beam spot of the pulsed laser beam has a long shape in the x direction.

The operation of operating the beam scanner <NUM> to move the beam spot in the x direction is referred to as a "sweep operation. " When the control device <NUM> controls the movable stage <NUM> to move the annealed target <NUM> in the y direction, the position of the beam spot is displaced in the y direction (shifted in the y direction) on the surface of the annealed target <NUM>. The operation of displacing the position of the beam spot in the y direction is referred to as a "step operation.

By driving either the beam scanner <NUM> or the movable stage <NUM> in this way, the beam spot can be moved in the x direction or the y direction on the surface of the annealed target <NUM>. The beam scanner <NUM> and the movable stage <NUM> constitute a movement mechanism <NUM> that moves the beam spot in a two-dimensional direction on the surface of the annealed target <NUM>.

The maximum length by which the beam spot can be swept in the x direction depends on the swing angle of the pulsed laser beam of the beam scanner <NUM> and the performance of the fθ lens <NUM>. If the maximum length of the sweep is shorter than the dimensions of the annealed target <NUM>, almost the entire area of the annealed target <NUM> can be annealed by repeating the sweep operation and the step operation and executing a plurality of times the procedure of annealing in a part of the range in the x direction by shifting the annealed target <NUM> in the x direction.

Next, a temporal change of the surface temperature when the pulsed laser beam is incident on the annealed target <NUM> will be described with reference to <FIG>.

For simplicity, a case where a laser pulse having a uniform power density P is incident on the annealed target <NUM> will be described. The surface temperature T of the laser irradiation surface of the annealed target <NUM> can be expressed by the following equation. [Equation <NUM>] <MAT>.

Here, t is the elapsed time from the start of heating, C is the specific heat of the annealed target <NUM>, ρ is the density of the annealed target <NUM>, and λ is the thermal conductivity of the annealed target <NUM>. For example, the unit of the surface temperature T is "K," the unit of the power density P is "W/cm<NUM>," the unit of the elapsed time t is "second," the unit of the specific heat C is "J/g·K," the unit of the density ρ is "g/cm<NUM>," and the unit of the thermal conductivity λ is "W/cm-K.

When the pulse width of the pulsed laser beam is marked as t<NUM>, the reached maximum temperature Ta of the laser irradiation surface is expressed by the following equation. [Equation <NUM>] <MAT>.

When the target value of the reached maximum temperature Ta of the laser irradiation surface is determined, the power density P and the pulse width t<NUM> required to raise the temperature to the target value are determined.

<FIG> is a graph showing the calculated value of the temporal change of the surface temperature T when one shot of the pulsed laser beam is incident on the silicon wafer. The horizontal axis represents the elapsed time t from the rising point of the laser pulse in the unit "ns," the left vertical axis represents the surface temperature T of the annealed target <NUM> in the unit "°C," and the right vertical axis represents the power density P of the pulsed laser beam in the unit "MW/cm<NUM>. " The broken line in the graph shows the temporal change of the power density P of the pulsed laser beam, and the solid line shows the temporal change of the surface temperature T of the annealed target <NUM>. The pulse width of the pulsed laser beam is t<NUM>, and the peak power density is <NUM> MW/cm<NUM>.

During the period (<NUM> ≦ t ≦ t<NUM>) in which the laser pulse is incident, the surface temperature T rises according to Equation (<NUM>). The surface temperature T at the point (t = t<NUM>) when the time corresponding to the pulse width t<NUM> elapses from the rising point of the laser pulse is equal to the reached maximum temperature Ta. After the laser pulse falls (t ≧ t<NUM>), the surface temperature T gradually decreases.

Next, the temperature rise of the surface (hereinafter referred to as a back surface) opposite to the laser irradiation surface of the annealed target <NUM> will be described with reference to <FIG>.

<FIG> is a cross-sectional view of the annealed target <NUM> on which the pulsed laser beam is incident. The incident position of the laser beam is a heat source Pf. For simplicity, considering the temperature distribution directly under the heat source of an infinitely thick plate, the temperature rise amount ΔT of the position Pr on the back surface directly under the heat source Pf is expressed by the following equation. [Equation <NUM>] <MAT>.

Here, Q is the heat input from the heat source Pf to the annealed target <NUM>, h is the thickness of the annealed target <NUM>, v is the sweep speed of the heat source Pf, and k is the thermal diffusivity of the annealed target <NUM>. For example, the unit of the input heat Q is "W," the unit of the thickness h of the annealed target <NUM> is "cm," the unit of the sweep speed v is "cm/s," and the unit of the thermal diffusivity k is "cm<NUM>/s.

From Equation (<NUM>), it can be seen that the slower the sweep speed v of the heat source Pf, the larger the temperature rise amount ΔT of the point Pr on the back surface. In particular, when the thickness h of the annealed target <NUM> is thin, the increase in the temperature rise amount ΔT becomes remarkable.

<FIG> are graphs showing an example of the calculation result of the temperature distribution in the cross section of the annealed target <NUM>. <FIG> show the temperature distribution in the cross section of the annealed target <NUM> under the conditions of a finite thickness and the back surface being adiabatic. The horizontal axis represents the position of the heat source Pf in the sweep direction. The current position of the heat source Pf is defined as the origin of the horizontal axis, and the movement direction of the heat source is defined as positive. The vertical axis represents the depth from the beam irradiation surface in the unit "µm. " <FIG> show the temperature distribution when the sweep speed v of the heat source Pf is different. <FIG> shows the temperature distribution when the sweep speed v of the heat source Pf is faster than that of <FIG>. The curves in the graph represent isotherms, and the numerical values attached to the curves represent the temperatures in the unit "°C.

It can be seen that when the sweep speed v is slow (<FIG>), the temperature gradient in the thickness direction is gentler than when the sweep speed v is fast (<FIG>). That is, when the sweep speed v is slow, the temperature rise amount ΔT of the back surface is larger than when the sweep speed v is fast. In other words, by increasing the sweep speed v, the temperature rise amount ΔT of the back surface can be reduced.

Next, an annealing method according to an embodiment will be described with reference to <FIG> is a flowchart showing the procedure of the annealing method according to the embodiment. First, the control device <NUM> (<FIG> and <FIG>) controls the laser light source <NUM> and the beam scanner <NUM> to execute the sweep operation of moving the beam spot in the longitudinal direction thereof while causing the pulsed laser beam to be incident on the annealed target <NUM> (step S1).

When one sweep operation is completed, the control device <NUM> controls the movable stage <NUM> to execute the step operation of shifting the annealed target <NUM> in the direction intersecting the longitudinal direction of the beam spot (step S2). The sweep operation of step S1 and the step operation of step S2 are repeated until almost the entire area of the surface of the annealed target <NUM> is annealed (step S3).

Next, with reference to <FIG>, the excellent effects of the embodiment will be described in comparison with a comparative example. <FIG> are schematic views showing the trajectory of the beam spot <NUM> when annealing is performed by the annealing methods according to the embodiment and the comparative example, respectively. The white arrows in <FIG> indicate the direction of movement of the beam spot <NUM> with respect to the annealed target <NUM>.

In both the embodiment and the comparative example, the sweep direction of the beam spot <NUM> is parallel to the x direction. The sweep of the beam spot <NUM> is performed by operating the beam scanner <NUM> (<FIG> and <FIG>). When one sweep is completed, the step operation of shifting the beam spot <NUM> in the y direction is performed. The annealing is performed by alternately repeating the sweep operation and the step operation.

The dimension of the beam spot <NUM> in the x direction is marked as Lx, and the dimension in the y direction is marked as Ly. The distance by which the beam spot <NUM> moves in the x direction during one cycle of the pulsed laser beam is marked as Wx. The distance by which the beam spot <NUM> moves in the y direction in one step operation is marked as Wy. The overlap rate OVx in the x direction and the overlap rate OVy in the y direction are expressed by the following equations. [Equation <NUM>] <MAT>.

In the embodiment, the dimension Lx of the beam spot <NUM> in the x direction is larger than the dimension Ly in the y direction. In the embodiment, the beam spot <NUM> is swept in the longitudinal direction thereof. In contrast, in the comparative example, the dimension Ly in the y direction is larger than the dimension Lx in the x direction. In the comparative example, the beam spot <NUM> is swept in a direction orthogonal to the longitudinal direction thereof.

The case where the dimension Lx in the x direction and the dimension Ly in the y direction of the beam spot <NUM> according to the embodiment are respectively equal to the dimension Ly in the y direction and the dimension Lx in the x direction of the beam spot <NUM> according to the comparative example is considered. That is, the beam spot <NUM> according to the embodiment and the beam spot <NUM> according to the comparative example have the same size and shape. Further, the overlap rates OVx in the x direction and the overlap rates OVy in the y direction are the same in the embodiment and the comparative example.

Under this condition, the numbers of shots required to anneal almost the entire area of the annealed target <NUM> are substantially the same in the embodiment and the comparative example. Further, the pulse energy densities are the same in the embodiment and the comparative example. Therefore, as shown in <FIG>, the reached maximum temperatures of the surface of the annealed target <NUM> are also substantially the same in the embodiment and the comparative example. Therefore, when activation annealing of the annealed target <NUM> is performed, the activation rates are also substantially the same.

Since the sizes of the beam spot <NUM> and the overlap rates OVx in the x direction are the same in the embodiment and the comparative example, the sweep speed of the beam spot <NUM> is faster in the embodiment than in the comparative example. Therefore, as described with reference to <FIG>, the reached maximum temperature of the back surface of the annealed target <NUM> is lower in the embodiment than in the comparative example. When annealing is performed under the condition of similar numbers of shots of the pulsed laser beam and similar activation rates, the present embodiment can suppress the temperature of the back surface of the annealed target <NUM> from rising as compared with the comparative example.

In order to confirm the excellent effects of the present embodiment, the sweep speed of the beam spot <NUM>, the activation rate, and the reached maximum temperature of the back surface of the annealed target <NUM> were obtained by calculation. Hereinafter, the calculation result will be described with reference to <FIG>.

<FIG> is a graph showing the relationship between the sweep speed of the beam spot <NUM> and the activation rate, and the relationship between the sweep speed of the beam spot <NUM> and the reached maximum temperature of the back surface of the annealed target <NUM>. The horizontal axis represents the sweep speed of the beam spot, the vertical axis of the upper graph represents the activation rate in the unit "%," and the vertical axis of the lower graph represents the reached maximum temperature of the back surface of the annealed target <NUM>. The thick solid line and the thin solid line in the graph show the calculation result when annealing was performed by the methods according to the embodiment (<FIG>) and the comparative example (<FIG>), respectively.

In the embodiment, the dimension Lx of the beam spot <NUM> in the x direction was double the dimension Ly in the y direction, and in the comparative example, the dimension Ly of the beam spot <NUM> in the y direction was double the dimension Lx in the x direction. Moreover, the repetition frequencies of the pulse and the pulse energy densities were set to be the same in the embodiment and the comparative example. In both the embodiment and the comparative example, the activation rate decreases as the sweep speed of the beam spot <NUM> increases, and the reached maximum temperature of the back surface of the annealed target <NUM> also decreases.

It can be seen that, when the beam spot <NUM> is swept under the condition that the activation rate is <NUM>%, the reached maximum temperature of the back surface of the annealed target <NUM> is lower when the annealing method according to the embodiment is adopted than when the annealing method according to the comparative example is adopted. As described above, by adopting the annealing method according to the present embodiment, it is possible to achieve the desired activation rate while keeping the reached maximum temperature of the back surface low.

Next, a preferred shape of the beam spot <NUM> (<FIG>) will be described with reference to <FIG> is a graph showing the relationship between the aspect ratio of the beam spot <NUM>, the reached maximum temperature of the back surface of the annealed target <NUM>, and the activation rate. Here, the aspect ratio is defined as the ratio of the dimension Lx in the x direction to the dimension Ly in the y direction of the beam spot <NUM>. The horizontal axis of the graph shown in <FIG> represents the aspect ratio, the left vertical axis represents the reached maximum temperature of the back surface of the annealed target <NUM> in the unit "°C," and the right vertical axis represents the activation rate in the unit "%. " The solid line in the graph shown in <FIG> indicates the reached maximum temperature of the back surface of the annealed target <NUM>, and the broken line indicates the activation rate.

The graph shown in <FIG> was obtained by calculation. As the preconditions for the calculation, the annealed target <NUM> was a silicon wafer having a thickness of <NUM>, and the repetition frequency of the pulse of the pulsed laser beam was set to <NUM>. Phosphorus ions were used as the dopant, and the acceleration energy at the time of ion implantation was set to <NUM> MeV. Even though the aspect ratio of the beam spot <NUM> was changed, the area of the beam spot <NUM> and the overlap rate OVx (Equation (<NUM>)) in the sweep direction were kept constant.

As the aspect ratio increases, the reached maximum temperature of the back surface of the annealed target <NUM> gradually decreases. This is because the sweep speed becomes faster. The activation rate drops sharply at the point when the aspect ratio exceeds about <NUM>. The aspect ratio of the beam spot <NUM> is preferably smaller than the aspect ratio at the point when the activation rate starts to drop sharply. In accordance with the invention, the aspect ratio is <NUM> or less.

In addition, compared with the case where the aspect ratio is <NUM>, that is, the beam spot <NUM> is square, in order to sufficiently achieve the effect of suppressing the rise of the reached maximum temperature of the back surface of the annealed target <NUM>, the aspect ratio is set to <NUM> or more.

Next, the relationship between the sweep speed of the beam spot <NUM>, the reached maximum temperature of the back surface of the annealed target <NUM>, and the activation rate will be described with reference to <FIG> is a graph showing the relationship between the sweep speed when the aspect ratio of the beam spot <NUM> is <NUM>, the reached maximum temperature of the back surface of the annealed target <NUM>, and the activation rate. The horizontal axis of the graph shown in <FIG> represents the sweep speed in the unit [m/s], the left vertical axis represents the reached maximum temperature of the back surface of the annealed target <NUM> in the unit "°C," and the right vertical axis represents the activation rate in the unit "%. " The solid line in the graph shown in <FIG> indicates the reached maximum temperature of the back surface of the annealed target <NUM>, and the broken line indicates the activation rate.

As the sweep speed increases, the reached maximum temperature of the back surface of the annealed target <NUM> decreases, and the activation rate also decreases. In the example shown in <FIG>, when the allowable upper limit of the reached maximum temperature of the back surface of the annealed target <NUM> is <NUM>, the sweep speed has to be <NUM>/s or more. From the calculation result shown in <FIG>, it can be seen that, when the aspect ratio is increased from <NUM> to <NUM>, the reached maximum temperature of the back surface of the annealed target <NUM> decreases by about <NUM>. Also, in the example shown in <FIG>, when the aspect ratio of the beam spot <NUM> is set to about <NUM>, the reached maximum temperature of the back surface is expected to decrease by about <NUM>. Then, even if the sweep speed is reduced to <NUM>/s, the reached maximum temperature of the back surface can be suppressed to about <NUM>.

By optimizing the aspect ratio of the beam spot <NUM> in this way, the selectable range of the sweep speed can be expanded. In the above embodiment, the beam spot <NUM> (<FIG>) is swept in the x direction by operating the beam scanner <NUM> (<FIG>). Therefore, it is possible to increase the sweep speed as compared with the case where the movable stage <NUM> is operated to move the annealed target <NUM> in the x direction for sweeping.

In order to reduce the temperature rise amount ΔT of the back surface of the annealed target <NUM>, it is preferable to increase the sweep speed v as much as possible, as can be seen from Equation (<NUM>). However, if the sweep speed v is increased under the condition that the repetition frequency of the pulse of the pulsed laser beam and the dimension Lx of the beam spot <NUM> (<FIG>) in the x direction are constant, the overlap rate OVx in the x direction may become small or there may be no overlap.

If the dimension Lx of the beam spot <NUM> in the x direction is increased in order to maintain the overlap rate OVx when the sweep speed v is increased, the power density P on the surface of the annealed target <NUM> may be lowered. In order to maintain the reached maximum temperature Ta of the laser irradiation surface under the condition of a low power density P, the pulse width t<NUM> has to be lengthened. If the pulse width t<NUM> is lengthened, the amount of heat transfer conducted in the thickness direction during the period in which the laser pulse is incident increases. As a result, the temperature of the irradiated surface becomes high. Accordingly, the dimension Lx cannot be increased unconditionally.

In order to maintain a sufficient overlap rate OVx without increasing the dimension Lx, the repetition frequency f of the pulse may be increased. For example, in order to suppress an excessive rise of the temperature of the back surface of the semiconductor wafer having a thickness of <NUM> or less, the repetition frequency f of the pulse is preferably set to <NUM> or more, more preferably <NUM> or more.

Claim 1:
An annealing device, comprising:
a laser light source (<NUM>) that outputs a pulsed laser beam;
a beam shaping optical element (<NUM>) that shapes a beam spot (<NUM>) of the pulsed laser beam output from the laser light source on a surface of an annealed target (<NUM>) into an elongated shape
a movement mechanism (<NUM>) that scans the pulsed laser beam and moves the beam spot in a longitudinal direction of the beam spot; and
a control device (<NUM>) that controls the laser light source and the movement mechanism,
wherein the control device performs annealing by performing a sweep operation of moving the beam spot in the longitudinal direction of the beam spot with respect to the annealed target while causing the pulsed laser beam to be incident on the annealed target, characterized in that an aspect ratio of the shaped elongated beam spot (<NUM>) is within a range of <NUM> to <NUM>.