JP5252505B2 - Laser annealing equipment - Google Patents

Description

The present invention relates to a laser annealing apparatus used in such process to recover the crystals to remove the ion-implanted crystal defect activation and wafer in the surface layer of impurities on the back surface of the power device IGBT (Insulated Gate Bipolar Transistor) It is.

In the heat treatment for activating the impurities implanted into the back surface of the power device IGBT (Insulated Gate Bipolar Transistor), the semiconductor substrate is heated by irradiating a laser beam, and the heat treatment is performed by raising the temperature.
In such a heat treatment, it is desirable that the substrate is effectively heated to a certain depth position so that the activation is good. However, since a conventionally used laser has a narrow pulse width (half-value width of a pulse), the heating time is short and it is difficult to perform sufficient activation. For this reason, a method has been proposed in which a plurality of pulses are irradiated continuously to increase the apparent pulse width for activation.

For example, an activation technique has been proposed in which activation of a shallow ion implantation layer and a deep ion implantation layer is performed by a CW (continuous oscillation) laser having two wavelengths, that is, a short wavelength and a long wavelength (see Patent Document 1).
In this technique, a CW type LD (wavelength ≦ 900 nm) and a CW type YAG laser harmonic laser (wavelength ≧ 370 nm) are simultaneously irradiated onto the same substrate surface, and the irradiation time of each laser beam (determined by the beam scanning speed and beam size). ), The temperature distribution in the depth direction is controlled to realize a deep activation of 3 μm level. The shallow part of the impurity implantation layer is activated by a short-wavelength solid-state laser, and the deep part is activated by a semiconductor laser. The non-melting activation is performed with the back surface below the melting point and the surface below 200 ° C.

Further, a technique has been proposed in which a shallow implanted impurity layer is activated in a molten state and a deep implanted impurity layer is activated in a non-molten state using a double pulse laser annealing apparatus (see Non-Patent Documents 1 and 2).
The double pulse laser annealing apparatus according to this technology uses two green pulse lasers to activate a deep pn junction, and provides a delay time between two laser pulses having a short pulse width of 100 ns level. The annealing time is increased by increasing the pulse width in a pseudo manner. By optimizing the delay time, the shallow boron implantation layer and the deep phosphorus implantation layer are activated collectively. The activation depth reaches 1.8 μm, and a high activation rate is obtained. The activation process of the pn junction is as follows. First, the deep phosphorus implanted layer recovers in a solid state, then the crystal recovered phosphorus layer becomes a seed crystal, and the shallow boron implanted layer undergoes liquid phase epitaxial growth. Step by step into the liquid phase.

In addition, a technique has been proposed in which an amorphous layer formed by ion implantation using a combination of two-wavelength lasers is activated in a molten state (see Non-Patent Document 3).
In this technique, two wavelengths of laser having an infrared wavelength of 1060 nm (pulse width of 40 ns) and a green wavelength of 530 nm (pulse width of 30 ns) are simultaneously irradiated, and first, As ions (30 keV, E + 15 / cm ² ) are emitted by a green wavelength pulse laser. Is a melt activation method in which the surface of an amorphous layer (48 nm) implanted with is melted shallowly, and then the absorption of infrared wavelength is enhanced, and the entire amorphous layer is melted with a pulse laser of infrared wavelength. The green pulse laser plays a trigger role in the light absorption of the infrared pulse laser.

International Publication No. 2007/015388

Toshio Kudo and Naoki Wakabayashi, “PN Junction Formation for High-Performance Insulated Gate Bipolar Transistors (IGBT) Double-Pulsed Green Laser Annealing Technique”, Mater Res. Soc. Symp. Proc. , Material research Society, Vol 912, 2006 Toshio Kudo, “Double-pulse control type solid-state laser annealing technology”, application to the backside activation process of high-power transistors, Journal of Laser Processing Society, vol. 14, no. 1. May 2007 D.H. Auston and J.A. Golovchenko, “Dual-wavelength laser annealing”, Appl. Phys. Lett., 34, (1979) 558.

As shown in Patent Document 1, a long wavelength light penetration length can be effectively utilized by using a CW laser having a long wavelength and activating it below the melting point. However, for example, when laser irradiation with a wavelength of 805 nm is performed, the light penetration length L _α = 10.7 μm at room temperature (300 ° K) and L _α = 2.1 μm at 1000 ° K. The light penetration length is further shortened near the melting point. If a rapid temperature rise is accompanied even by irradiation with a long wavelength laser, securing of the activation temperature is hindered up to a target deep region.

In addition, as shown in Non-Patent Documents 1 and 2, even if a delay time is provided for two pulse lasers to increase the pulse width in a pseudo manner, light absorption by phonons occurs as the substrate surface temperature rapidly increases. A sudden increase in the light penetration length is unavoidable. For example, in the case of irradiation of the green laser of wavelength 515 nm, the light penetration depth at room temperature (300 ° K) with L α ₌ 0.79μm, 1000 ° K in L _alpha = 0.16 [mu] m, and the temperature increase of 1000 ° K from room temperature Thus, the light penetration length decreases rapidly to about 1/5. In particular, when the surface melts, the light penetration length becomes extremely shallow at 8 nm, and since the reflectance rapidly increases from 36% to 72% by melting the surface, the laser light is prevented from penetrating deeply, This causes a loss of irradiation energy. Therefore, reaching the melting point in a short time due to a rapid temperature rise hinders securing the activation temperature to the target deep region.

Further, as shown in Non-Patent Document 3, the light penetration length can be increased by increasing the wavelength of the irradiation laser. On the other hand, for example, when laser irradiation with a wavelength of 805 nm is performed, the light penetration L _α = 10.7 μm at room temperature (300 ° K) and L _α = 2.1 μm at 1000 ° K. The light penetration length rapidly decreases to about 1/5 as it rises. However, from the viewpoint of the light penetration length, the long wavelength is advantageous in activating a region deeper than the short wavelength. However, when activated in the molten state, the light penetration length is extremely shallow at 8 nm and the irradiation energy is lost due to the rapid increase in reflectance, so if the temperature rises rapidly and the time to reach the melting point is shortened, It is disadvantageous for securing the activation temperature up to a deep region.

In order to activate low thermal budget (low temperature) on the back surface of the power device IGBT, it is important to secure a light penetration length and a thermal diffusion length covering the target activation region.
The present invention has been made in view of the above circumstances, and provides a laser annealing apparatus capable of effectively performing a heat treatment such as an impurity activation process by sufficiently securing a light penetration length and a thermal diffusion length. The purpose is to do.

That is, the laser annealing apparatus of the present invention is a laser annealing apparatus for heat-treating the substrate surface, and the rising time for reaching 10% to 90% of the maximum intensity of the pulse waveform is 160 ns or more. A laser light source that generates a laser pulse having a ratio of the fall time reaching 90% to 10% of the maximum intensity of the laser beam greater than 1 , and irradiation of the laser pulse onto the substrate by beam shaping of the laser pulse An optical system that enables the scanning, and a moving device that enables scanning of the laser pulse irradiation by relatively moving the substrate and the laser pulse,
When the laser pulse is applied to the surface of the substrate, the laser pulse is not melted and the processing layer remains in an insoluble state, or only the surface layer is melted and the processing layer excluding the surface layer is not dissolved. Irradiation is performed so as to maintain the state .

  A laser pulse with a slow rise time has a slow rise compared to a conventional laser pulse with a sharp rise. Specifically, for example, a substrate having a pulse waveform with a rise time of 160 ns or more that reaches 10% to 90% of the maximum intensity of the pulse waveform is irradiated on the substrate. The rise time is more preferably 180 ns or more, and even more preferably 300 ns or more.

When a laser pulse with a slow rise is irradiated onto the substrate, it is possible to suppress a rapid increase in the temperature of the substrate at the initial irradiation stage and to mitigate a rapid decrease in the light penetration length due to the temperature increase.
In the present invention, the laser light source that outputs a laser pulse with a slow rise is not limited to a specific one. For example, a laser light source equipped with a second harmonic of an LD-pumped Yb: YAG laser is preferable. Can show.

In a laser annealing apparatus according to another aspect of the present invention, the laser pulse has a pulse waveform having a half width of 600 ns or more and is irradiated onto the substrate .

The laser pulse desirably has a long pulse width as well as a slow rise time. Specifically, it is preferable that the substrate has a pulse waveform with a half width of 600 ns or more and the substrate is irradiated, and more preferably 1000 ns or more.
By controlling (lengthening) the pulse width of the pulse laser, it is possible to secure a thermal diffusion length commensurate with the light penetration depth, and effectively realize a low thermal budget process (low temperature activation process).

In a laser annealing apparatus according to another aspect of the present invention, the laser pulse has an energy density at which heat treatment of the substrate is performed in a state where the surface layer of the substrate is not melted or only the surface layer is melted when the substrate is irradiated. It is characterized by that .

In the present invention, the laser irradiation is effective for deep activation regardless of melting and non-melting processes. However, to obtain a more efficient light penetration depth, heat treatment Ru place in the molten state of non-melting or surface only. These states can be obtained by adjusting the energy density of the laser pulse by adjusting means such as an attenuator. A known attenuator can be used.

  As the laser pulse, a very long pulse cut can be used. In this case, it is desirable that the rising time has an asymmetric pulse waveform longer than the falling time for reaching 90% to 10% of the pulse intensity at the cut position. That is, the ratio between the rise time and the fall time is preferably greater than 1, and more preferably 2 or more.

  The laser pulse used in the present invention has a pulse waveform with a slow rise time, but the fall time is not limited. However, from the viewpoint of effective use of pulse energy, it is desirable that the laser pulse applied to the substrate has an asymmetric pulse waveform whose rise time is longer than the fall time.

As explained above, according to the present invention,
1) By slowing the rise time of the laser pulse, the time-averaged light penetration length of the pulse laser to be irradiated can be lengthened, and the sudden decrease of the light penetration length accompanying the temperature rise at the initial stage of irradiation, which has been a problem in the past, can be mitigated. . For this reason, the light penetration length due to the wavelength of the pulse laser can be secured, and the activation can be performed up to a target deep region (for example, 2 μm or more).
2) Regardless of the melting and non-melting activation process, any process is effective for deep activation.
3) It is effective for deep activation by a pulse laser of any wavelength.
4) By controlling the pulse width of the pulse laser, a thermal diffusion length corresponding to the light penetration length can be secured and a low thermal budget process can be realized.
5) The effect of the present invention can also be exhibited when the amount of ion implantation increases to increase light absorption and make deep activation difficult.

It is the schematic which shows the laser annealing apparatus of one Embodiment of this invention. Similarly, it is the cross-sectional schematic which shows an example of power device IGBT which is an example of irradiation object. It is a schematic diagram of a laser pulse waveform in which the rise of the present invention and the conventional example are contrasted. Similarly, it is a figure which shows the pulse waveform of LD excitation solid-state laser. Similarly, it is a figure which shows the time change of the substrate temperature by pulse laser irradiation with a sharp rise and pulse laser with a slow rise. Similarly, it is a figure which shows the effect which the rise time in a pulse waveform has on average light penetration | invasion length. Similarly, it is a figure which shows the carrier concentration distribution profile in an Example.

Hereinafter, an embodiment of the present invention will be described.
As shown in FIG. 1, the laser annealing device 1 includes a processing chamber 2, a moving device 3 that can move in the XY direction, and a base 4 on the upper portion. Yes. On the base 4, a target object placement base 5 is provided. During the laser annealing process, the semiconductor substrate 20 is placed on the object placement table 5. The moving device 3 is driven by a motor (not shown) or the like.

  A laser light source 10 on which a second harmonic of an LD pumped Yb: YAG laser is mounted is installed outside the processing chamber 2. The laser light 15 output from the laser light source 10 is adjusted in energy density by an attenuator 11 as necessary, and is subjected to beam shaping and deflection by an optical system 12 including a lens, a reflection mirror, a homogenizer, and the like. The semiconductor substrate 20 in 2 is irradiated.

The pulsed laser beam 15 output from the laser light source 10 has a pulse waveform with a slow rise time, and preferably reaches the rise time (from 10% to 90% of the maximum intensity of the pulse waveform). (Time) is 160 ns or more, and the half-value width is 600 ns or more. When the laser beam is irradiated to the semiconductor substrate 20, the impurity layer can be maintained in an unmelted state and adjusted to an energy density that can bring the surface layer to a high temperature up to the vicinity of the melting point or an energy density that only melts the surface layer. It is desirable. As described above, the laser beam 15 is shaped into, for example, a line beam shape by the optical system 12.
The state in which only the surface layer is melted is a state in which only the surface layer is partially melted in the latter half of the irradiation period of the laser beam, not the region that needs to be activated. It penetrates into and effectively heats up to the depth region.

FIG. 2A shows an example of a cross-sectional structure of an FS (Field Stop) IGBT that can be processed in the present invention. A p-type base region 23 in which boron is implanted is formed on the surface side of the semiconductor substrate 20, and an n ⁺ -type emitter region 24 in which phosphorus is implanted is formed on a part of the surface side of the p-type base region 23. Yes. A p ⁺ -type collector layer 22 into which boron is implanted is formed in the surface layer on the back side of the semiconductor substrate 20. In a region deeper than the collector layer 22, an n ⁺ type buffer layer 21 in which phosphorus is implanted so as to be in contact with the collector layer 22 is formed, and an n − type substrate 25 is located inside the n ⁺ type buffer layer 21. In the figure, 26 is a collector electrode, 27 is an emitter electrode, 29 is a gate oxide film, and 28 is a gate electrode.

  As shown in FIG. 2B, the semiconductor impurity layer is repeatedly irradiated with pulsed laser light from the back side before the collector electrode 26 is formed, so that the thickness becomes 2 μm or more. The impurity layer is activated over this. The overlapping rate (overlap rate) of laser light can be appropriately selected as necessary. At this time, by controlling the moving speed of the base 4 by the moving device 3, the semiconductor substrate 20 can be scanned with the laser light 15 at a predetermined speed.

  Next, how the present invention can realize the activation of a target deep region by effectively utilizing the light penetration length originating from the wavelength of the pulse laser will be described below.

FIG. 3 schematically shows pulse waveforms of the conventional and the pulse lasers of the present invention. The rise time from 10% to 90% of the maximum intensity of the pulse waveform is defined as a rise time t _A , and the fall time from 90% to 10% of the maximum intensity is defined as a fall time t _B. Conventional pulsed laser has a short rise time t _A2, the fall time t _B2 is long asymmetrical pulse waveform. On the other hand, the pulse laser of the present invention has a long rise time t _A1 , preferably a short fall time t _B1 , and has an asymmetric pulse waveform opposite to the conventional one. When the rise times are compared, the present invention is characterized by being much longer than the conventional example.

FIG. 4 shows a specific example of the pulse waveform of the second harmonic of the conventional example and the LD-pumped solid state laser according to the present invention. The pulse of the present invention in this example is obtained by cutting a long pulse, and has an inflection point at the cut position. Conventional examples include Nd: YLF laser and Nd: YAG second harmonic. As the former pulse laser, as shown in FIG. 4, a laser having a rise time of 42 ns and a fall time of 120 ns with respect to a pulse width of 83 ns can be mentioned. Other examples include those having a rise time of 48 ns and a fall time of 365 ns with respect to a pulse width of 171 ns. An example of the latter pulse laser is one having a rise time of 45 ns and a fall time of 200 ns with respect to a pulse width of 105 ns.
As an example of the present invention, there is a second harmonic of a Yb: YAG laser, and as shown in FIG. 4, a laser having a rise time of 308 ns and a fall time of 92 ns with respect to a pulse width of 1200 ns is exemplified. Other examples include those having a rise time of 182 ns and a fall time of 85 ns for a pulse width of 301 ns, and those having a rise time of 250 ns and a fall time of 100 ns for a pulse width of 504 ns.
The rise time of the second harmonic pulse laser of the conventional LD-pumped solid state laser is less than 50 ns, while the rise time of the example of the present invention is greater than 180 ns. The rise time of the example of the present invention is about 1 digit longer than the conventional example.

  In the present invention, as a degree of asymmetry of the pulse waveform, a value obtained by dividing the symmetry of the pulse waveform by the rising time divided by the falling time can be used as a guide. When the symmetry of the pulse waveform is less than 1, it means that the rise is steep and the fall is slow. On the contrary, when it is greater than 1, it means that the rise is slow and the fall is steep. The second harmonics of Nd: YLF and Nd: YAG in the conventional example have a pulse waveform symmetry much smaller than 1. The symmetry of the pulse waveform of the second harmonic of Yb: YAG of the present invention is greater than 2.

  FIG. 5 shows a schematic diagram of the rise of the wafer surface temperature when irradiating a silicon wafer as a substrate using the laser pulse. The arrival time from the room temperature to melting can be introduced as a measure of the rise time of the pulse waveform in the pulse laser. In the conventional laser pulse, the substrate temperature rapidly increases and reaches the melting point early. On the other hand, in the laser pulse of the present invention, the increase in the substrate temperature is slow and the time until the melting point is reached is also long. In the heat treatment of the present invention, the substrate surface may be processed without reaching the melting point, or may be melted during the processing. Even when melting occurs, the time to reach this can be lengthened, and the penetration length of the laser beam can be sufficiently secured.

Next, FIG. 6 is a schematic diagram of the temperature change of the light penetration depth with respect to the silicon wafer of the conventional and the pulse laser of the present invention. Light penetration depth L _alpha is defined as the reciprocal of the linear absorption coefficient alpha. The linear absorption coefficient of the silicon wafer is expressed by equation (1) depending on the temperature. However, ₀ alpha in the _formula, T _R is a parameter determined at the time of fitting the experimental data.

Equation (1) is in good agreement with the experimental results in the temperature region 300K ≦ T ≦ 1000K. In the figure, L _α (T _RM ) represents the light penetration length at room temperature, and L _α (T _m ) represents the light penetration length at the melting point.

In order to examine the influence of the rise time of the light penetration length pulse waveform, a time average of the light penetration length is introduced. FIG. 6 shows the time averages L _α1 and L _α2 of the penetration length when irradiated with the conventional and the pulse laser of the present invention having a contrasting rise time. Light penetration depth of time-averaged ^* L _alpha is, L [alpha-t time chart _{0 to t} 1, by integrating to 0 to t _2, can be calculated from the rectangular therewith same area shown in FIG. As the rise time becomes longer as in the pulse laser of the present invention, the area calculated from the Lα-t graph becomes larger, and the average light penetration depth becomes deeper than the conventional example with a short rise time. Therefore, deep activation can be effectively performed by increasing the rise time of the pulse waveform of the pulse laser.

Example 1
Examples of the present invention will be described below.
An LD-excited solid laser second harmonic was used as the laser pulse, and Yb: YAG, Nd: YLF, or Nd: YAG was used as the laser light source. The pulse width, rise time, and fall time of the laser pulse output from each laser light source were adjusted as shown in Table 1 and irradiated. For the second harmonic of the Yb: YAG laser, a laser pulse having a long pulse width (2000 ns or longer) was originally cut out with an EO switch to form a pulse having a long rise time and a short fall time. Regarding the second harmonic laser of Nd: YEF, the pulse width can be increased by increasing the oscillation signal frequency.
The activation depth in the substrate is determined from the SRP depth profile, that is, the depth concentration distribution of carriers. For example, it can be determined how deeply the bottom of the field stop layer into which phosphorus is implanted is distributed. The results are shown in Table 1.

  As is clear from the table, the substrate irradiated by the present invention was effectively activated to a depth exceeding 1 μm and reaching 3 μm. On the other hand, if it deviates from the conditions of the present invention, the activation depth did not reach 1 μm, and the laser beam penetration depth was not sufficiently obtained.

(Example 2)
FIG. 7 shows an actual irradiation result with a pulse width of 1200 ns, a rise time of 308 ns, and a fall time of 92 ns on a substrate having a thickness of 150 μm. A pulse width of 1200 ns and an energy density (4.7 J / cm ² ) are the deepest and most active. The activation rates of the boron layer and phosphorus layer are 89% and 62%, and the activation depth reaches 2.7 μm. The activation rate is obtained by integrating the SIMS and SRP depth profiles and dividing the number of SRP carriers by the SIMS impurity amount and expressed as a percentage. Although there is no other pulse width example shorter than 1200 ns, there is a problem in limiting the pulse width. However, the data of Innovabent's paper, melting activation with pulse widths of 325 ns and 600 ns, and an activation depth of 1 μm Achieved. Therefore, since the pulse width must be larger than 600 ns, the range of the pulse width is preferably limited.

DESCRIPTION OF SYMBOLS 1 Laser annealing apparatus 2 Processing chamber 3 Moving apparatus 4 Base 5 To-be-processed object arrangement | positioning base 10 Laser light source 11 Attenuator 12 Optical system 15 Laser light 20 Semiconductor substrate

Claims (6)

A laser annealing apparatus for heat-treating a substrate surface,
The rise time to reach 10% to 90% of the maximum intensity of the pulse waveform is 160 ns or more, and the ratio between the rise time and the fall time to reach 90% to 10% of the maximum intensity of the pulse waveform is 1 A laser light source that generates a large laser pulse, an optical system that performs beam shaping of the laser pulse to enable irradiation of the laser pulse to the substrate, and a relative movement of the substrate and the laser pulse. A moving device that enables scanning of the laser pulse irradiation,
When the laser pulse is applied to the surface of the substrate, the laser pulse is not melted and the processing layer remains in an insoluble state, or only the surface layer is melted and the processing layer excluding the surface layer is not dissolved. Irradiation is performed so as to maintain the state . 2. The laser annealing apparatus according to claim 1 , wherein the laser pulse has a pulse waveform with a half width of 600 ns or more and is applied to the substrate. 3. The energy density according to claim 1, wherein the laser pulse has an energy density at which heat treatment of the substrate is performed in a state where the surface layer of the substrate is not melted or only the surface layer is melted when the laser pulse is irradiated. Laser annealing equipment. The laser annealing apparatus according to any one of claims 1 to 3, characterized in that it comprises adjustable means energy density of the laser pulse. The laser pulse is formed by cutting a long pulse, and has an asymmetric pulse waveform in which the rise time is longer than the fall time reaching 90% to 10% of the pulse intensity at the cut position. the laser annealing apparatus according to any one of claims 1 to 4, characterized in that it is irradiated to the substrate Te. It said laser light source, LD pumped Yb: laser annealing apparatus according to any one of claims 1 to 5, characterized in that mounting the second harmonic of the YAG laser.

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