JP6465679B2 - Laser annealing method and laser annealing apparatus - Google Patents

Description

  The present invention relates to a laser annealing method and a laser annealing apparatus.

  A laser annealing method for activating a dopant by making a laser beam incident on a semiconductor substrate containing the ion-implanted dopant is known. Patent Document 1 below discloses a laser annealing method applied to the formation of an insulated gate bipolar transistor (IGBT). With this laser annealing method, it is possible to efficiently activate dopants in both the high concentration layer (collector layer) in the surface layer portion and the low concentration layer (buffer layer) disposed deeper than the high concentration layer. It is.

JP 2013-258288 A

  In the IGBT, an element structure such as a gate and an emitter is disposed on the front surface of the semiconductor substrate, and a collector is disposed on the back surface. When laser annealing is performed to form a collector on the back side surface, element structures such as a gate and an emitter are already formed on the front side surface. The front side surface has irregularities reflecting the element structure. At the time of laser annealing, a laser beam is incident on the back side surface with the front side surface adsorbed to the adsorption table.

  The inventors of the present application conducted various evaluation experiments using the laser annealing method described above. As a result, it was found that the laser processing quality may be uneven with respect to the in-plane direction of the substrate. An object of the present invention is to provide a laser annealing method and a laser annealing apparatus that are less likely to cause unevenness in processing quality.

According to one aspect of the invention,
Preparing a semiconductor wafer having an uneven surface on the front side;
A step of filling the front surface of the concave portion with a filler so as not to generate a cavity ;
Holding the semiconductor wafer filled with the filler on the annealing table in a posture in which the surface on the front side faces the annealing table;
There is provided a laser annealing method including a step of performing annealing by making a laser beam incident on a back surface opposite to the front surface of the semiconductor wafer held on the anneal table.

According to another aspect of the invention,
Affixing table;
In a state where a semiconductor wafer having an uneven surface is placed on the sticking table, the adhesive layer of the adhesive member with an adhesive layer is opposed to the front surface, and the surface on the front side with respect to the adhesive member A pressure mechanism for applying the pressure-sensitive adhesive member to the front side surface while applying pressure so that no cavity is generated in the concave portion of the surface of the semiconductor wafer in the direction of tightly contacting the surface ,
An annealing table for holding the semiconductor wafer to which the adhesive member is bonded by the pasting table;
There is provided a laser annealing apparatus having a laser optical system for making a laser beam incident on the semiconductor wafer held on the annealing table.

  By filling the concave portion with the filler, a cavity is hardly generated between the semiconductor wafer and the annealing table. If a cavity is formed between the semiconductor wafer and the annealing table, heat transfer from the semiconductor wafer to the annealing table is hindered. When the heat transfer is hindered, the temperature of the semiconductor wafer varies in the in-plane direction. When the above-described configuration is employed, the generation of cavities is suppressed, so that temperature variations are reduced. Thereby, the nonuniformity of processing quality can be suppressed.

FIG. 1 is a schematic diagram of a laser annealing apparatus according to an embodiment. 2A to 2C are cross-sectional views of the sticking table, the semiconductor wafer, and the adhesive tape in the step of attaching the adhesive tape to the semiconductor wafer. 3A to 3C are cross-sectional views of the semiconductor wafer and the adhesive tape in the middle of the laser annealing method according to the embodiment. 3D to 3E are cross-sectional views of the semiconductor wafer and the adhesive tape in the middle of the laser annealing method according to the embodiment. 4A and 4B are cross-sectional views of a semiconductor wafer and an annealing table when laser annealing is performed by a method according to a comparative example and an example, respectively. FIG. 5 is a cross-sectional view of a semiconductor wafer subjected to laser annealing by a method according to a comparative example. 6A and 6B are cross-sectional views of the simulation model, and FIG. 6C is a graph showing a simulation result. FIG. 7A is a cross-sectional view of a simulation model, and FIG. 7B is a graph showing a simulation result. FIG. 8A is a cross-sectional view of an IGBT shown as an example of a semiconductor device manufactured by the method according to the embodiment, and FIG. 8B is a cross-sectional view of a semiconductor wafer at a stage where laser annealing is performed.

FIG. 1 shows a schematic diagram of a laser annealing apparatus according to an embodiment. The semiconductor laser oscillator 21 outputs a first pulse laser beam having a wavelength of 808 nm, for example. A semiconductor laser oscillator that outputs a pulse laser beam having a wavelength of 690 nm or more and 950 nm or less may be used. The solid-state laser oscillator 31 outputs a second pulse laser beam in the green wavelength region. For the solid-state laser oscillator 31, for example, a Q-switched laser such as Nd: YAG, Nd: YLF, Nd: YVO ₄ or the like that outputs the second harmonic is used.

  The first pulse laser beam output from the semiconductor laser oscillator 21 and the second pulse laser beam output from the solid-state laser oscillator 31 are incident on the semiconductor wafer 50 that is an object to be annealed via the propagation optical system 27. To do. The first pulse laser beam and the second pulse laser beam are incident on the same region of the surface of the semiconductor wafer 50.

  Next, the configuration and operation of the propagation optical system 27 will be described. The first pulse laser beam output from the semiconductor laser oscillator 21 is incident on the semiconductor wafer 50 via the attenuator 22, the beam expander 23, the homogenizer 24, the dichroic mirror 25, and the condenser lens 26.

  The second pulse laser beam output from the solid-state laser oscillator 31 passes through the variable attenuator 32, the beam expander 33, the homogenizer 34, the bending mirror 35, the dichroic mirror 25, and the condenser lens 26 to the semiconductor wafer 50. Incident.

  The attenuator 22 attenuates the intensity of the first pulse laser beam. The variable attenuator 32 attenuates the intensity of the second pulse laser beam. The attenuation amount of the second pulse laser beam by the variable attenuator 32 is variable. The beam expanders 23 and 33 collimate the incident pulse laser beam and expand the beam diameter. The homogenizers 24 and 34 and the condenser lens 26 shape the beam cross section on the surface of the semiconductor wafer 50 into a long shape, and uniformize the light intensity distribution in the beam cross section. The first pulse laser beam and the second pulse laser beam are incident on substantially the same long region on the surface of the semiconductor wafer 50.

  The semiconductor wafer 50 is held on the annealing table 40. The annealing table 40 sucks the semiconductor wafer 50. For example, a vacuum chuck is used as the suction mechanism. An xyz orthogonal coordinate system is defined in which a plane parallel to the surface of the semiconductor wafer 50 is defined as an xy plane, and a normal direction of the surface of the semiconductor wafer 50 is defined as a z direction. The moving mechanism 41 receives the control from the control device 10 and moves the annealing table 40 in the x direction and the y direction. The control device 10 controls the suction mechanism of the semiconductor laser oscillator 21, the solid state laser oscillator 31, the variable attenuator 32, and the annealing table 40 in addition to the moving mechanism 41.

  The laser annealing apparatus further includes a pasting table 45. In a state where the semiconductor wafer 50 is held on the sticking table 45, the adhesive tape 51 is attached to the surface (surface on the front side) where the element structure of the semiconductor wafer 50 is formed. The pressurizing mechanism 46 applies a pressure in a direction in close contact with the semiconductor wafer 50 to the adhesive tape 51.

  The semiconductor wafer 50 to which the adhesive tape 51 is bonded is transferred from the sticking table 45 to the annealing table 40 by the transfer mechanism 49. For the transfer mechanism 49, for example, a robot arm that can be expanded, contracted, moved up and down, and rotated is used.

  With reference to FIGS. 2A to 2C, the process of attaching the adhesive tape 51 to the semiconductor wafer 50 will be described.

  As shown in FIG. 2A, the semiconductor wafer 50 is placed on the sticking table 45 with the element forming surface (front surface 501) of the semiconductor wafer 50 facing upward. For example, the adhesive tape 51 is prepared in a state of being wound around the feeding roll 48. By feeding the adhesive tape 51 from the feeding roll 48, the adhesive layer of the adhesive tape 51 with the adhesive layer is made to face the front surface 501 of the semiconductor wafer 50.

  In this state, the pressure-sensitive adhesive mechanism 51 applies the pressure-sensitive adhesive tape 51 to the front surface 501 of the semiconductor wafer 50 while applying a pressure in a direction in close contact with the semiconductor wafer 50 to the pressure-sensitive adhesive tape 51. For example, a pressure roller is used as the pressure mechanism 46. The pressure-sensitive adhesive tape 51 is attached to the semiconductor wafer 50 by sandwiching the pressure-sensitive adhesive tape 51 between the pressure roller and the semiconductor wafer 50 and moving the pressure roller against the semiconductor wafer 50.

  FIG. 2B shows a state where the adhesive tape 51 is attached to the entire front surface of the semiconductor wafer 50.

  As shown in FIG. 2C, the adhesive tape 51 is cut by the cutting machine 47. Thereby, the adhesive tape 51 affixed to the semiconductor wafer 50 is cut off from the portion wound around the feeding roll 48 (FIG. 2A).

  With reference to FIG. 3A to FIG. 3E, the laser annealing method will be described by paying attention to the bonded portion between the semiconductor wafer 50 and the adhesive tape 51.

  As shown in FIG. 3A, the semiconductor wafer 50 is held on the sticking table 45 in a posture in which the front surface 501 is directed upward. Concavities and convexities are formed on the front surface 501. A recess 503 appears on the surface 501 on the front side. The back surface 502 is substantially flat. A resin protective film 55 is formed on the front surface 501. The surface of the protective film 55 has a shape reflecting the unevenness of the front surface 501 of the semiconductor wafer 50. The adhesive layer 512 of the adhesive tape 51 faces the surface 501 on the front side of the semiconductor wafer 50. As the base material of the adhesive layer 512, for example, acrylic is used.

  FIG. 3B shows a cross-sectional view of the semiconductor wafer 50 with the adhesive tape 51 attached. When the pressure-sensitive adhesive tape 51 is applied, the pressure mechanism 46 (FIG. 2A) presses the pressure-sensitive adhesive tape 51 against the semiconductor wafer 50, so that the adhesive of the adhesive layer 512 is filled in the concave portion 503 of the front surface 501. The adhesive plays a role as a filler. The outer surface of the adhesive tape 51 remains flat.

  As shown in FIG. 3C, the semiconductor wafer 50 to which the adhesive tape 51 is attached is transferred from the attachment table 45 (FIG. 3B) to the annealing table 40. The semiconductor wafer 50 is held on the annealing table 40 in a posture in which the front surface 501 faces the annealing table 40. The outer surface of the adhesive tape 51 is in close contact with the annealing table 40 in almost the entire area.

  As shown in FIG. 3D, the pulse laser beam 38 is incident on the back surface 502 of the semiconductor wafer 50 while moving the annealing table 40 in the x direction or the y direction (FIG. 1). Thereby, annealing of the semiconductor wafer 50 is performed.

  As shown in FIG. 3E, the semiconductor wafer 50 is taken out from the annealing table 40, and the adhesive tape 51 is peeled from the semiconductor wafer 50. When the semiconductor wafer 50 has sufficient mechanical strength, the adhesive tape 51 can be peeled from the semiconductor wafer 50 in a state where the semiconductor wafer 50 is adsorbed on the adsorption table. When the mechanical strength of the semiconductor wafer 50 is not sufficient, it is preferable to use the adhesive tape 51 having an ultraviolet curable adhesive layer. The adhesive tape 51 can be easily peeled from the semiconductor wafer 50 by irradiating the adhesive layer with ultraviolet rays.

  Next, with reference to FIGS. 4A to 7B, the excellent effect of the laser annealing method according to the above embodiment will be described.

  FIG. 4A shows a cross-sectional view of the semiconductor wafer 50 and the annealing table 40 when laser annealing is performed by the method according to the comparative example. A recess 503 is formed on the front surface 501 of the semiconductor wafer 50. Although the adhesive tape 51 is affixed on the surface 501 on the front side, the concave portion 503 is not filled with adhesive, and the concave portion 503 remains hollow.

  When a laser beam is incident on the back surface 502 of the semiconductor wafer 50, the surface layer portion of the back surface 502 is heated. As a result, a heat flow 57 from the back surface 502 toward the front surface 501 is generated. In a portion where the recess 503 is not provided, heat is transferred from the semiconductor wafer 50 to the annealing table 40 via the adhesive tape 51. On the other hand, air exists in the heat transfer path in the portion where the recess 503 is provided. For this reason, the heat transfer rate of the portion where the recess 503 is provided is lower than the heat transfer rate of the portion where the recess 503 is not provided. For this reason, the temperature of the bottom surface of the recess 503 tends to be high.

  FIG. 5 shows an example of a cross-sectional view of a semiconductor wafer 50 that has been subjected to laser annealing by a method according to a comparative example. A protective film 55 made of resin is formed on the surface 501 on the front side. If the temperature of the bottom of the recess 503 rises excessively during laser annealing, bubbles 551 or altered portions 552 may be generated in the protective film 55.

  FIG. 4B shows a cross-sectional view of the semiconductor wafer 50 and the annealing table 40 when laser annealing is performed by the method according to the embodiment. In the method according to the embodiment, the adhesive of the adhesive tape 51 is filled in the recess 503. The adhesive has a higher thermal conductivity than air. The heat transfer coefficient at the interface between the semiconductor wafer 50 and the adhesive is higher than the heat transfer coefficient at the interface between the semiconductor wafer 50 and air. For this reason, heat is transferred from the semiconductor wafer 50 to the annealing table 40 via the adhesive tape 51 even in the portion where the recess 503 is provided. Thereby, an excessive increase in the temperature of the bottom surface of the recess 503 can be suppressed. Since the temperature rise is suppressed, the deterioration of the protective film 55 (FIG. 3E) can be suppressed, and the generation of bubbles can also be suppressed.

  With reference to FIGS. 6A to 6C, simulation results of temperature rises in the portion where the recess 503 (FIGS. 4A and 4B) is provided and the portion where the recess 503 is not provided will be described. 6A and 6B show cross-sectional views of the simulation model.

The simulation model A shown in FIG. 6A includes a semiconductor wafer having a thickness of 100 μm and an air layer having a thickness of 60 μm disposed below the semiconductor wafer. The simulation model B shown in FIG. 6B includes a semiconductor wafer having a thickness of 100 μm and a resin layer having a thickness of 60 μm disposed below the semiconductor wafer. The simulation was performed under the condition that SiO ₂ (glass) was disposed under the air layer (FIG. 6A) and the resin layer (FIG. 6B). This condition corresponds to a state where a glass plate with an adhesive layer is attached to a semiconductor wafer.

  The simulation model A corresponds to a portion where the recess 503 (FIG. 4A) of the semiconductor wafer 50 is provided, and the simulation model B corresponds to a portion where the recess 503 (FIG. 4A) is not provided. The laser beam was incident on the upper surface of the semiconductor wafer of the simulation model A and the simulation model B, and the maximum temperature reached immediately after the third shot was incident was calculated by simulation.

  FIG. 6C shows the simulation result. The horizontal axis represents the depth from the upper surface of the semiconductor wafer in the unit of “μm”, and the vertical axis represents the maximum temperature reached in the unit of “K”. A circle symbol and a square symbol in the graph of FIG. 6C indicate temperatures in the semiconductor wafer of the simulation model A and the simulation model B, respectively.

  It can be seen that the maximum temperature reached by simulation model A is higher than the maximum temperature reached by simulation model B. This is because the heat transfer efficiency is lowered due to the air layer disposed at the interface between the semiconductor wafer and the annealing table.

  In general, the collector layer of the IGBT reaches about 1 μm to 5 μm from the surface of the semiconductor wafer. From the simulation results shown in FIG. 6C, it can be seen that there is a difference in the maximum temperature reached even in a region where the depth of the semiconductor wafer is less than 5 μm. The difference in the maximum temperature reached in the region shallower than the depth of 5 μm of the semiconductor wafer between the simulation model A and the simulation model B is that in the comparative example shown in FIG. 4A, the region where the recess 503 is provided and the recess 503 This suggests that the activation rate of the dopant differs in the region where no is provided.

  In the embodiment, since the recess 503 (FIG. 4B) is filled with the adhesive, no cavity is generated. For this reason, the maximum temperature reached is not easily affected by the recess 503. Thereby, the activation rate of the dopant can be made uniform in the in-plane direction.

  With reference to FIG. 7A and FIG. 7B, the simulation result of the highest temperature achieved when the thickness of the resin layer disposed under the semiconductor wafer is changed will be described.

FIG. 7A shows a cross-sectional view of the simulation model. A resin layer having a thickness of 10 μm to 200 μm is disposed under a semiconductor wafer having a thickness of 100 μm. This resin layer corresponds to the adhesive tape 51 with the adhesive layer shown in FIG. 3C. The simulation was performed under the condition that SiO ₂ (glass) was disposed under the resin layer.

  FIG. 7B shows the simulation result. The horizontal axis represents the thickness of the resin layer in the unit “μm”, and the vertical axis represents the maximum temperature reached in the unit “K”. The circle symbol, square symbol, triangle symbol, pentagon symbol, hexagon symbol, and star symbol in the graph of FIG. 7B have a depth from the top surface of the semiconductor wafer of 0 μm, 1 μm, 2 μm, 3 μm, 5 μm, and 10 μm, respectively. The highest temperature reached at the position of. In the cross-sectional view shown in FIG. 3C, a portion where the recess 503 is provided corresponds to a simulation model with a thick resin layer, and a portion where no recess 503 is provided corresponds to a simulation model with a thin resin layer.

  At any depth, the maximum temperature reached decreases as the resin layer becomes thicker. This is presumably because as the resin layer becomes thicker, the heat capacity of the resin layer increases and more heat is absorbed from the semiconductor wafer into the resin layer.

  The gradient of the change in the maximum temperature achieved with respect to the thickness change of the resin layer becomes more gradual as the resin layer becomes thicker. This means that when the pressure-sensitive adhesive tape 51 becomes thick, the maximum temperature reached is less affected by the presence or absence of the recess 503 (FIG. 3C). In order to reduce the variation in temperature in the in-plane direction, the thickness of the adhesive tape 51 is preferably 100 μm or more. When the thickness of the adhesive tape 51 exceeds 200 μm, even if the adhesive tape 51 is thickened, there is no significant change in the effect of reducing the temperature variation. Therefore, the thickness of the adhesive tape 51 is preferably 200 μm or less.

  FIG. 8A shows a cross-sectional view of an IGBT as an example of a semiconductor device manufactured by the method according to the embodiment. An emitter and a gate are formed on the front surface 501 of the semiconductor wafer 50 made of n-type silicon. A backside surface 502 collector is formed. As the semiconductor wafer 50, a silicon single crystal wafer is usually used. Structures such as an emitter and a gate formed on the surface 501 on the front side are manufactured in the same process as a general MOSFET manufacturing process. For example, as shown in FIG. 8A, a p-type base region 61, an n-type emitter region 62, a gate electrode 63, a gate insulating film 64, and an emitter electrode 65 are disposed on the surface layer portion of the front surface 501 of the semiconductor wafer 50. Is done. Current on / off control can be performed by the voltage between the gate and the emitter.

  A p-type collector layer 67 and a low-concentration n-type buffer layer 66 are formed on the surface layer portion of the back surface 502 of the semiconductor wafer 50. The buffer layer 66 is disposed at a position deeper than the collector layer 67. The collector layer 67 and the buffer layer 66 are formed by implanting boron and phosphorus, for example, as dopants and performing activation annealing. The laser annealing apparatus shown in FIG. 1 is applied to this activation annealing. A collector electrode 68 is formed on the surface of the collector layer 67 after the activation annealing.

  The depth from the back surface 502 to the interface between the collector layer 67 and the buffer layer 66 is, for example, about 0.3 μm. The depth from the back surface 502 to the deepest position of the buffer layer 66 is, for example, in the range of 1 μm to 5 μm.

  FIG. 8B shows a cross-sectional view of the semiconductor wafer 50 at a stage where laser annealing is performed. Boron is ion-implanted into a relatively shallow portion 67 a near the surface 502 of the back surface 502 of the semiconductor wafer 50. Phosphorus ions are implanted into the relatively deep portion 66a. Boron in the shallow portion 67a and phosphorus in the deep portion 66a are not activated.

  A recess 503 is formed on the front surface 501 of the semiconductor wafer 50. The recess 503 is filled with the adhesive of the adhesive layer 512 of the adhesive tape 51. The adhesive tape 51 is adsorbed on the annealing table 40. Laser annealing is performed by causing the pulse laser beam 38 to enter the back surface 502 of the semiconductor wafer 50. By this laser annealing, boron in the shallow portion 67a and phosphorus in the deep portion 66a are activated.

  At the time of laser annealing, since the adhesive of the adhesive layer 512 is filled in the concave portion 503 of the front surface 501, uniform annealing can be performed in the wafer in-plane direction.

  8A and 8B, the laser annealing apparatus according to the embodiment is applied to the manufacture of the IGBT. However, the laser annealing apparatus according to the embodiment can be applied to the manufacture of other semiconductor elements, for example, an element for an image sensor. It is.

  In the above embodiment, the adhesive layer of the pressure-sensitive adhesive tape 51 (FIG. 3A) is used as the filler that fills the recesses 503 of the semiconductor wafer 50, but an adhesive layer of a pressure-sensitive adhesive member other than the pressure-sensitive adhesive tape 51 may be used. . For example, a glass plate with an adhesive layer can be used as the adhesive member. When a glass plate with an adhesive layer is used as the adhesive member, a press device that applies a uniform load to the plane can be used as the pressurizing mechanism 46 (FIGS. 1 and 2A).

  Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

DESCRIPTION OF SYMBOLS 10 Control apparatus 21 Semiconductor laser oscillator 22 Attenuator 23 Beam expander 24 Homogenizer 25 Dichroic mirror 26 Condensing lens 27 Propagation optical system 31 Solid laser oscillator 32 Variable attenuator 33 Beam expander 34 Homogenizer 35 Bending mirror 38 Pulse laser beam 40 Annealing table 41 Moving mechanism 45 Adhering table 46 Pressurizing mechanism 47 Cutting machine 48 Feeding roll 49 Transfer mechanism 50 Semiconductor wafer 501 Front side surface 502 Back side surface 503 Recess 51 Adhesive tape 512 Adhesive layer 55 Protective film 551 Bubble 552 Altered portion 57 Heat flow 61 p Type base region 62 n type emitter region 63 gate electrode 64 gate insulating film 65 emitter electrode 66 n type buffer layer 66a deep portion 67 p type collector layer 67a shallow portion Min 68 Collector electrode

Claims (5)

Preparing a semiconductor wafer having an uneven surface on the front side;
A step of filling the front surface of the concave portion with a filler so as not to generate a cavity ;
Holding the semiconductor wafer filled with the filler on the annealing table in a posture in which the surface on the front side faces the annealing table;
And a step of annealing the semiconductor wafer held on the annealing table by causing a laser beam to be incident on a back surface opposite to the front surface.   The filler is an adhesive for an adhesive member with an adhesive layer. In the step of filling the filler, the adhesive is applied while applying pressure in a direction in close contact with the surface on the front side to the adhesive member. The laser annealing method according to claim 1, wherein a member is attached to the front side surface.   The laser annealing method according to claim 1, wherein a protective film made of a resin is formed on the front side surface, and the surface of the protective film has a shape reflecting the irregularities of the front side surface. Affixing table;
In a state where a semiconductor wafer having an uneven surface is placed on the sticking table, the adhesive layer of the adhesive member with an adhesive layer is opposed to the front surface, and the surface on the front side with respect to the adhesive member A pressure mechanism for applying the pressure-sensitive adhesive member to the front side surface while applying pressure so that no cavity is generated in the concave portion of the surface of the semiconductor wafer in the direction of tightly contacting the surface ,
An annealing table for holding the semiconductor wafer to which the adhesive member is bonded by the pasting table;
A laser annealing apparatus comprising: a laser optical system that causes a laser beam to be incident on the semiconductor wafer held on the annealing table. The laser annealing apparatus according to claim 4, further comprising a transfer mechanism that transfers the semiconductor wafer held on the sticking table to the annealing table.

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