CN112216603A

CN112216603A - Laser processing method for wafer and semiconductor device

Info

Publication number: CN112216603A
Application number: CN201910620259.9A
Authority: CN
Inventors: 陈伟钿; 周永昌; 张永杰
Original assignee: Alpha Power Solutions Ltd
Current assignee: Alpha Power Solutions Ltd
Priority date: 2019-07-10
Filing date: 2019-07-10
Publication date: 2021-01-12

Abstract

The invention discloses a laser processing method for a wafer and a semiconductor device. The laser processing method for the wafer includes: providing a wafer comprising a semiconductor material and having a first side and a second side opposite the first side; thinning the second side to generate a damaged area on the thinned second side; irradiating the damaged area with a laser beam so that the semiconductor material of the damaged area is melted to form a melted area; and cooling the melted region to recrystallize, wherein a cooling rate of the melted region is adjusted by controlling a removal mode of the laser beam, thereby controlling a size of a crystal grain formed by recrystallization. The invention also provides a corresponding semiconductor device. The method and the device can improve the adhesion of the wafer and the metal layer and the ohmic contact between the wafer and the metal layer, and are very beneficial to the performance, the service life and the application of a semiconductor device.

Description

Laser processing method for wafer and semiconductor device

Technical Field

The present invention relates to the field of semiconductors, and more particularly, to a laser processing method for a wafer and a semiconductor device.

Background

Semiconductor devices, or integrated circuits, are typically fabricated on a semiconductor substrate or base plate (e.g., a wafer). For example, most semiconductor devices are fabricated on a shallow surface layer of a semiconductor substrate. Due to the complexity of the manufacturing process, which can reach hundreds of process flows, thin wafers cannot be adopted generally, but wafers with certain thickness can be used for transferring and flowing in the process. It is often necessary to remove a certain thickness of excess substrate material from the back side of the wafer prior to packaging the semiconductor device.

For thinning, the back side of the wafer is typically ground. The polishing may cause damage to the back layer, for example, generation of defects such as cracks or fissures. These defects tend to create mechanical stress that can cause wafer deformation, such as warpage, etc., which can be detrimental to device performance. To address this problem, additional process steps, such as chemical etching, are typically employed to remove the damaged layer, and then a metal layer is deposited on the back side of the wafer from which the damaged layer was removed. However, this not only adds process steps, but also great care must be taken to avoid chemical agents from contacting the semiconductor devices on the front side of the wafer during the removal of the damaged layer. In addition, the back side of the wafer from which the damaged layer is removed may be too smooth, have an unsatisfactory adhesion to the metal layer, and easily fall off in subsequent packaging or use, thereby reducing the lifetime of the semiconductor device or limiting its application field.

Disclosure of Invention

The present invention is directed to a laser processing method for a wafer and a semiconductor device, which solve one or more of the problems set forth above in the prior art.

According to an aspect of the present invention, a method of laser processing a wafer is provided. The method comprises the following steps: providing a wafer comprising a semiconductor material and having a first side and a second side opposite the first side; thinning the second side to generate a damaged area on the thinned second side; irradiating the damaged area with a laser beam so that the semiconductor material of the damaged area is melted to form a melted area; and

and cooling the melted region to recrystallize, wherein the cooling rate of the melted region is adjusted by controlling the removal mode of the laser beam, thereby controlling the size of the crystal grains formed by recrystallization.

According to another aspect of the present invention, a method of laser processing a wafer is provided. The method comprises the following steps: providing a wafer comprising silicon carbide, the wafer having a first side and a second side opposite the first side, the first side having a semiconductor device disposed thereon; grinding the second side, wherein the ground second side generates a damaged area; forming a metal layer on the damaged area, wherein the metal layer and the damaged area have an interface; irradiating the metal layer with a laser beam such that the laser beam passes through the metal layer and into the damaged region to cause the damaged region to melt; by controlling the removal mode of the laser beam, the melted damaged region is cooled to be recrystallized, thereby forming a crystalline state having crystal grains.

According to still another aspect of the present invention, a semiconductor device is provided. The semiconductor apparatus includes a wafer, at least one semiconductor device, and a metal layer. The wafer includes a semiconductor material, the wafer having a first side and a second side opposite the first side. The at least one semiconductor device is disposed on the first side. A metal layer is disposed on the second side, the metal layer having an interface with the second side, a region of the wafer proximate the interface having a polycrystalline state different from a crystalline state of other portions of the wafer and including grains.

The present invention has many advantages over the prior art. For example, the wafer laser processing method according to one or more embodiments of the present invention does not require an additional special process step to remove the damaged region in the wafer, which not only reduces the process steps, but also avoids the risk of damage to the device caused by the chemical agent during the process of chemically removing the damaged region in the conventional art. Furthermore, in accordance with the laser processing method of a wafer according to one or more embodiments of the present invention, the laser irradiation is used to remove defects (e.g., cracks) in the damaged area by first melting the defect area and then recrystallizing the defect area, thereby also removing stress and deformation caused by the defects. In addition, the wafer laser processing method according to one or more embodiments of the invention is very flexible and controllable. The depth of laser beam irradiation can be adjusted by adjusting a parameter associated with the laser beam. By controlling the removal mode of the laser beam, the cooling speed of the melted damage area can be flexibly controlled, so that the size of the recrystallized grains can be controlled. Since the size of the crystal grain is closely related to the interface of the wafer and the metal layer, the adhesion or adherence of the wafer and the metal layer can be controlled with great flexibility, which is advantageous for increasing the electrical properties, mechanical stability, lifetime, and application fields of the semiconductor device.

Other embodiments and further technical effects of the present invention will be described in detail below.

Drawings

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings. One or more embodiments are illustrated by the corresponding figures in the drawings, which are not meant to be limiting. For convenience, the same or similar elements are identified with the same or similar reference numerals in the drawings, and the drawings in the drawings are not to scale unless otherwise specified. Wherein the content of the first and second substances,

FIG. 1 shows a flow diagram of a method for laser processing of a wafer according to an embodiment of the invention;

FIGS. 2a-2e show schematic diagrams of laser processing methods for wafers according to some embodiments of the invention;

3a-3e show schematic diagrams of a removal pattern of a laser beam according to some embodiments of the invention;

FIGS. 4a-4c are schematic diagrams illustrating laser processing methods for wafers according to further embodiments of the present invention;

fig. 5 shows a flowchart of a laser processing method for a wafer according to yet another embodiment of the present invention.

Detailed Description

To facilitate an understanding of the present invention, a number of exemplary embodiments will be described below in conjunction with the associated drawings. It will be understood by those skilled in the art that the examples herein are for the purpose of illustrating the invention and are not in any way limiting.

As used herein, the term "removal mode" refers to how a laser beam is removed or ablated after a certain area is irradiated with the laser beam for a certain time.

Fig. 1 and 2a-2e illustrate a method of laser processing a wafer, according to an aspect of the present invention. At block 12, a wafer 200 (fig. 2a) is provided. Wafer 200 includes a semiconductor material, such as a monocrystalline semiconductor material. In this particular embodiment, the semiconductor material is silicon carbide (e.g., 4H-SiC). The wafer 200 has a first side 210 and a second side 220. The first side 210 is opposite the second side 220.

The first side 210 may, for example, serve as a front side for fabricating or providing semiconductor devices, integrated circuits, etc. The second side 220 may, for example, serve as a backside for providing a back metal electrode connection.

At block 14, the wafer 200 is subjected to a thinning process. For example, the second side 220 may be thinned. The thinning process may result in a thinning of the wafer 200 (i.e., a reduction in the wafer thickness) while creating a thickness or depth of the damaged region 230 (fig. 2b) on the second side 220 from its surface toward the first side 210.

The thinning process may employ a suitable method, such as mechanical methods, chemical methods, or a combination thereof. The thinning process can be realized by, for example, grinding, lapping, chemical mechanical polishing, dry polishing, or the like. The thinning process is advantageous, for example, to improve heat dissipation of the semiconductor device, chip, or apparatus, and to reduce resistance, facilitate a post-packaging process, and the like.

The damaged region 230 has defects 232 such as cracks, fissures, dislocations, and the like. The presence of the defect 232 can create undesirable mechanical stress, causing wafer warpage, which ultimately affects the electrical performance and lifetime of the semiconductor device.

At block 16, the damaged region 230 is irradiated with a laser beam 280, thereby causing the semiconductor material of the damaged region 230 to melt, thereby forming a melt region 240 (fig. 2 c). The laser beam 280 may be chosen to have a suitable wavelength and energy density according to the actual requirements. The wavelength may be, for example, 305 nanometers (nm), 355nm, or the like. The energy density may be, for example, at 2.0 joules per square centimeter (J/cm)²) To 5.0J/cm²E.g. 4.0J/cm². The laser beam may be generated by a laser generating device. In this particular embodiment, krypton fluoride (KrF) and argon fluoride (ArF) excimer lasers pulsed at 30 nanoseconds (ms) were used to generate an energy of 4.0J/cm at a wavelength of 248nm²The laser beam of (1) irradiates the damaged area 230. In one embodiment, a xenon chloride (Xenon chloride) excimer laser is used to generate a wavelength of 308nmAnd (4) laser. In another embodiment, the laser beam has a wavelength of 351-353 nm.

By way of example, the laser beam may be directed to the damaged area of the wafer by providing appropriate optics (e.g., optical fibers, mirrors, lenses, etc.). The optics may be mounted, for example, on a stage movable in the X-Y plane. The laser beam is focused by a lens to the damage region, where the intensity of the focused spot, which may have a gaussian distribution, may have a diameter in the order of a few to several hundred microns, for example.

In this particular embodiment, the laser energy is such that it can penetrate into the entire damaged area 230 and reach beneath the damaged area 230, such that the resulting melt zone 240 covers the entire damaged area 230. For example, the thickness of the damaged region 230 may be, for example, in the range of 0.2 micrometers (um) to 2 um. In one embodiment, the thickness of the damaged region is about 0.6um, and the laser beam can penetrate into the wafer by about 1um, so that the irradiation of the laser beam can sufficiently cover the damaged region, so that the semiconductor material in the whole damaged region is melted.

At block 18, the melt zone 240 is cooled to recrystallize. The semiconductor material of the melted region 240 thus forms a crystalline state having grains, such as polycrystalline silicon carbide grains 242 (fig. 2d) in this particular embodiment. As the semiconductor material of the damaged region is melted, defects 232 such as cracks, fissures, and the like are eliminated and, as a result of recrystallization, a relatively regular structure is formed, eliminating or reducing mechanical stress, thereby eliminating or mitigating undesirable deformation caused by mechanical stress, such as wafer warpage. In addition, due to the existence of the die 242, the surface roughness of the second side 220 of the wafer 200 is increased, so that the mechanical adhesion between the second side and the backside metal layer can be increased, which is beneficial to the subsequent packaging, service life and application field of the semiconductor device.

Advantageously, the inventors have found that by controlling the removal pattern of the laser beam, the cooling rate of the melt zone can be adjusted or controlled, thereby controlling the size of the grains formed by recrystallization, and thus the surface characteristics of the second side of the wafer. For example, the inventors have found that by cooling the melt zone at a faster rate, smaller sized grains can be obtained (e.g., fig. 2 d). By cooling the melted region at a slower rate, larger sized grains (e.g., grains 242' in fig. 2 e) may be obtained.

Fig. 3a-3e illustrate some embodiments of a removal pattern of a laser beam according to the present invention. These exemplary removal patterns may be applied to the embodiments of fig. 2a-2e, for example. As illustrated in fig. 3a, the laser beam is irradiated in pulses 310. The peak energy density of pulse 310 is E0. At time t1, the laser beam is removed, i.e., irradiation of the laser beam is stopped. This can be done, for example, by turning off the laser-generating source, placing a shield (e.g., a shutter) in the optical path of the laser beam, or moving away the laser beam or the wafer. Since there is no longer irradiation of the laser beam since t1, the melted region is rapidly cooled at ambient temperature (e.g., room temperature), thereby re-crystallizing. In some embodiments, the grain size formed is between 1 micrometer (um) to 10 um. In other embodiments, the grains are between 0.1um and 1um in size. In other embodiments, grains larger than 10um are possible, depending on the actual situation.

In fig. 3b-3e, the grain size of the formed crystalline state can be more finely controlled by adjusting the removal pattern of the laser beam. In fig. 3b, starting at time t1, the duty cycle of the laser pulse 320 is gradually decreased such that the energy density 322 of the laser beam is linearly decreased, decreasing to zero at time t 2. The three points to the right in fig. 3b indicate the omission of one or more laser pulses, which is only for the sake of simplicity of the drawing.

In fig. 3c, starting at time t1, the peak energy density of laser pulse 330 gradually decreases, causing the energy density 332 of the laser beam to decrease linearly, decreasing to zero at time t 3.

In fig. 3d, starting at time t1, the duty cycle of laser pulse 340 is gradually decreased such that the energy density 342 of the laser beam decreases non-linearly, decreasing to zero at time t 4.

In fig. 3e, starting at time t1, the peak energy density of laser pulse 350 gradually decreases, causing the energy density 352 of the laser beam to decrease non-linearly, decreasing to zero at time t 5.

In the embodiment of fig. 3b-3e, the melting zone is cooled at a slower rate than the laser beam removal mode of fig. 3a due to the decreasing energy density of the laser beam, resulting in larger sized grains under otherwise identical or substantially identical conditions. Furthermore, by controlling various parameters, such as E0, t1, laser beam removal mode (gradual duty cycle reduction, reduced energy density peak, reduced amplitude, and combinations thereof), the manner in which the melt zone is cooled (e.g., cooling rate) can be controlled, resulting in crystalline states with different grain sizes.

Fig. 4a-4c show schematic views of a laser processing method for a wafer according to another embodiment of the present invention. As shown in fig. 4a-4c, wafer 400 has a first side 410 and a second side 420. After thinning the wafer 400 from the second side, a damaged region 430 is created. In contrast to fig. 2a-2e, in fig. 4a, a metal layer 450 is formed on the second side 420 of the wafer 400 before the second side 420 is laser processed. The metal layer 450 may comprise titanium, nickel, or other suitable metal. In this particular embodiment, the metal layer 450 is a titanium metal layer having a thickness between 100 nanometers (nm) and 300 nm. An interface 452 is provided between the metal layer 450 and the damaged region 430. It is desirable that the metal layer 450 form an ohmic or low resistance contact with the wafer 400 at the interface 452 and have good adhesion or adherence to the wafer 400.

In fig. 4b, the metal layer 450 is irradiated with a laser beam 480. Laser beam 480, having a suitable wavelength and energy, may pass through metal layer 450 and into damaged region 430, causing at least a portion of damaged region 430 to melt. In this particular embodiment, laser beam 480 penetrates damage region 430 and reaches the area below damage region 430, thereby forming melt region 440.

The removal pattern of the laser beam 480 may be, for example, one or more of those shown above in connection with fig. 3a-3e, depending on the actual requirements. By selecting the removal pattern of the laser beam, the melt region 440 may be cooled in different cooling patterns (e.g., cooling rates) to control the grain size of the crystalline state formed after recrystallization of the melt region 440. The grains 442 formed at the interface are shown in fig. 4 c. As shown, at the interface with the metal layer, the die 442 has a surface profile that forms the contact surface of the die with the metal layer. By controlling the size of the grains, different surface profiles can be obtained. The different surface profiles correspond to different surface roughness and different contact surfaces with the metal layer.

Fig. 5 shows a flowchart of a laser processing method for a wafer according to yet another embodiment of the present invention. This embodiment may be, for example, one specific implementation of the embodiment shown in fig. 1.

At block 51, a wafer is provided. The wafer is, for example, a wafer containing silicon carbide. The wafer has a first side and a second side. A semiconductor device is disposed on the first side of the wafer. Examples of semiconductor devices are schottky diodes, bipolar transistors, insulated gate bipolar transistors, metal oxide field effect transistors, junction field effect transistors, semiconductor memory devices, semiconductor photovoltaic devices, and integrated circuits incorporating one or more of these devices. Semiconductor devices may be fabricated by suitable semiconductor processes including, for example and without limitation, one or more of epitaxial growth, ion implantation, photolithography, etching, metal deposition, interconnection, passivation, and the like.

At block 52, the second side is lapped. The grinding causes the thickness of the wafer to decrease and a damaged area to be created on the second side of the wafer.

At block 53, a metal layer is formed on the second side. The metal layer may be formed, for example, by deposition, sputtering, or the like in a suitable manner. The metal layer may be a single metal layer or a stack of a plurality of metal layers. The metal layer may be formed of a single metal (e.g., titanium), or may be formed of two or more metals, such as a titanium-nickel alloy.

At block 54, the metal layer is irradiated with a laser beam such that the laser beam passes through the metal layer and into the damaged area to cause the damaged area to melt. In this step, one or both of the wavelength and energy density of the laser beam may be adjusted, thereby adjusting the depth of the laser beam into the damage region.

At block 55, the melted damaged area is cooled to recrystallize, thereby forming a crystalline state with grains. The removal pattern of the laser beam can be controlled to obtain crystalline states having different grain sizes.

According to still another aspect of the present invention, a semiconductor device is also provided. The semiconductor arrangement comprises a wafer processed by the method according to fig. 1 and 2a-2e or 3a-3e, and a semiconductor device arranged on a first side of the wafer. A metal layer is disposed on the second side of the wafer, the metal layer having an interface with the second side. An ohmic or low resistance contact is formed between the second side of the wafer and the metal layer. The region of the wafer near the interface has a polycrystalline state different from the crystalline state of other portions of the wafer and includes grains. The size of the grains can alter the electrical (e.g., conductivity) and mechanical (e.g., adhesion) properties of the wafer to metal contact.

It will be further appreciated by those of ordinary skill in the art that for purposes of clarity of illustration, elements (e.g., elements, regions, layers, etc.) in the figures have not necessarily been drawn to scale. Nor are the individual elements of the drawings necessarily the actual shapes thereof. For example, the grains in fig. 2d, 2e and 4c are only for illustrating the idea of the corresponding embodiment, and do not necessarily have the shown shape and distribution.

In the embodiments of fig. 2c, 2d, 4b, 4c, the melt zone is shown as filling the damage zone and extending to an area of the wafer outside (shown below) the

damage zone

230, 430. It will be appreciated by those skilled in the art that in some embodiments, the melt zone may just fill the entire lesion field (i.e., coincide with the lesion field). In still other embodiments, the melt zone may fill only a portion of the damage zone.

Further, in the drawings, the damaged and melted regions are shown in the form of layers with clear boundaries, e.g., the damaged region 230 has clear boundaries with other portions of the wafer 200, and the depth of the damaged region into the wafer 200 at different locations is also the same. It will be understood by those skilled in the art that this is only for the purpose of clearly illustrating the idea of an embodiment of the invention. In a real wafer, during thinning of the second side, in most cases the damage to the wafer is uncertain and the distribution of the damage is also non-uniform. In some locations, the damage may extend deeper into the wafer (e.g., up to 2um or more), and in some locations, the damage may be shallower. In some locations, there may be no damage. Thus, in the drawings, the boundary of the damaged area parallel to the second side is only an idealized illustration for the purpose of clarity of description, or may be considered an illustrative boundary with reference to the location of the deepest damage. Furthermore, the melt zone usually does not have a very clear boundary.

In the above embodiments, silicon carbide is exemplified for convenience of description. It will be appreciated by those skilled in the art that the wafer processing methods disclosed herein are not limited to wafers of silicon carbide materials, but may be applied to semiconductor wafers of other suitable materials, such as silicon (Si), germanium (Ge), silicon germanium (SiGe), gallium nitride (GaN), gallium arsenide (GaAs), and the like.

In the above embodiments, an excimer laser is taken as an example for convenience of description. It will be appreciated by those skilled in the art that in some other embodiments, other suitable laser generating devices may be used, such as solid state lasers, gas state lasers, and the like. The laser beam may be irradiated according to actual requirements.

Furthermore, it will be appreciated by those skilled in the art that the above embodiments are intended to illustrate the invention in different respects, and that they are not intended to be in isolation; rather, those skilled in the art can combine the different embodiments appropriately according to the above examples to obtain other technical solution examples.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Embodiments of the present invention are illustrated in non-limiting examples. Variations that may occur to those skilled in the art upon consideration of the above-disclosed embodiments are within the scope of the invention.

Claims

1. A method of laser processing a wafer, the method comprising:

providing a wafer comprising a semiconductor material and having a first side and a second side opposite the first side;

thinning the second side to generate a damaged area on the thinned second side;

irradiating the damaged region with a laser beam so as to melt the semiconductor material of the damaged region, thereby forming a melted region; and

cooling the melted region to recrystallize, wherein a cooling rate of the melted region is adjusted by controlling a removal pattern of the laser beam, thereby controlling a size of a grain formed by recrystallization.

2. The method of claim 1, further comprising: a metal layer is formed on the thinned second side before irradiating the damaged area with a laser beam.

3. The method of claim 1 or 2, wherein the removal pattern of the laser beam comprises: after irradiating the damaged region with a laser beam for a period of time, the irradiation of the laser beam is stopped, thereby allowing the melted region to cool at ambient temperature for recrystallization.

4. The method of claim 1 or 2, wherein the removal pattern of the laser beam comprises: gradually reducing a duty cycle of the pulses of the laser beam to zero.

5. The method of claim 4, wherein tapering the duty cycle decreases the energy density of the laser beam linearly to zero.

6. The method of claim 4, wherein tapering the duty cycle reduces the energy density of the laser beam non-linearly to zero.

7. The method of claim 1 or 2, wherein the removal pattern of the laser beam comprises: gradually reducing an energy density peak of a pulse of the laser beam to zero.

8. The method of claim 7, wherein tapering the peak energy density causes the energy density of the laser beam to decrease linearly to zero.

9. The method of claim 7, wherein tapering the peak energy density causes the energy density of the laser beam to decrease non-linearly to zero.

10. A method according to claim 1 or 2, wherein the semiconductor material is silicon carbide or silicon.

11. A method of laser processing a wafer, the method comprising: providing a wafer comprising silicon carbide, the wafer having a first side and a second side opposite the first side, the first side having a semiconductor device disposed thereon;

grinding the second side, wherein the ground second side generates a damage area;

forming a metal layer on the damaged region, wherein the metal layer and the damaged region have an interface;

irradiating the metal layer with a laser beam such that the laser beam passes through the metal layer and into the damaged region to cause the damaged region to melt;

by controlling the removal pattern of the laser beam, the melted damaged region is cooled to be recrystallized, thereby forming a crystalline state having crystal grains.

12. The method of claim 11, wherein irradiating the metal layer with a laser beam comprises adjusting one or both of a wavelength and an energy density of the laser beam, thereby adjusting a depth of the laser beam into the wafer.

13. A semiconductor device, characterized in that the semiconductor device comprises:

a wafer comprising a semiconductor material, the wafer having a first side and a second side opposite the first side;

at least one semiconductor device disposed on the first side; and

a metal layer disposed on the second side, the metal layer having an interface with the second side, a region of the wafer proximate the interface having a polycrystalline state different from a crystalline state of other portions of the wafer and including grains.

14. The semiconductor device according to claim 13, wherein the size of the crystal grains is between 0.1 and 10 micrometers.

15. The semiconductor device of claim 13, wherein the die at the interface has a surface profile that forms a contact surface for the die to contact the metal layer.

16. The semiconductor device according to any one of claims 13 to 15, wherein the metal layer is a titanium metal layer or a nickel metal layer.

17. A semiconductor arrangement according to any of claims 13-15, characterized in that the at least one semiconductor device is selected from one or more of the following group: schottky diodes, bipolar transistors, insulated gate bipolar transistors, metal oxide field effect transistors, junction field effect transistors, semiconductor memory devices, semiconductor photovoltaic devices.