Laser assisted bonding method and semiconductor package using the same

Abstract

A laser-assisted bonding method includes: placing a semiconductor wafer on a substrate, wherein the semiconductor wafer has an anti-reflective layer for infrared light on its front surface, and forming solder bumps between the substrate and the semiconductor wafer; and irradiating the semiconductor wafer with an infrared laser beam through the anti-reflective layer, so as to reflow the solder bumps by heat generated by the energy of the infrared laser beam absorbed by the semiconductor wafer, wherein the anti-reflective layer is used to reduce the reflection of the infrared laser beam by the semiconductor wafer.

Description

Laser-assisted bonding method This application relates generally to semiconductor technology, and more specifically to laser-assisted bonding methods. Semiconductor packaging is a crucial part of semiconductor manufacturing and design. Semiconductor packaging technology affects the power consumption, performance, cost, and basic functionality of all semiconductor wafers. Typically, semiconductor wafers are first manufactured at the wafer level, and then each semiconductor wafer can be attached to a packaging substrate to form an integrated semiconductor package. Attachment techniques for semiconductor wafers to a package substrate can be performed, for example, by laser-assisted bonding equipment. During bonding, the equipment uses a vacuum to pick up the semiconductor wafer and then places it onto the package substrate. Infrared lasers can be used to heat the solder bumps beneath the semiconductor wafer, enabling the solder bumps to form an electrical connection between the semiconductor wafer and the package substrate. However, it should be noted that some laser-assisted bonding equipment is less efficient. Therefore, further improvements are needed to the laser-assisted bonding method described above. One objective of this application is to provide a laser-assisted bonding method with improved efficiency. According to one aspect of this application, a laser-assisted bonding method is disclosed, the method comprising: placing a semiconductor wafer on a substrate, wherein the semiconductor wafer has an anti-reflective layer for infrared light on its front surface, and forming solder bumps between the substrate and the semiconductor wafer; and irradiating the semiconductor wafer with an infrared laser beam through the anti-reflective layer, so as to reflow the solder bumps by the heat generated by the energy of the infrared laser beam absorbed by the semiconductor wafer, wherein the anti-reflective layer is used to reduce the reflection of the infrared laser beam by the semiconductor wafer. According to another aspect of this application, a laser-assisted bonding method is provided, the method comprising: placing a semiconductor wafer on a substrate, wherein the semiconductor wafer has an anti-reflective layer for infrared light on its front surface, and solder bumps are formed between the substrate and the semiconductor wafer; pressing the semiconductor wafer against the substrate using an infrared-transmissive mold; and, while the semiconductor wafer is pressed against the substrate, irradiating the semiconductor wafer with an infrared laser beam from an infrared laser source through the infrared-transmissive mold and the anti-reflective layer, so that the heat generated by the energy of the infrared laser beam absorbed by the semiconductor wafer is used to backflow the solder bumps, wherein the anti-reflective layer is used to reduce the reflection of the infrared laser beam by the semiconductor wafer. It should be understood that the foregoing general description and the following detailed description are merely exemplary and explanatory and do not limit the invention. Furthermore, the drawings incorporated in and forming a part of this specification illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. The following detailed description of exemplary embodiments of this application refers to the drawings that form part of the description. The drawings illustrate specific exemplary embodiments in which this application may be practiced. The detailed description, including the drawings, describes these embodiments in sufficient detail to enable those skilled in the art to practice this application. Those skilled in the art can further utilize other embodiments of this application and make logical, mechanical, and other changes without departing from the spirit or scope of this application. Therefore, the reader of the following detailed description should not interpret the description in a limiting sense, and only the appended claims define the scope of the embodiments of this application. In this application, unless otherwise expressly stated, the use of the singular includes the plural. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "including" and other forms such as "includes" and "included" is not restrictive. Additionally, unless specifically stated otherwise, terms such as "element" or "component" cover elements and components comprising one unit and elements and components comprising more than one sub-unit. Furthermore, the section headings used herein are for organizational purposes only and should not be construed as limiting the subject matter described. As used herein, for ease of description, spatially relative terms such as "below," "below," "above," "over," "upper," "lower," "left," "right," "vertical," "horizontal," and "side" may be used to describe the relationship between an element or feature and another element (or feature) or feature (or feature), as shown in the figures. In addition to the orientations depicted in the figures, the spatially relative terms are intended to cover different orientations of the device in use or operation. The device may be oriented in other ways (rotated 90 degrees or in other orientations), and the spatially relative descriptive terms used herein shall be interpreted accordingly. It should be understood that when an element is referred to as being "connected to" or "coupled to" another element, the element may be directly connected to or coupled to another element, or there may be intermediate elements present. As mentioned earlier, semiconductor wafers are typically first manufactured at the wafer level, and then each semiconductor wafer can be attached to a packaging substrate after the semiconductor wafer is diced into multiple pieces or units to form an integrated semiconductor package. The attachment technology for attaching semiconductor wafers to the packaging substrate can be performed using laser-assisted bonding equipment, which utilizes vacuum to pick up the semiconductor wafer and move it onto the substrate, and uses an infrared laser beam to heat the solder bumps beneath the semiconductor wafer to bond the semiconductor wafer to the substrate. However, the inventors of this application discovered that when solder bumps are heated by laser-assisted bonding equipment, the efficiency of laser heating is relatively low, thus requiring increased laser power or heating time during the heating process to ensure the solder bumps reach a satisfactory reflow temperature. The inventors also discovered that during laser heating, a significant amount of infrared laser energy is wasted due to the substantial difference in refractive index between the semiconductor wafer and the environment. In other words, most of the infrared laser energy directed at the semiconductor wafer is likely to be reflected off the top surface of the wafer instead of being absorbed. Therefore, less laser energy can be converted into heat for reflow of the solder bumps. For example, the inventors found that only about 30% of the infrared laser energy can be absorbed by the semiconductor wafer during laser-assisted bonding, meaning that 70% of the infrared laser energy is reflected by the semiconductor wafer and cannot be used for heating. To address the aforementioned problems, this application provides a novel laser-assisted bonding method. An anti-reflection layer is formed on the front surface of the semiconductor wafer to be bonded. The anti-reflection layer can reduce reflection loss during laser-assisted bonding, thereby improving the efficiency of laser-assisted bonding. For example, the anti-reflection layer may have a refractive index between air (for infrared light) and the refractive index of the semiconductor wafer material. Figures 1A to 1C illustrate a laser-assisted bonding method according to one embodiment of this application. In some embodiments, the laser-assisted bonding method can be used to bond a semiconductor wafer, such as a silicon wafer, to a substrate using solder bumps. As shown in FIG1A, a semiconductor wafer 102 having a front surface and a rear surface can be provided. The semiconductor wafer 102 will be attached to a substrate 104 by bonding technology. Solder bumps 106 are formed below the rear surface of the semiconductor wafer 102, which can be formed of tin or other suitable materials. In some embodiments, conductive pads may be formed on the front surface of the substrate 104 for attaching the semiconductor wafer 102 thereto. It is understood that these conductive pads may be exposed portions of conductive leads formed in the substrate 104. Similarly, conductive pads may be formed on the rear surface of the semiconductor wafer 102, i.e., at the location where the solder bumps 106 are formed. In some embodiments, the solder bumps 106 may be formed on the substrate 104, rather than on the rear surface of the semiconductor wafer 102, before the substrate 104 and the semiconductor wafer 102 are placed together. An antireflective layer 108 can be formed on the front surface of the semiconductor wafer 102, which can be used to reduce the refractive index mismatch between the semiconductor wafer 102 and the environment (typically air) for infrared light. Infrared light typically has wavelengths ranging from 0.75 micrometers to 1000 micrometers, but short-wave infrared light with wavelengths ranging from 0.75 to 3 micrometers is more suitable for heating because it has higher energy compared to mid-wave and far-infrared light. In this embodiment, infrared light with wavelengths ranging from 900 nanometers to 1100 nanometers is used for laser-assisted bonding. Therefore, the antireflective layer 108 can have a refractive index between that of the semiconductor material and air for infrared light with wavelengths ranging from 900 nanometers to 1100 nanometers. Specifically, for infrared light with wavelengths of 900 nm to 1100 nm, silicon has a refractive index of 3.5, while air has a refractive index of 1. Therefore, the antireflective layer 108 can have a refractive index in the range of 1 to 3.5 for such infrared light, preferably in the range of 1.2 to 3.3, or more preferably in the range of 1.5 to 3.0. For example, the antireflective layer 108 can be formed from one or more of the following materials: ZnS, TiO2, SiO2, CeF3, MgF2, or other suitable materials. Specifically, the antireflective layer 108 can generally be formed as a thin layer with superior uniformity and quality, for example, using electron beam evaporation, sputtering, or spin coating techniques. Furthermore, depending on the thickness of the antireflective layer 108, different techniques can also be used to form the antireflective layer 108. For example, spin coating can be used to deposit and form antireflective layers with a thickness ranging from 1 to 10 micrometers, while electron beam evaporation and sputtering can be used to form thinner antireflective layers, such as less than 1 micrometer. In some embodiments, the antireflective layer can be formed by electron beam evaporation, sputtering, dispensing, spin coating, or thin-film attachment. For example, a mixture of liquid, emulsion, or suspension can be dispensed onto a semiconductor wafer, such as a silicon wafer, to form the antireflective layer 108. After the active material has cured and the antireflective layer 108 has been formed, the semiconductor wafer can be diced into individual units, i.e., semiconductor wafers. In some embodiments, the thickness of the antireflective layer 108 formed on the semiconductor wafer 102 can be from 500 nanometers to 20 micrometers, or preferably from 1 micrometer to 10 micrometers. In some embodiments, the thickness of the antireflective layer 108 can be N times the wavelength of 1 / 4 of infrared light, where N is an odd number. In some embodiments, the antireflective layer 108 may remain on the semiconductor wafer 102 after the bonding process. However, in some other embodiments, the antireflective layer 108 may be removed from the semiconductor wafer 102 after the bonding process. Therefore, in some preferred embodiments, the antireflective layer 108 can subsequently be easily removed from the semiconductor wafer 102 without causing any substantial damage to the semiconductor wafer 102 and the substrate 104. For example, an antireflective layer with a thickness of 10 micrometers or less can be removed using a suitable chemical etchant or by mechanical means such as polishing. In some embodiments, the antireflective layer 108 may be a single layer, but in other alternative embodiments, the antireflective layer 108 may consist of multiple sublayers, each of which may have a different refractive index for infrared light. For example, the antireflective layer 108 may include at least a first sublayer and a second sublayer. The second sublayer is closer to the semiconductor wafer 102 than the first sublayer, and the refractive index of the second sublayer is greater than that of the first sublayer. In this way, the antireflective layer 108 may have a gradually increasing refractive index as it gets closer to the semiconductor wafer 102 to compensate for the refractive index mismatch between air and the matrix material of the semiconductor wafer 102. Referring again to FIG. 1A, substrate 104 can be placed on stage 110, such as a platform or mechanical support. Furthermore, semiconductor wafer 102 can be placed on substrate 104, with solder bumps 106 in direct contact with substrate 104. The anti-reflective layer 108 located at the top of semiconductor wafer 102 can be exposed to a laser source 112 above semiconductor wafer 102. During bonding, laser source 112 can be activated to irradiate the semiconductor wafer 102 with an infrared laser beam. The infrared laser beam can penetrate the anti-reflective layer 108 to reach semiconductor wafer 102. Thus, semiconductor wafer 102 can receive laser energy and convert it into heat due to the indirect band gap charateristic properties of the substrate material of semiconductor wafer 102. As more heat accumulates within semiconductor wafer 102, the heat can be transferred to the solder bumps 106 below semiconductor wafer 102, allowing solder bumps 106 to be reflowed. Specifically, as the temperature of the semiconductor wafer 102 and the solder bump 106 increases, the solder bump 106 can begin to melt and penetrate into the conductive pads on the substrate 104. Due to the presence of the anti-reflective layer 108 on the front surface of the semiconductor wafer 102, compared to existing laser-assisted bonding methods that do not form an anti-reflective layer on the semiconductor wafer, less laser light is reflected at the interface between the air and the semiconductor wafer 102, and therefore the efficiency of laser energy can be improved. Referring to Figure 1B, after the semiconductor wafer 102 is irradiated by a laser, the laser source can be removed, and the semiconductor wafer 102 and substrate 104, bonded together by reflowed solder bumps 106, remain on the stage 110. Subsequently, the anti-reflective layer can be removed from the semiconductor wafer 102. For example, an anti-reflective layer formed of silicon oxide can be removed using a chemical etchant such as hydrofluoric acid. However, it will be understood that in some embodiments, the anti-reflective layer may remain on the semiconductor wafer 102. Referring to FIG1C, one or more additional process treatments can be performed on the semiconductor wafer 102 and the substrate 104. For example, a sealant layer 114 can be formed on the substrate 104 to seal the semiconductor wafer 102 and prevent it from being damaged by the environment. In some embodiments, a metal layer (not shown in the figure), such as a copper layer, can be formed on the sealant layer 114, which can serve as an electromagnetic interference shielding layer for a semiconductor package thus formed. Figures 2A to 2C illustrate a laser-assisted bonding method according to another embodiment of this application. In this embodiment, the laser-assisted bonding method is a pressure bonding method, that is, pressure is applied to the semiconductor wafer to assist the bonding process. As shown in Figure 2A, a semiconductor wafer 202 is provided, with an anti-reflective layer 208 formed on its front surface and solder bumps 206 formed on its rear surface. A mold 216 is used to pick up the semiconductor wafer 202 and move it to a bonding position on a substrate 204, wherein the substrate 204 is placed on a stage 210. In some embodiments, the mold 216 can use a vacuum to pick up the semiconductor wafer 202. The mold 216 is transmissive to infrared light, so that infrared light can pass through the mold 216. Next, as shown in FIG2B, the semiconductor wafer 202 can be pressed against the substrate 204 by the mold 216. Simultaneously, the laser source 212 placed above the mold 216 can irradiate the semiconductor wafer 202 with an infrared laser beam through the infrared-transmissible mold 216. Since the infrared-transmissible mold 216 is in direct contact with the anti-reflective layer 208 on the semiconductor wafer 202, the anti-reflective layer 208 can reduce the degree to which the semiconductor wafer 202 reflects infrared light back to the mold 216, thereby increasing the amount of light entering the semiconductor wafer 202, where laser energy can be converted into heat. In this way, the solder bumps 206 can be reflowed more efficiently than existing bonding techniques, thereby forming sufficient bonding between the semiconductor wafer 202 and the substrate 204 through the solder bumps 206. In some embodiments, flux material can be formed on the surface of the solder bumps 206. As described above, since the infrared-transmissive mold 216 is in direct contact with the anti-reflective layer 208 on the semiconductor wafer 202, the refractive index of the anti-reflective layer 208 for infrared light can be between the refractive index of the infrared-transmissive mold 216 and the refractive index of the matrix material of the semiconductor wafer 202. It is understood that, depending on the materials of the mold 216 and the semiconductor wafer 202, a suitable material can be selected as the anti-reflective layer 208. In some embodiments, the anti-reflective layer 208 may include at least a first sublayer and a second sublayer, wherein the second sublayer is closer to the semiconductor wafer 202, and the refractive index of the second sublayer is greater than that of the first sublayer. Subsequently, as shown in FIG2C, after the solder bumps 206 are reflowed and penetrated into the conductive pads on the substrate 204, the laser source 212 can be turned off, and the infrared-transmissive mold 216 can be lifted and moved away from the semiconductor wafer 202 and substrate 204 on the stage 210. The bonding process is thus completed. In some embodiments, subsequent packaging techniques such as sealing and electromagnetic interference shielding can be applied to the bonded semiconductor wafer 202 and substrate 204. Optionally, an anti-reflective layer can be removed from the semiconductor wafer 202 before performing these subsequent packaging techniques. This paper includes numerous illustrative diagrams showing various parts of the laser-assisted bonding method. For clarity, such diagrams do not represent all aspects of each example method. Any of the example methods provided in this paper may share any or all of the characteristics with any or all other methods provided herein. Various embodiments have been described herein with reference to the accompanying drawings. However, it will be apparent that various modifications and alterations can be made thereto, and other embodiments can be implemented, without departing from the broader scope of the invention as set forth in the appended claims. Furthermore, other embodiments will become apparent to those skilled in the art upon consideration of the description and practice of one or more embodiments of the invention disclosed herein. Therefore, it is intended that this application and the examples herein be considered exemplary only, and the true scope and spirit of the invention are indicated by the list of exemplary claims appended. 102: Semiconductor wafer; 104: Substrate; 106: Solder bump; 108: Anti-reflective layer; 110: Stage; 112: Laser source; 114: Sealant layer; 202: Semiconductor wafer; 204: Substrate; 206: Solder bump; 208: Anti-reflective layer; 210: Stage; 212: Laser source; 216: Mold The drawings referenced herein form part of this specification. The features shown in the drawings are only illustrative of some embodiments of this application, and not all embodiments of this application, unless the detailed description clearly indicates otherwise, and the reader of this specification should not infer the contrary. Figures 1A to 1C illustrate a laser-assisted bonding method according to an embodiment of this application. Figures 2A to 2C illustrate a laser-assisted bonding method according to another embodiment of this application. Throughout the diagram, the same reference numerals will be used to refer to the same or similar parts. 102: Semiconductor wafers 104: Base 106: Solder bump 108: Anti-reflective layer 110: Platform 112: Laser source

Claims

1. A laser-assisted bonding method, the method comprising: A semiconductor wafer is placed on a substrate, wherein the semiconductor wafer has an anti-reflective layer for infrared light on its front surface, and a solder bump is formed between the substrate and the semiconductor wafer; an infrared laser beam is irradiated onto the semiconductor wafer through the anti-reflective layer, and the solder bump is reflowed by the heat generated by the energy of the infrared laser beam absorbed by the semiconductor wafer, wherein the anti-reflective layer is used to reduce the reflection of the infrared laser beam by the semiconductor wafer; after the solder bump is reflowed, the anti-reflective layer is removed from the semiconductor wafer.

2. The method according to request item 1, wherein, The refractive index of the anti-reflective layer for infrared light is between that of air and that of the substrate material of the semiconductor wafer.

3. The method according to request item 2, wherein, The semiconductor wafer comprises a silicon matrix material, and the antireflective layer has a refractive index ranging from 1 to 3.5 for infrared light.

4. The method according to request item 1, wherein, The anti-reflective layer includes at least a first sublayer and a second sublayer, wherein the second sublayer is closer to the semiconductor wafer and the refractive index of the second sublayer is greater than that of the first sublayer.

5. The method according to request item 1, wherein, The wavelength range of the infrared laser beam is 900 nanometers to 1100 nanometers.

6. The method according to request item 1, wherein, The anti-reflective layer is formed by electron beam evaporation, sputtering, dispensing, spin coating, or thin-film attachment.

7. A semiconductor package formed using the method described in any one of claims 1 to 6.

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