Patent Application
20220397536
The present invention generally relates to sensors, imaging systems, and methods for forming a sensor. Certain embodiments relate to sensor shape control via sensor assembly.
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Backside illuminated image sensors can achieve high quantum efficiency (QE) and good modulation transfer function (MTF) and are widely employed for inspection of a variety of semiconductor and other substrates. To enable fast operation, these sensors are closely connected to application-specific integrated circuits (ASICs), which can perform one or several of the following functions: analog-to-digital (A/D) conversion, signal conditioning, digital signal processing, and communication to an external computer.
Such a sensor configuration poses challenges for proper control of the shape of the photo-active area of the sensor. For example, in the case of a backside illuminated sensor, the photo-active area is a thin membrane and can become mechanically unstable. Flip-chip assembly of the sensor die enables fast operation compatible with backside illumination. Upon flip-chip assembly of a sensor onto a ceramic substrate, the sensor may become convex, concave or wrinkled. The effects of flip-chip assembly on sensor shape can pose challenges for optical systems in which highly controlled and simple shape is required. In optical applications, a curved image plane may be preferred with specific curvature. It can be therefore critical to the imaging performance of such systems to be able to control sensor shape during the assembly and to be able to design the assembly to target a specific sensor shape.
Optical system designs typically produce negative curvature, positive curvature, or flat image fields. Image sensors, on the other hand, may be assembled on a ceramic substrate, and the shape of ceramic substrates is difficult to control due to the substrate manufacturing process. Being unable to control the shape of a sensor assembly can decrease the useful field of view, reduce system level optical tolerances, and increase the amount of optical aberration.
The disadvantages of currently used sensor assembly methods therefore include that the ceramic substrate shape cannot be easily controlled, while high-performance optical design may require high sensor planarity or a certain sensor shape. An additional disadvantage of currently-used assembly methods is that relatively poor die co-planarity can make it difficult to properly attach the sensor die, which impacts the thermal performance of the assembly. Another disadvantage of currently used sensor assembly methods is heat dissipation, which is important for high-speed, low-noise operation. A further disadvantage of the currently used sensor assembly methods is that field curvature can make it difficult to achieve a telecentric image space, which can be important in applications of metrology. Furthermore, current assembly methods do not allow good control or repeatability of sensor shape
There are several currently proposed methods for controlling the shape of a backside illuminated thinned sensor die for applications in mobile phones and astronomy. However, in all those applications, wire-bonded sensor dies are used. Wire-bonded sensor dies limit the number of interconnects and the readout speed and are not well-suited for applications of optical inspection described above. Such currently used methods for sensor shape control may also not be suitable for enabling good thermal contact, flip-chip with high density interconnect, or for applications in vacuum.
Accordingly, it would be advantageous to develop systems and methods for sensors, imaging systems, and methods for forming a sensor that do not have one or more of the disadvantages described above.
The following description of various embodiments is not to be construed in any way as limiting the subject matter of the appended claims.
One embodiment relates to a sensor that includes a substrate and one or more components attached to the substrate. The sensor also includes a sensor die having a thinned backside and energy responsive elements configured for detecting energy illuminating the thinned backside of the sensor die. The sensor further includes discrete thermally-conductive structures formed between a frontside of the sensor die and the substrate by a flip-chip process, thereby bonding the sensor die to the substrate and causing the thinned backside of the sensor die to have a pre-selected shape. At least a portion of the discrete thermally-conductive structures electrically connect the sensor die to the one or more components. The sensor may be further configured as described herein.
Another embodiment relates to an imaging system that includes an energy source configured for generating energy directed to a specimen by an illumination subsystem. The imaging system also includes a sensor configured for detecting energy from the specimen and generating output responsive to the detected energy. The sensor is further configured as described above. The imaging system may be further configured as described herein.
Another embodiment relates to a method for forming a sensor. The method includes forming discrete thermally-conductive structures on a substrate and altering a shape of the discrete thermally-conductive structures based on a pre-selected shape of a thinned backside of a sensor die. The method also includes bonding a frontside of the sensor die to the substrate via the discrete thermally-conductive structures thereby causing the thinned backside of the sensor die to have the pre-selected shape. At least a portion of the discrete thermally-conductive structures electrically connect the sensor die to one or more components attached to the substrate. The sensor die has energy sensitive elements configured for detecting energy illuminating the thinned backside of the sensor die.
Each of the steps of the method may be further performed as described herein. The method may include any other step(s) of any other method(s) described herein. The method may be performed by any of the systems described herein.
Further advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description of the preferred embodiments and upon reference to the accompanying drawings in which:
a are schematic diagrams illustrating side views of embodiments of an imaging system configured as described herein; and
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and are herein described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
Turning now to the drawings, it is noted that the figures are not drawn to scale. In particular, the scale of some of the elements of the figures is greatly exaggerated to emphasize characteristics of the elements. It is also noted that the figures are not drawn to the same scale. Elements shown in more than one figure that may be similarly configured have been indicated using the same reference numerals. Unless otherwise noted herein, any of the elements described and shown may include any suitable commercially available elements.
In general, the embodiments described herein are sensors, imaging systems, and methods for forming a sensor. More specifically, the embodiments described herein are methods of sensor assembly and shape control for applications such as inspection and metrology. The embodiments provide image sensors including, but not limited to, time delay integration (TDI) sensors for deep ultraviolet (DUV) and extreme ultraviolet (EUV) applications that can operate in partial vacuum or other controlled environment and that are assembled on a variety of ceramic substrates. The embodiments described herein advantageously show how to implement such sensors with controlled sensor shape, while maintaining relatively high-speed, relatively low-noise operation. In addition, the embodiments described herein provide methods for sensor shape control while enabling substantially good thermal contact for the sensor die, flip-chip assembly with substantially high density interconnects, and suitability for applications in a vacuum.
The term “energy sensitive elements” as used herein is defined as sensor elements that are sensitive or responsive to one of the types of energy described herein including light, electrons, other charged particles, and the like. These energy sensitive elements may be formed of different components depending on the type of energy that they will be used to detect. Although many embodiments and examples are described herein using the term “light sensitive elements,” any use of that term is not meant to exclude those embodiments and examples to any other types of energy sensitive elements described herein. In other words, the terms “energy sensitive elements” and “light sensitive elements” are used interchangeably herein for ease, and any instance of the term “light sensitive elements” should be interpreted more broadly as “energy sensitive elements” described herein.
The embodiments also include imaging systems such as inspection systems based on such sensors to thereby achieve superior imaging performance and consequently higher defect sensitivity and throughput compared to currently available inspection systems. Enabling sensor assembly with a controlled curvature as described herein can advantageously increase the optical field of view (FOV) of an imaging system, relax imaging system level optical tolerances, and reduce the amount of optical aberrations in the systems. The embodiments described herein also advantageously enable relatively large sensors and tiled sensor arrays designed for curved image spaces or other non-flat image spaces, which provide higher sensitivity for applications such as inspection with sufficient throughput (and thus lower cost-of-ownership).
As will be seen from the following description of various embodiments, the sensors described herein have a number of additional advantages over currently used sensors. These additional advantages include that although the shape of a ceramic substrate may not be easily controlled, sensor dies can be bonded to such substrates as described herein with substantially high sensor die planarity or a pre-selected, certain sensor die shape thereby rendering the sensors particularly suitable for substantially high performance optical designs. Another advantage of the embodiments described herein is that regardless of a pre-selected sensor die shape, the embodiments described herein enable sufficient contact with the sensor die, which in turn optimizes thermal performance of the assembly thereby enabling relatively high-speed, relatively low-noise operation of the sensor die. A further advantage of the embodiments described herein is that despite any field curvature in an imaging system, the sensor dies can enable a telecentric image space, which is important for metrology applications. These and other advantages described herein are provided by the sensor assembly methods described herein that allow substantially accurate control of the sensor die shape.
As described further herein, the sensor die embodiments have a thinned backside and light sensitive elements configured for detecting energy illuminating the thinned backside of the sensor die. The energy that is detected by the sensor die is directed to the thinned backside and then through the body of the sensor die so that charge can be collected by elements formed on the frontside. The back surface profile defines the location of the image plane of the sensor. As a result, the shape of the sensor die is arguably one of the most important characteristics of the sensor die for reasons including those described further herein.
As used herein, the term “pre-selected shape of the thinned backside of the sensor die” is used interchangeably herein with the terms “sensor shape” and “sensor die shape.” The “pre-selected shape of the thinned backside of the sensor die” is defined herein as a position of the thinned backside of the sensor die with respect to some reference or coordinate system and as a function of position across the sensor die. For example, the “pre-selected shape of the thinned backside of the sensor die” can be defined by the depth or vertical height of the thinned backside of the sensor die as a function of position across the sensor die. This depth or height function may be defined in two dimensions (2D) across the sensor die. Therefore, the “pre-selected shape of the thinned backside of the sensor die” also defines a 2D profile of the sensor backside height or depth. In other words, the “pre-selected shape of the thinned backside of the sensor die” effectively defines the position of the backside of the sensor die in z and as a function of x and y.
As described further herein, the “pre-selected shape of the thinned backside of the sensor die” is enabled by the novel and advantageous sensor assembly methods described herein, which preferably do not alter the thickness or vertical height of the sensor membrane (the light sensitive elements) in any way. The shape of the membrane is constant, and the shape in other dimensions is fixed. In other words, the sensor assembly methods described herein do not alter a shape of the sensor die by altering any characteristics of the light sensitive elements (although some negligible alterations may occur). Instead, the light sensitive elements preferably have the same characteristics prior to sensor assembly and after. In this manner, the changes in the height (or depth) of the backside of the sensor die after sensor assembly will result in a similar change in height (or depth) of the frontside of the sensor die. Conversely, changes in the frontside profile due to attachment to another component will result in similar changes to the backside profile.
One embodiment of a sensor includes a substrate, one or more components attached to the substrate, and a sensor die bonded to the substrate. In one embodiment, therefore, the image sensor die is co-packaged with other dies assembled onto a common substrate as shown in
As further shown in side view 100, sensor die 108 has backside 114 that is thinned (not shown in
As described further herein, the embodiments are particularly suitable for situations in which the sensor die is operating within a vacuum such as embodiments in which the energy sensitive elements are configured for detecting DUV light, vacuum or extreme ultraviolet (VUV/EUV) light, an electron beam, and/or x-rays. The embodiments are also suitable for non-vacuum applications such as when the energy sensitive elements are configured for detecting visible or infrared (IR) light.
The sensor die can be configured as a charge coupled device (CCD), a TDI sensor, or a complementary metal-oxide-semiconductor (CMOS) image sensor die. The sensor die can also be made of silicon (Si), indium gallium arsenide (InGaAs), indium antimonide (InSb), cadmium telluride (CdTe), or any other suitable compounds for energy detection across a spectrum including, but not limited to, x-rays, VUV light, DUV light, visible light, and IR light. While some of the embodiments may be described herein with respect to silicon-based sensor dies, the embodiments described herein can be applied to sensors made of any other suitable material(s).
In one embodiment, one or more components 110 are configured for performing one or more functions on output generated by energy sensitive elements 116 responsive to energy detected by the energy sensitive elements. The one or more other components (or other dies) can be analog-to-digital (A/D) chips, digital-to-analog components (DAC), image signal processing dies, application-specific integrated circuits (ASICs), or a combination thereof. The one or more functions performed by the one or more components may include, for example, amplification, A/D conversion, signal conditioning, digital image processing, and communication to an external computer. Therefore, the one or more functions may be as simple as transferring the output of the sensor die to a component external to the sensor assembly or may involve transforming the output from one type to another, altering the sensor output in some manner, etc. The assembly can use a variety of interfaces (not shown in
As shown in step a), ceramic substrate 200 is made. In this step, the substrate will generally be made thicker than the nominal design to include sacrificial material that is removed in the next step. In step b), the top side of the substrate is polished to the desired shape thereby forming ceramic substrate 202 having a polished top side. This polishing process may expose internal vias (not shown in
In step d), metal 206 is deposited and patterned on the top side of the substrate. Patterning on a flat surface can be achieved using a standard lithographic method known in the art. Patterning on a concave surface can be achieved using a direct imaging method for patterning such as Direct Imaging, also known in the art. In step e), the same process as in step d) may be implemented on the bottom side of the substrate thereby forming metal 208 on the bottom side of the substrate. Metals 206 and 208 may be formed of any suitable material known in the art and may have any suitable configuration known in the art. In step f), one or more components 210 such as ASIC chips are assembled onto the bottom side of the substrate using any suitable flip-chip process known in the art.
The method includes forming discrete thermally-conductive structures on a substrate. For example, in step g), solder bumping is performed on the top surface to thereby form discrete thermally-conductive structures 212 on the top side of the substrate. The method also includes altering a shape of the discrete thermally-conductive structures based on a pre-selected shape of a thinned backside of a sensor die. For example, in step h), the discrete thermally-conductive structures, e.g., solder balls, may be stamped (coined) by tool 214 with surface 216 polished into the desired shape (a curved shape shown in
The method further includes bonding the frontside of the sensor die to the substrate via the discrete thermally-conductive structures thereby causing the thinned backside of the sensor die to have the pre-selected shape. In this manner, the sensor includes discrete thermally-conductive structures formed between the frontside of the sensor die and the substrate by a flip-chip process thereby bonding the sensor die to the substrate and causing the thinned backside of the sensor die to have a pre-selected shape. At least a portion of the discrete thermally-conductive structures electrically connect the sensor die to one or more components 210 attached to substrate 204. For example, not all of the discrete thermally-conductive structures may connect electrically to components or devices. In the region where the light sensitive elements are formed (e.g., the membrane region), the conductive structures may provide mechanical and thermal benefits, but may not have an electrical purpose. The sensor die may be further configured as described herein. For example, as shown in step i), sensor die 220 may have thinned backside 222, frontside 224, and energy sensitive elements (not shown in
In step i), the periphery of sensor die 220 is soldered to the substrate using a contact method such as hot-bar or similar. For example, hot-bar 226 may press down on backside 222 of the sensor die in the direction indicated by arrows 228 so that the periphery of the sensor die comes into contact with and is soldered to only the discrete thermally-conductive structures near the periphery of the sensor die. In this manner, after this step, the sensor die may be bonded to only a portion of the discrete thermally-conductive structures, and the sensor die may be bonded to the remaining discrete thermally-conductive structures in a later step.
In one embodiment, the sensor includes an underfill material formed around the discrete thermally-conductive structures and between the frontside of the sensor die and the substrate. In one such embodiment, the underfill material is configured to stabilize the sensor die when the sensor die is subject to a vacuum. For example, one new and advantageous feature of the embodiments described herein is that a curved image sensor packaged onto a ceramic substrate with an underfill allows for vacuum based operation. As shown in step j), underfill resin 230 is applied between sensor die 220 and the substrate to reinforce the solder joints. In this manner, the underfill resin may stabilize the solder joints thereby helping to maintain the shape of the sensor die even in the presence of a vacuum or other pressure applied to the sensor or to which the sensor is otherwise exposed. Such vacuum exposure of the sensor may be needed, for example, if the energy being detected is VUV light, EUV light, electrons, etc. In step k), flow cell 232 is used to pressurize sensor die 220 and establish a contact between the thinner portion of the sensor die and the substrate. Curing of the underfill can be applied at this step as well.
As shown in steps j and k), the backside of the sensor die may have a curved shape after the sensor die has been bonded to all of the discrete thermally-conductive structures, which have been stamped or coined to have, in combination, the pre-selected shape. One new and advantageous feature of the embodiments described herein is therefore that a back-thinned curved image sensor can be assembled via a flip-chip process, which allows both backside illumination for DUV, VUV, etc., and substantially high speed operation.
The steps shown in
Each of the steps of the method may be further performed as described herein. The method may also include any other step(s) that can be performed by the sensor, imaging system, computer subsystem, component(s), etc., described herein. The sensor formed by the method described above and an imaging system in which it is included may be configured according to any of the embodiments described herein. The method may be performed by any of the system embodiments described herein.
The general idea of assembly of an integrated circuit (IC) onto a substrate with coined solder bumps is described in U.S. Patent Application Publication No. 2012/0309187 by Sri-Jayantha et al. published Dec. 6, 2012, which is incorporated by reference as if fully set forth herein. However, a challenge of implementing such a method for an image sensor stems from the fact that mechanical contacting of the sensor die surface is undesired as it may damage the pixel array and result in relatively low assembly yield.
In step 300 of
The interface material will then be coined using a forming tool. For example, in step 312, solder bumps 306 may be stamped or coined by tool 314 that is moved in a direction shown by arrow 316 to bring the tool into contact with and exert force upon the solder bumps. In a similar manner, in step 318, gold studs 310 may be stamped or coined by tool 320 that is moved in a direction shown by arrow 322 to bring the tool into contact with and exert force upon the gold studs.
One new and advantageous feature of the embodiments described herein is that they enable substantially precise control of back-thinned image sensor shape using stamping/coining of under-membrane bumps, which allows tailoring the sensor shape to a particular field curvature. For example, in one embodiment, prior to bonding the sensor die to the substrate in the flip-chip process, the discrete thermally-conductive structures are formed on the substrate and a shape of one or more of the discrete thermally-conductive structures is modified so that the discrete thermally-conductive structures in combination have a shape that is substantially the same as the pre-selected shape. In another embodiment, the pre-selected shape is determined prior to the flip-chip process, and a shape of one or more of the discrete thermally-conductive structures formed on the substrate prior to bonding the sensor die to the substrate in the flip-chip process is altered based on the pre-selected shape. As shown in
In one embodiment, a surface of the substrate on which the discrete thermally-conductive structures are formed has a shape different than the pre-selected shape. For example, in the embodiment shown in
In contrast, as shown in
The forming tool and the shape of the discrete thermally-conductive structures will determine the exact shape of the sensor die that will consequently be assembled onto the substrate. When the sensor die is placed onto the shaped discrete thermally-conductive structures, the discrete thermally-conductive structures are heated to reflow the discrete thermally-conductive structures and establish a permanent connection between the sensor die and the substrate. While heated, the sensor die has to be pressed against the substrate, which can be performed by a variety of means such as pressing with a hot bar on the periphery of the sensor die or by applying a gas pressure through a dedicated fixture. In the embodiments of
The embodiments described further herein show practical examples where the sensor die is designed to have particular shapes. As described further herein, in one embodiment, the pre-selected shape is a curved shape. In another embodiment, the pre-selected shape is defined by a higher order polynomial. For example, it is worth highlighting the example where the sensor shape can be described by high-order polynomials, enabling greater flexibility in an optical system design.
In some embodiments, a surface of the substrate on which the discrete thermally-conductive structures are formed has a shape determined based on the pre-selected shape. For example, if the ceramic substrate substantially deviates from the desired shape and the solder balls cannot bridge the gap between the desired sensor die shape and the shape of the ceramic, the ceramic substrate can be polished to the desired shape and the top metal pattern that defines the solder pads can be patterned on the surface. This may be performed as described and shown in steps b) and d) described above with reference to
In some embodiments, the substrate is formed of a material selected based on a coefficient of thermal expansion (CTE) for the material determined from a size of the sensor die and the pre-selected shape. In another embodiment, the discrete thermally-conductive structures are formed of a material selected based on a reflow temperature for the material determined from a size of the sensor die and the pre-selected shape. Methods for selecting the substrate and solder materials are now described that can help ensure that the assembled sensor is capable of meeting performance requirements.
Since the reflow of solder happens at an elevated temperature, both the sensor die and the ceramic substrate shrink when cooling to room temperature or the operating temperature. This change in temperature poses several challenges to both achieve the desired die shape and also assure solder joint reliability due to stress. The conditions for substantially stress-free solder joints can be determined based on geometric considerations using a simple 1D model.
An embodiment showing an initially flat sensor die and fully compliant final shape are shown in
In this example, the stress in the outer solder joints will be minimized when the differences (Lc′−Ls′) can be minimized. Stress-free solder joints are achieved when Lc′−Ls′=0. The requirements to meet this condition can be modified from simple geometric considerations. For the example illustrated in 1D and assuming no bending of the ceramic substrate, we obtain
where R is the target radius of curvature of the die, αC and αS are the coefficients of thermal expansion (CTE) for the ceramic and the silicon die, and ΔT is the temperature difference from the reflow in step j) to step k) of
While the example is a simplified case with several approximations, it illustrates the method of selecting key process conditions to minimize solder stress in the proposed assembly. For an actual 2D geometry, the conditions may be obtained using numerical modeling. In the simple case of a flat sensor die (R is infinity), optimum conditions are obtained when there is a close CTE match between sensor die and ceramic substrate. Ceramic IC substrates with CTE closely matching silicon are also commercially available from multiple suppliers. These materials span a range of CTE and include both oxide and non-oxide ceramics. Non-oxide ceramics include aluminum nitride (CTE ˜4.4-4.7 ppm/° C.), silicon carbide (CTE ˜3.7-3.9 ppm/° C.), and silicon nitride (CTE ˜2.8-3.5 ppm/° C.). The ranges indicate a variety of compositions with different CTE for each type. Oxide-based ceramics such as those commercially available from Kyocera Corporation, Kyoto, Japan, include materials with a CTE ranging from 3.4 to 12.3 ppm/° C. For a silicon die, the CTE is 2.6 ppm/° C. For a given die size and curvature, the material with an optimum CTE can be chosen via any suitable numerical modeling known in the art.
To further minimize the mismatch, solder with the desired reflow temperature is used. A variety of solder materials are commercially available spanning the entire temperature range from ˜60° C. to over 220° C. These include indium-bismuth-tin (In—Bi—Sn), indium-bismuth (In—Bi), indium-tin (In—Sn), tin-silver-copper (SACx), and other alloys, commercially available from multiple suppliers. It becomes clear how choosing proper solder and ceramic materials enables to select parameters αC and ΔT and therefore approach the condition of substantially stress-free solder joints. The discrete thermally-conductive structures made of solder may also contain multiple materials with distinguishable differences in melting points. One of the materials may advantageously be a solder with a higher melting point that would more easily maintain its desired shape, and another of the materials may be a solder with a lower melting point that can make electrical connection to components or devices at a much lower temperature. Such combinations of materials may be selected in any suitable manner from any suitable commercially available solder materials.
For substantially fast operation, it is necessary to bring a relatively large number of interconnect signals from the sensor die to the one or more components, e.g., ASICs. Driving the many circuits requires transistors. An example of such an implementation is provided in U.S. Pat. No. 10,764,527 to Chuang et al. issued Sep. 1, 2020, which is incorporated by reference as if fully set forth herein. The sensors described herein may be further configured as described in this patent. To enable substantially fast operation and a relatively large number of signals, the packaging technology must offer a relatively high density of routing in the ceramic substrate and relatively low channel parasitic capacitance. Both are enabled simultaneously by flip-chip assembly of the sensor die on a low-temperature cofired ceramic (LTCC). Interconnects at 150 um pitch and below are available in such technology. Such interconnects can enable in excess of 10 Gigasamples/second (GS/s) of data to be transferred across the ceramic substrate to the ASICs.
Overlaid arrows in this figure indicate the magnitude and direction of heat flux within the sensor assembly. The method of assembly embodiments described herein have thermal advantages for the sensor assembly. For example, bumps 610 supporting the sensor die not only define the sensor die shape, but also serve as an efficient thermal conduit of the heat generated by the image sensor die. The gap between the sensor die and the ceramic substrate may be filled with a resin that solidifies and reinforces the solder connections for reliability. All resins developed in the electronics industry for that purpose exhibit relatively low thermal conductivity, typically below 1 W/mK (Watt per meter-Kelvin). The thermal conductivity of solder bumps or gold pillars is substantially higher, meaning most of the heat flux will be conducted by these discrete thermally-conductive structures.
In one embodiment, the sensor includes thermally and electrically conductive vias formed in the substrate with at least a subset configured for connecting the at least the portion of the discrete thermally-conductive structures to the one or more components thereby connecting the sensor die to the one or more components. For example, to further improve the thermal performance of the sensor assembly, ceramic substrate 602 may include an array of thermally and electrically conductive vias (not shown in
Another embodiment relates to an imaging system. In general, the imaging system includes an energy source (e.g., a light source, an electron beam source, etc.) configured for generating energy directed to a specimen by an illumination subsystem. Such an energy source and illumination subsystem may be configured as described further herein and shown in
The system also includes a sensor configured for detecting energy from the specimen and for generating output responsive to the detected energy. The sensor is configured as described further herein. The energy that is detected by the sensor may include any energy described herein such as electrons, charged particles, x-rays, VUV light, EUV light, DUV light, visible light, and IR light. As described further herein, the type of energy that is detected by the sensor may also include specularly reflected light, scattered light, or both, depending on the configuration of the system. The output that is generated by the sensor may include any suitable output such as image data, image signals, non-image data, non-image signals, etc., or some combination thereof. The sensor and one or more elements of the imaging system that are coupled to it may be further configured as described herein.
In one embodiment, the imaging system includes a camera lens subsystem configured to direct the energy from the specimen to the sensor. For example, an image sensor having a pre-selected curved shape may be used in a camera lens system as shown in
Although the camera lens subsystems are shown in
The aperture stop of a design based on a curved image sensor die can also be increased from that based on a flat sensor die. In the example provided in
In another embodiment, the system includes a tube lens subsystem configured to direct the energy from the specimen to the sensor. Such a configuration may be used for a tube lens in a DUV camera used with a microscope. In this embodiment, an image sensor having a pre-selected shape is used with a tube lens subsystem, as shown in
Although the tube lens subsystems are shown in
A tube lens subsystem that includes three elements was optimized for two cases, one where sensor die curvature is allowed as shown in
Aberrations of such designs are about 10 times lower compared with an equivalent design based on a planar image sensor die, as shown in plot 832 of RMS spot size in
As to the exact values shown in the plot of
The embodiments described herein may include an array of curved sensors. For example, the embodiments described herein may include two or more of the sensors described herein, whose sensor dies may have the same characteristics such as the same pre-selected shape, size, etc. or may have one or more different characteristics, such as different pre-selected shapes and/or different sizes. In one such embodiment, the imaging system includes an additional sensor configured for detecting additional energy from the specimen and for generating output responsive to the additional detected energy. The additional sensor may be configured for detecting the additional energy and generating the output as described further herein. The energy detected by the two sensors may have one or more different characteristics such as type of energy (scattered vs. specularly reflected), wavelength, polarization, etc. For example, as described further below, different detection channels may include different sensors, and each of the sensors may be configured as described herein. However, one particularly advantageous implementation of the multiple sensor embodiment is to have multiple sensors coupled to the same collector or collection subsystem so that the multiple sensors detect energy in the same image plane, even if that image plane is curved or has some other non-flat shape.
The additional sensor includes an additional substrate and one or more additional components attached to the additional substrate. The additional sensor also includes an additional sensor die having a thinned backside and additional energy sensitive elements configured for detecting the additional energy from the specimen illuminating the thinned backside of the additional sensor die. In addition, the additional sensor includes additional discrete thermally-conductive structures formed between a frontside of the additional sensor die and the additional substrate by a flip-chip process thereby bonding the additional sensor die to the additional substrate and causing the thinned backside of the additional sensor die to have an additional pre-selected shape. At least a portion of the additional discrete thermally-conductive structures electrically connect the additional sensor die to the one or more additional components. Each of these elements of the additional sensor may be further configured as described herein.
Using more than one of the sensor embodiments described herein in a single detection channel can be particularly advantageous in some situations such as when the light from the specimen is directed to a relatively large area in the image plane and/or when the image plane has a curvature that is not easily achievable by a single sensor. In any case, the pre-selected shape and the additional pre-selected shape (the pre-selected shapes of the backsides of the sensor dies in different sensors) may be different or the same. In addition, as described further herein, in some embodiments, the imaging system is configured to independently control positions of the sensor and the additional sensor in the imaging system. The imaging system may be configured to control the position of each of the sensors in any suitable manner using any appropriate software and/or hardware known in the art of image system control.
The amount of surface sag shown in
While
It is also noted that the various numerical values in
The checker-board sensor pattern shown in this embodiment is generally useful in both raster-scan and step-and-repeat inspection systems. In the former case of a raster-scan, the specimen being inspected is moving in the y-direction (referenced in
The embodiments described herein are also effective for reducing distortion-induced blur in scanning inspection systems. In some embodiments, the imaging system includes a scanning subsystem configured to scan the energy directed to the specimen by the illumination subsystem over the specimen, the illumination subsystem has a FOV on the specimen having substantially no field curvature, and the pre-selected shape of the sensor die is a curved shape. The scanning and illumination subsystems may be further configured as described herein.
To illustrate the advantages of such an embodiment,
Since the specimen is scanned with respect to the optics and the sensor, the defects of interest (DOI) at the edges of the field may be smeared across multiple pixels. For example, if light from defects 1004 and 1006 on a specimen (not shown in
In one such embodiment, the sensor is configured as a TDI sensor. For example, the imaging systems described herein configured for inspection will typically employ a TDI sensor, which accumulates the optical signal as the image is scanned across the pixel array. This will result in a larger effective point spread function (PSF) and therefore lower resolution and lower signal-to-noise ratio (SNR). Conversely, assembling an image sensor onto a curved surface will mimic the distortion of the system. Therefore, such a sensor projected onto the object plane will correspond to a nearly perfect grid as shown by perfect grid 1002 thereby improving the resolution and the SNR away from the center of the field. For example, as shown in
One embodiment of an imaging system is shown in
In general, the imaging systems described herein include at least an energy source, a sensor, and a scanning subsystem. The energy source is configured for generating energy directed to a specimen by an illumination subsystem. The sensor is configured for detecting energy from the specimen and for generating output responsive to the detected energy. The scanning subsystem is configured to change a position on the specimen to which the energy is directed and from which the energy is detected. In one embodiment, as shown in
In the embodiment of the imaging system shown in
The illumination subsystem may be configured to direct the light to the specimen at different angles of incidence at different times. For example, the imaging system may be configured to alter one or more characteristics of one or more elements of the illumination subsystem such that the light can be directed to the specimen at an angle of incidence that is different than that shown in
In some instances, the imaging system may be configured to direct light to the specimen at more than one angle of incidence at the same time. For example, the illumination subsystem may include more than one illumination channel, one of the illumination channels may include light source 1108, optical element 1110, and lens 1112 as shown in
In another instance, the imaging system may include only one light source (e.g., source 1108 shown in
Light source 1108 may include a narrowband source such as a laser or a plasma source such as an EUV or broadband plasma (BBP) light source. In this manner, the light generated by the light source and directed to the specimen may include narrowband or broadband light. The light source may also include a laser design known in the art and configured to generate light at any suitable wavelength(s). The laser may be configured to generate light that is monochromatic or nearly monochromatic. In this manner, the laser may be a narrowband laser. The light source may also include a polychromatic light source that generates light at multiple discrete wavelengths or wavebands.
Light from optical element 1110 may be focused onto specimen 1106 by lens 1112. Although lens 1112 is shown in
The imaging system may also include a scanning subsystem configured to change the position on the specimen to which the light is directed and from which the light is detected and possibly to cause the light to be scanned over the specimen. For example, the imaging system may include stage 1114 on which specimen 1106 is disposed during imaging. The scanning subsystem may include any suitable mechanical and/or robotic assembly (that includes stage 1114) that can be configured to move the specimen such that the light can be directed to and detected from different positions on the specimen. In addition, or alternatively, the imaging system may be configured such that one or more optical elements of the imaging system perform some scanning of the light over the specimen such that the light can be directed to and detected from different positions on the specimen. In instances in which the light is scanned over the specimen, the light may be scanned over the specimen in any suitable fashion such as in a serpentine-like path or in a spiral path.
The imaging system further includes one or more detection channels. At least one of the detection channel(s) includes a sensor configured to detect light from the specimen due to illumination of the specimen by the imaging system and to generate output responsive to the detected light. For example, the imaging system shown in
As further shown in
Although
As described further above, each of the detection channels included in the imaging system may be configured to detect scattered light. Therefore, the imaging system shown in
The sensors included in the one or more detection channels may be configured according to any of the embodiments described herein. The output that is generated by each of the sensors included in each of the detection channels of the imaging system may be image signals or image data or any other suitable output known in the art. In addition, although each of the detection channels is shown in
It is noted that
Computer subsystem 1102 may be coupled to the sensors of the imaging system in any suitable manner (e.g., via one or more transmission media, which may include “wired” and/or “wireless” transmission media) such that the computer subsystem can receive the output generated by the sensors. Computer subsystem 1102 may be configured to perform a number of functions with or without the output of the sensors including the steps and functions described further herein. As such, the steps described herein may be performed “on-tool,” by a computer subsystem that is coupled to or part of an imaging system. In addition, or alternatively, computer system(s) 1104 may perform one or more of the steps described herein. Therefore, one or more of the steps described herein may be performed “off-tool,” by a computer system that is not directly coupled to an imaging system. Computer subsystem 1102 and computer system(s) 1104 may be further configured as described herein.
Computer subsystem 1102 (as well as other computer subsystems described herein) may also be referred to herein as computer system(s). Each of the computer subsystem(s) or system(s) described herein may take various forms, including a personal computer system, image computer, mainframe computer system, workstation, network appliance, Internet appliance, or other device. In general, the term “computer system” may be broadly defined to encompass any device having one or more processors, which executes instructions from a memory medium. The computer subsystem(s) or system(s) may also include any suitable processor known in the art such as a parallel processor. In addition, the computer subsystem(s) or system(s) may include a computer platform with high-speed processing and software, either as a standalone or a networked tool.
If the system includes more than one computer subsystem, then the different computer subsystems may be coupled to each other such that images, data, information, instructions, etc. can be sent between the computer subsystems. For example, computer subsystem 1102 may be coupled to computer system(s) 1104 as shown by the dashed line in
Although the imaging system is described above as including an optical or light-based energy source, in another embodiment, the energy source is configured as an electron beam source. In such an imaging system, the energy directed to the specimen includes electrons, and the energy detected from the specimen includes electrons. In one such embodiment shown in
As also shown in
Electrons returned from the specimen may be focused by one or more elements 1138 to sensor 1140. One or more elements 1138 may include, for example, a camera lens subsystem or a tube lens subsystem, which may be configured as described herein. Sensor 1140 may be configured according to any of the embodiments described herein. In addition, sensor 1140 may be replaced with a sensor array such as that shown in
The electron column may include any other suitable elements known in the art. In addition, the electron column may be further configured as described in U.S. Pat. No. 8,664,594 issued Apr. 4, 2014 to Jiang et al., U.S. Pat. No. 8,692,204 issued Apr. 8, 2014 to Kojima et al., U.S. Pat. No. 8,698,093 issued Apr. 15, 2014 to Gubbens et al., and U.S. Pat. No. 8,716,662 issued May 6, 2014 to MacDonald et al., which are incorporated by reference as if fully set forth herein.
Although the electron column is shown in
Computer subsystem 1130 may be coupled to sensor 1140 as described above. The sensor may detect electrons returned from the surface of the specimen thereby forming images of (or other output for) the specimen. Computer subsystem 1130 may be configured to perform one or more functions on output generated by sensor 1140, which may be performed as described further herein. Computer subsystem 1130 may be configured to perform any additional step(s) described herein. A system that includes the imaging system shown in
It is noted that
Although the imaging system is described above as including a light or electron beam energy source, the imaging subsystem may include an ion beam energy source. Such an imaging system may be configured as shown in
As further noted above, the imaging system may be configured to have multiple modes. In general, a “mode” is defined by the values of parameters of the imaging system used to generate output for the specimen. Therefore, modes that are different may be different in the values for at least one of the imaging parameters of the imaging system (other than position on the specimen at which the output is generated). For example, for a light-based imaging system, different modes may use different wavelengths of light. The modes may be different in the wavelengths of light directed to the specimen as described further herein (e.g., by using different light sources, different spectral filters, etc. for different modes). In another embodiment, different modes may use different illumination channels. For example, as noted above, the imaging system may include more than one illumination channel. As such, different illumination channels may be used for different modes.
The multiple modes may also be different in illumination and/or collection/detection. For example, as described further above, the imaging system may include multiple sensors. Therefore, one of the sensors may be used for one mode and another of the sensors may be used for another mode. Furthermore, the modes may be different from each other in more than one way described herein (e.g., different modes may have one or more different illumination parameters and one or more different detection parameters). In addition, the multiple modes may be different in perspective, meaning having either or both of different angles of incidence and angles of collection, which are achievable as described further above. The imaging system may be configured to scan the specimen with the different modes in the same scan or different scans, e.g., depending on the capability of using multiple modes to scan the specimen at the same time.
In some instances, the imaging systems described herein may be configured as inspection systems. However, the imaging systems described herein may be configured as another type of semiconductor-related quality control type system such as a defect review system and a metrology system. For example, the embodiments of the imaging systems described herein and shown in
As noted above, the imaging system is configured for directing energy (e.g., light, electrons) to and/or scanning energy over a physical version of the specimen thereby generating actual images for the physical version of the specimen. In this manner, the imaging system may be configured as an “actual” imaging system, rather than a “virtual” system. However, a storage medium (not shown) and computer system(s) 1104 shown in
In one embodiment, the imaging system includes a computer subsystem configured to determine information for the specimen based on the output generated by the sensor. For example, the imaging system shown in
In another embodiment, the imaging system includes a computer subsystem configured to detect defects on the specimen based on the output generated by the sensor. In general, the output generated by the sensor may be used for defect detection in the same manner as any other images. In other words, the output generated by the sensors described herein is not defect detection algorithm or method specific, and detecting the defects using the output may be performed using any suitable defect detection algorithm or method known in the art. For example, defect detection may be performed by subtracting a reference from the output to thereby generate a difference image and applying a threshold to the difference image. Any pixels in the difference image having a value above the threshold may be identified as a defect, and all other pixels may not be identified as a defect. Of course, this is possibly the most simple way in which defect detection can be performed and is included herein as merely a non-limiting example.
In some embodiments, therefore, detecting defects on the specimen may include generating or determining information for the specimen, which may include information for any defects detected on the specimen. In such instances, the information may include, for example, a type of defect detected, a position of a detected defect with respect to one or more of the specimen image, the specimen, the imaging system, and a design for the specimen, and any other information generated for the defect by the defect detection method or algorithm and/or the computer subsystem. The information determined by the computer subsystem may also or alternatively include any suitable defect attributes, e.g., classification, size, shape, etc., (other than reported defect location) that can be determined from the output described herein and/or its alignment to other information for the specimen such as design data. Such information may be output and/or stored by the computer subsystem as described further herein.
Unlike inspection processes, a defect review process generally revisits discrete locations on a specimen at which a defect has been detected. An imaging system configured for defect review may generate specimen images as described herein, which may be input to the computer subsystem as described herein for one or more defect review functions such as defect re-detection, defect attribute determination, defect classification, and defect root cause determination. For defect review applications, the computer subsystem may also be configured for using any suitable defect review method or algorithm used on any suitable defect review tool to determine information for the defect or the specimen from the sensor output, possibly in combination with any other information determined by the defect review process or from the sensor output.
In some embodiments, the imaging system may be configured for metrology of the specimen. In one such embodiment, the information includes a measurement of one or more structures formed on the specimen. For example, the imaging systems described herein may be configured as metrology tools, and the sensor output generated by such a metrology tool can be used to determine metrology information for the specimen. The metrology information may include any metrology information of interest, which may vary depending on the structures on the specimen. Examples of such metrology information include, but are not limited to, critical dimensions (CDs) such as line width and other dimensions of the specimen structures. For metrology applications, the computer subsystem may also be configured for using any suitable metrology method or algorithm used on any suitable metrology tool to determine information for the specimen from the sensor output, possibly in combination with any other information determined by the metrology process or from the sensor output.
The computer subsystem may also be configured for generating results that include the determined information, which may include any of the results or information described herein. The results of determining the information may be generated by the computer subsystem in any suitable manner. All of the embodiments described herein may be configured for storing results of one or more steps of the embodiments in a computer-readable storage medium. The results may include any of the results described herein and may be stored in any manner known in the art. The results that include the determined information may have any suitable form or format such as a standard file type. The storage medium may include any storage medium described herein or any other suitable storage medium known in the art.
After the results have been stored, the results can be accessed in the storage medium and used by any of the method or system embodiments described herein, formatted for display to a user, used by another software module, method, or system, etc. to perform one or more functions for the specimen or another specimen of the same type. For example, results produced by the computer subsystem may include information for any defects detected on the specimen such as location, etc., of the bounding boxes of the detected defects, detection scores, information about defect classifications such as class labels or IDs, any defect attributes determined from any of the images, etc., predicted specimen structure measurements, dimensions, shapes, etc. or any such suitable information known in the art. That information may be used by the computer subsystem or another system or method for performing additional functions for the specimen and/or the detected defects such as sampling the defects for defect review or other analysis, determining a root cause of the defects, etc.
Such functions also include, but are not limited to, altering a process such as a fabrication process or step that was or will be performed on the specimen in a feedback or feedforward manner, etc. For example, the computer subsystem may be configured to determine one or more changes to a process that was performed on the specimen and/or a process that will be performed on the specimen based on the determined information. The changes to the process may include any suitable changes to one or more parameters of the process. In one such example, the computer subsystem preferably determines those changes such that the defects can be reduced or prevented on other specimens on which the revised process is performed, the defects can be corrected or eliminated on the specimen in another process performed on the specimen, the defects can be compensated for in another process performed on the specimen, etc. The computer subsystem may determine such changes in any suitable manner known in the art.
Those changes can then be sent to a semiconductor fabrication system (not shown) or a storage medium (not shown) accessible to both the computer subsystem and the semiconductor fabrication system. The semiconductor fabrication system may or may not be part of the system embodiments described herein. For example, the imaging subsystem and/or the computer subsystem described herein may be coupled to the semiconductor fabrication system, e.g., via one or more common elements such as a housing, a power supply, a specimen handling device or mechanism, etc. The semiconductor fabrication system may include any semiconductor fabrication system known in the art such as a lithography tool, an etch tool, a chemical-mechanical polishing (CMP) tool, a deposition tool, and the like.
Each of the embodiments of each of the systems described above may be combined together into one single embodiment.
An additional embodiment relates to a non-transitory computer-readable medium storing program instructions executable on a computer system for performing a computer-implemented method for determining information for a specimen. One such embodiment is shown in
Program instructions 1202 for the algorithms implementing methods such as those described herein may be stored on computer-readable medium 1200. The computer-readable medium may be a storage medium such as a magnetic or solid-state disk, a magnetic tape, or any other suitable non-transitory computer-readable medium known in the art.
The algorithms may be implemented in any of various ways, including procedure-based techniques, component-based techniques, object-oriented techniques, implementation of neural network architectures, among others. For example, the program instructions may be implemented using suitable programming frameworks and languages known in the art such as C, C++, or Python, and executed on local, remote, or centrally managed computation systems, or a combination of these systems. Custom accelerators may be implemented in application-specific integrated circuit devices (ASIC chips), in field-programmable gate arrays (FPGAs) with custom configurations, or in graphics processing units (GPUs), separately or in combination as desired.
Computer system(s) 1204 may be configured according to any of the embodiments described herein.
Further modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. For example, sensors, imaging systems, and methods for forming a sensor are provided. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims.
| Number | Date | Country | |
|---|---|---|---|
| 63210082 | Jun 2021 | US |
