The present application relates to image sensors suitable for sensing radiation in deep UV (DUV) and vacuum UV (VUV), and to methods for fabricating/producing such image sensors. Some embodiments of the sensors are suitable for sensing electrons and other charged particles. All of the sensors are suitable for use in photomask, reticle, or wafer inspection systems.
The integrated circuit industry requires inspection tools with increasingly higher resolution to resolve ever smaller features of integrated circuits, photomasks, reticles, solar cells, charge coupled devices etc., as well as detect defects whose sizes are of the order of, or smaller than, those feature sizes.
Inspection systems operating at short wavelengths, e.g., wavelengths shorter than about 250 nm, can provide such resolution in many cases. In other cases, electrons or other charged particles, such as helium (He) nuclei (i.e., alpha particles) may be used. Specifically, for photomask or reticle inspection, it is desirable to inspect using a wavelength identical, or close, to the wavelength that will be used for lithography, i.e., close to 193.4 nm for current generation lithography and close to 13.5 nm for future EUV lithography, as the phase-shifts of the inspection light caused by the patterns will be identical or very similar to those caused during lithography. For inspecting semiconductor patterned wafers, inspection systems operating over a relatively broad range of wavelengths, such as a wavelength range that includes wavelengths in the near UV, DUV, and/or VUV ranges, can be advantageous because a broad range of wavelengths can reduce the sensitivity to small changes in layer thicknesses or pattern dimensions that can cause large changes in reflectivity at an individual wavelength.
In order to detect small defects or particles on photomasks, reticles, and semiconductor wafers, high signal-to-noise ratios are required. High photon or particle flux densities are required to ensure high signal-to-noise ratios when inspecting at high speed because statistical fluctuations in the numbers of photons detected (Poisson noise) is a fundamental limit on the signal-to-noise ratio. In many cases, approximately 100,000 or more photons per pixel are needed. Because inspection systems are typically in use 24 hours per day with only short stoppages, the detectors are exposed to large doses of radiation after only a few months of operation.
A photon with a vacuum wavelength of 250 nm has energy of approximately 5 eV. The bandgap of silicon dioxide is about 10 eV. Although it may appear such wavelength photons cannot be absorbed by silicon dioxide, silicon dioxide as grown on a silicon surface must have some dangling bonds at the interface with the silicon because the silicon dioxide structure cannot perfectly match that of the silicon crystal. Furthermore, because the single dioxide is amorphous, there are likely also some dangling bonds within the material. In practice, there will be a non-negligible density of defects and impurities within the oxide, as well as at the interface to underlying semiconductor, that can absorb photons with deep UV wavelengths, particularly those shorter than about 250 nm in wavelength. Furthermore, under high radiation flux density, two high-energy photons may arrive near the same location within a very short time interval (nanoseconds or picoseconds), which can lead to electrons being excited to the conduction band of the silicon dioxide by two absorption events in rapid succession or by two-photon absorption.
A further requirement for sensors used for inspection, metrology and related applications is high sensitivity. As explained above, high signal-to-noise ratios are required. If the sensor does not convert a large fraction of the incident photons into signal, then a higher intensity light source would be required in order to maintain the same inspection or measurement speed compared with an inspection or metrology system with a more efficient sensor. A higher intensity light source would expose the instrument's optics and the sample being inspected or measured to higher light intensities, possibly causing damage or degradation over time. A higher intensity light source would also be more expensive or, particularly at DUV and VUV wavelengths, may not be available. Silicon reflects a high percentage of DUV and VUV light incident upon it. For example, near 193 nm in wavelength, silicon with a 2 nm oxide layer on its surface (such as a native oxide layer) reflects approximately 65% of the light incident on it. Growing an oxide layer of about 21 nm on the silicon surface reduces the reflectivity to close to 40% for wavelengths near 193 nm. A detector with 40% reflectivity is significantly more efficient than one with 65% reflectivity, but lower reflectivity, and hence higher efficiency, is desirable.
Anti-reflection (AR) coatings (aka, anti-reflection, anti-reflective or antiglare coatings) are commonly used on optical elements such as lenses and mirrors to increase efficiency by reducing the optical elements' reflectivity. However, many AR coating materials and processes commonly used for optical elements are often not compatible with silicon-based sensors. For example, electron and ion-assisted deposition techniques are commonly used to generate AR and other optical coatings. Such coating processes cannot generally be used to coat semiconductor devices because the electrons or ions can deposit sufficient charge on the surface of the semiconductor device to cause electrical breakdown, resulting in damage to the circuits fabricated on the semiconductor.
DUV and VUV wavelengths are strongly absorbed by silicon. Such wavelengths may be mostly absorbed within about 10 nm or a few tens of nm of the surface of the silicon. The efficiency of a sensor operating at DUV or VUV wavelengths depends on how large a fraction of the electrons created by the absorbed photons can be collected before the electrons recombine. Silicon dioxide can form a high-quality interface with silicon with a low density of defects. Most other materials, including many of those commonly used for anti-reflection coatings, if deposited directly on silicon, result in a very high density of electrical defects at the surface of silicon. A high density of electrical defects on the surface of silicon may not be an issue for a sensor intended to operate at visible wavelengths, as such wavelengths may typically travel about 100 nm or more into the silicon before being absorbed and may, therefore, be little affected by electrical defects on the silicon surface. However, DUV and VUV wavelengths are absorbed so close to the silicon surface that electrical defects on the surface and/or trapped charges within the layer (s) on the surface can result in a significant fraction of the electrons created recombining at, or near, the silicon surface and being lost, resulting in a low efficiency sensor.
U.S. Pat. Nos. 9,496,425, 9,818,887, and 10,121,914, describe boron-coated back-illuminated image sensors and methods of making image sensors that include at least one boron layer deposited on, at least, the exposed backside (light-receiving) surface of the image sensor that functions to increase the durability and quantum efficiency of the image sensor. Different ranges of temperature for deposition of the boron are disclosed, including a range of about 400-450° C., and a range of about 700-800° C. The inventors have discovered that one advantage of a higher deposition temperature for the boron, such as a deposition temperature between about 600° ° C. and about 900° ° C., is that, at such temperatures, boron diffuses into the silicon providing a very thin, heavily p-type doped silicon layer on the light-sensitive back surface. This p-type doped silicon layer is important for ensuring a high quantum efficiency to DUV and VUV radiation because it creates a static electric field near the surface that accelerates electrons away from the surface into the silicon layer. The p-type silicon also increases the conductivity of the back surface of the silicon, which is important for high-speed operation of an image sensor, since a return path is needed for ground currents induced by the switching of signals on electrodes on the front surface of the sensor. U.S. Pat. No. 11,114,491 describes image sensor structures with a very thin, low temperature (below 450° C.) epitaxial silicon grown over the back thinned surfaces of silicon sensors, prior to low temperature (below 450° C.) boron coating to achieve high quantum efficiency to DUV and VUV radiation because it can also create a static electric field near the surface that accelerates electrons away from the surface into the silicon layer. In all of these boron-coated back-illuminated image sensors, the boron coating increases the image sensors' durability by protecting against degradation caused by heavy doses of high energy radiation, such as DUV and/or VUV radiation.
As set forth above, silicon-based back-illuminated image sensors utilized for high energy wafer inspection applications (i.e., using wavelengths below 193 nm) require both a boron layer and an anti-reflection coating. That is, while the boron layer can improve the durability of these image sensors, an anti-reflection coating is required over the boron layer to improve the image sensors' quantum efficiency by increasing the amount of incident radiation received by the sensor (i.e., by decreasing the amount of radiation reflected from the silicon surface before it reaches the sensor's detection element). An ideal anti-reflection coating for such boron-coated image sensors would be one that can be safely formed over the boron layer and is transparent to wavelengths below 193 nm. Oxide-based anti-reflection coatings are used on boron-coated image sensors utilized to detect wavelengths above 193 nm, but oxide-based materials increasingly absorb radiation in inverse proportion to wavelengths below 193 nm. That is, the absorption of radiation by oxide-based anti-reflection coatings is very high at VUV wavelengths, such as 150 nm or below, as the photon energy at these wavelengths equals or exceeds the oxide material's band gap. Fluoride-based materials, such as magnesium fluoride (MgF2) and calcium fluoride (CaF2), are utilized to form anti-reflection coatings on optical elements used at wavelengths below 193 nm due to their higher band gap (i.e., in comparison to oxides). However, the use of such fluoride-based anti-reflection coatings on boron-coated image sensors is problematic because the fluoride-based atoms, ions, and free radicals involved in the fluoride-based material deposition procedures migrate into and damage the boron layer, thereby decreasing the durability of the image sensor. Other materials, such as metals, can be safely deposited on the boron layer without reducing the image sensor's durability, but are not transparent to wavelengths below 193 nm.
Therefore, a need arises for back-illuminated image sensors that are both durable and capable of sensing UV and/or VUV radiation with high quantum efficiency. In particular, what is needed is a method for generating back-illuminated image sensors that combine both the durability provided by a pure boron coating and the low reflectivity to UV/VUV radiation exhibited by fluoride-based anti-reflection coatings that overcomes some or all the above-mentioned fluoride-on-boron issues.
The present invention is directed to a back-illuminated image sensor for deep ultraviolet (DUV) radiation and vacuum ultraviolet (VUV) radiation that includes a pure boron coating disposed on the backside surface of a semiconductor membrane (e.g., an epitaxial silicon layer) and a two-part anti-reflective coating that includes a protection layer disposed on the pure boron coating and a fluoride-based coating disposed on the protection layer. According to an aspect, the protection layer includes at least one of a thin oxide film (e.g., one or more of Al2O3, MgO, La2O3, Li2O, CaO, BeO, and HfO2) and/or a thin nitride film (e.g., one or more of AlN, Li3N, LaN, Mg3N2, HAN and Ca3N2), wherein a total thickness of the oxide/nitride film(s) is within the range of 0.5 nm to 10 nm. Implementing the protection layer using oxide/nitride films that meet these specifications provides several advantages over other possible protection layer materials. First, when formed with a thickness of at least 0.5 nm, such oxide/nitride protection layers can be made thick enough to function as diffusion barriers that are capable of impeding the migration of fluoride ions/atoms/radicals from any fluoride-based material (e.g., one of AlF3, MgF2, CaF2, LaF3, LiF, and HfF4) subsequently deposited/formed thereon to the boron coating, thus facilitating optimization of the anti-reflective characteristics of the fluoride-based AR coating while avoiding the above-mentioned fluoride-on-boron issues. Second, limiting the total oxide/nitride film thickness to 10 nm minimizes any parasitic absorption of DUV and/or VUV radiation of the oxide/nitride protection layer. Third, such oxide/nitride thin films can be safely and reliably formed with high precision on pure boron coatings using several well-established semiconductor fabrication processes including: physical vapor deposition (PVD) methods such as thermal or e-beam evaporation; chemical vapor deposition (CVD) methods such as atmospheric pressure chemical vapor deposition (APCVD), plasma enhanced chemical vapor deposition (PECVD), or low pressure chemical vapor deposition (LPCVD); atomic layer deposition (ALD) which can also be thermal or plasma enhanced; or molecular beam epitaxy (MBE)). Fourth, such oxide/nitride thin films provide excellent protection for the underlying pure boron coating, thereby facilitating the use of thinner pure boron coatings while increasing production yields and image sensor operating lifetimes. Accordingly, the two-part anti-reflective coating provides the back-illuminated image sensors with both the durability of a pure boron coating and the low reflectivity to UV/VUV radiation of a fluoride-based anti-reflection coating while overcoming some or all the above-mentioned fluoride-on-boron issues.
In a presently preferred embodiment, the oxide/nitride protection layer includes one of an Al2O3 or AlN thin film having a thickness in the range of 0.5 nm and 5 nm (and even more preferably 2 nm or less as process allows) that is formed on an upper surface of the pure boron coating, and the fluoride-based anti-reflection layer is formed on the Al2O3 or AlN protection layer. An advantage to utilizing Al2O3 and AlN in the formation of the oxide/nitride protection layer is that both of these materials can be deposited with high precision (i.e., with atomic control over thickness) using well-established ALD processes. Moreover, their scalability and production worthiness on large area wafers, especially of Al2O3, is very well known. The deposition of other oxide/nitride materials using ALD is less common, but research literature indicates they may be fabricated with sufficient precision at smaller (lab) scales, and so may be practical in production in the future. In embodiments that utilize an Al2O3 and/or AlN protection layer, from among the various materials listed herein for forming the fluoride-based anti-reflection layer, MgF2 is presently preferred, followed by CaF2 and then the other fluoride-based material options.
In a generalized embodiment a method for fabricating the above-mentioned back-illuminated image sensors includes forming frontside circuit structures on a frontside (first) surface of a semiconductor membrane, then forming a pure boron coating on a backside (second) surface of the semiconductor membrane, then forming an oxide/nitride protective layer on the pure boron coating with an initial thickness in the range of 0.5 nm to 50 nm, and then forming a fluoride-based anti-reflection coating on the protective layer. By forming the oxide/nitride protective layer between the pure boron coating and the fluoride-based anti-reflection coating, the method facilitates the production of back-illuminated image sensors exhibiting both the durability of a pure boron coating and the low reflectivity to UV/VUV radiation exhibited by fluoride-based anti-reflection coatings while overcoming some or all the above-mentioned fluoride-on-boron issues.
In some embodiments forming the pure boron coating involves depositing amorphous boron on the backside surface until a total thickness in the range of 2 nm to 20 nm is achieved. In some embodiments a high temperature deposition process is utilized to form the pure boron coating with a suitably high quality, which requires the completion of front-end circuit elements (e.g., forming metal interconnects over the previously formed front-end circuit structures) after the boron formation process. In other embodiments the pure boron coating may be formed with a suitably high quality using high temperature deposition process, thereby facilitating the completion of front-end circuit elements before the boron formation process.
In some embodiments forming the protective layer involves depositing one or more of Al2O3, MgO, La2O3, Li2O, CaO, BeO, HfO2, AlN, Li3N, LaN, Mg3N2, HfN and/or Ca3N2 directly onto the pure boron coating. In alternative specific embodiments, the oxide/nitride deposition process involves performing one of a PVD, CVD, ALD or MBE deposition process. An initial thickness of the protection layer (i.e., immediately after deposition and before formation of the fluoride-based anti-reflective coating) is determined by the process utilized to form the fluoride-based anti-reflective coating. In some embodiments the fluoride-based anti-reflective coating is formed by depositing a fluoride-based materials (e.g., AlF3, MgF2, CaF2, LaF3, LiF and/or HfF4) onto the protection layer using, e.g., a PVD, CVD, ALD or MBE deposition process. In these cases, because the thickness of the protection layer does not change significantly during formation of the fluoride-based anti-reflective coating, the protection layer is formed with a relatively thin initial thickness (e.g., in the range of 0.5 nm to 10 nm). In other embodiments the fluoride-based anti-reflective coating is formed using a fluorination process in which the protection layer is exposed to one or more fluorine-containing gases (e.g., one or more of F2, HF, XeF2, CH3F, SF6, CF4, NbF5, and WF6) under conditions that convert an upper region (e.g., uppermost layers) of the protection layer from oxide/nitride material to fluoride-based material (i.e., such that an upper portion of the protection layer is used/converted to form the fluoride-based anti-reflective coating). In these cases, because the thickness of the protection layer is significantly reduced during formation of the fluoride-based anti-reflective coating, the protection layer is formed with a relatively thick initial thickness (e.g., in the range of 10 nm to 50 nm, depending on the targeted final thickness of the protection layer and the targeted final thickness of the fluoride-based anti-reflective coating. In some embodiments, the fluorination process is performed in a plasma chamber to enhance the conversion process.
The fabrication methods described herein may be incorporated into the fabrication flows associated with several types of boron-coated, back-illuminated image sensors. For example, the frontside circuit elements may be configured to implement charge coupled devices (CCDs), complementary metal oxide semiconductor (CMOS) imagers and/or photodiodes, and include other semiconductor devices such as transistors, diodes, resistors and capacitors that are configured to collectively perform image sensor operations. The fabrication methods may be used in combination with image sensor fabrication processes in which a silicon layer or SOI structure are partially or fully back-thinned and through-silicon vias are formed before formation of the pure boron layer.
By utilizing any of the fabrication methods mentioned above and/or described in additional detail below, the present invention provides back-illuminated image sensors capable of sensing DUV and VUV radiation (e.g., radiation below 193 nm) that exhibit both longer operating lifetimes (i.e., due to the pure boron coating) and high quantum efficiency (i.e., due to the fluoride-based AR coating) while overcoming the above-mentioned fluoride-on-boron issues (i.e., due to the oxide/nitride protective layer). The present invention is also directed to back-illuminated image sensors incorporating at least one fluorine-based anti-reflection coating disposed over at least one pure boron layer, and to an inspection system utilizing such back-illuminated image sensor.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention.
Although claimed subject matter will be described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the benefits and features set forth herein, are also within the scope of this disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the disclosure. Accordingly, the scope of the disclosure is defined only by reference to the appended claims.
The following description is presented to enable one of ordinary skill in the art to make and use the disclosure as provided in the context of a particular application and its requirements. As used herein, directional terms such as “top,” “bottom,”, “front,” “frontside”, “backside,” “over,” “under,” “upper,” “upward,” and “lower” are intended to provide relative positions for purposes of description and are not intended to designate an absolute frame of reference. Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present disclosure is not intended to be limited to the embodiments shown and described but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.
Referring to block 101 at the top of
Next, a pure boron coating 180 is formed on backside (second) surface 162 of semiconductor membrane 160 at least in locations opposite to active sensor areas defined by the location of front-end circuit structures 171 (block 110). Pure boron coating 180 comprises a boron concentration of 80% or higher with inter-diffused silicon atoms and oxygen atoms predominantly making up the remaining 20% or less. In one embodiment, the formation of pure boron coating 180 involves depositing one or more amorphous layers of pure boron on exposed backside surface 162 until pure boron coating 180 has a total thickness T180 in the range of 2 nm to 20 nm. In alternative embodiments, pure boron layer 180 may be formed using one of a physical vapor deposition (PVD) method such as thermal or ebeam evaporation; a chemical vapor deposition (CVD) method such as atmospheric (APCVD), plasma enhanced (PECVD), or low pressure (LPCVD); an atomic layer deposition (ALD) method, which can be thermal or plasma enhanced; or molecular beam epitaxy (MBE). In some embodiments, pure boron coating 180 is formed using a multi-cycle plasma ALD boron deposition process in which a plurality of plasma ALD cycles are performed to sequentially generate boron nanolayers that collectively form pure boron coating 180. This multi-cycle plasma ALD boron deposition process is described in co-owned and co-pending U.S. Published Patent Application No. 2022/0254829 entitled “Back Illuminated Sensor with Boron Layer Deposited Using Plasma Atomic Layer Deposition”, which is incorporated herein by reference.
A two-part anti-reflection coating 181 is then provided over pure boron coating 180 by forming a protective layer 182 on pure boron coating 180 (i.e., such that pure boron coating 180 is between protective layer 182 and semiconductor membrane 160; block 120), and then forming a fluoride-based anti-reflection (AR) coating 185 on protective layer 182 (i.e., such that protective layer 182 is between fluoride-based AR coating 185 and pure boron coating 180; block 130).
Referring to block 120, the formation of protective layer 182 involves forming one of an oxide film and a nitride film on pure boron coating 180, where the oxide/nitride film has a minimum thickness Tis: of about 0.5 nm (i.e., when properly fabricated, oxide/nitride films having this minimum thickness are able to substantially impede the migration of fluoride ions from the subsequently formed fluoride-based AR coating 185 to pure boron layer 180, thereby protecting pure boron coating 180 from the subsequently formed fluoride-based AR coating 185). In one embodiment, protective layer 182 is formed by depositing at least one of Al2O3, MgO, La2O3, Li2O, CaO, BeO, HfO2, AlN, Li3N, LaN, Mg3N2, HfN and Ca3N2 on an upper surface 1800 of pure boron coating 180 using one of the PVD, CVD, ALD or MBE methods mentioned above. A final maximum thickness Tis of protective layer 182 is preferably limited to 10 nm or less to minimize the absorption of DUV/VUV radiation by the selected oxide/nitride material. In some embodiments, such when fluoride-based AR layer 185 is formed using the fluorination process described below with reference to
Referring to block 130, in alternative exemplary embodiments the formation of fluoride-based AR coating 185 involves either depositing a fluoride-based material or converting oxide/nitride material into a fluoride-based material using a fluorine-containing gas. In some embodiments (e.g., as described below with reference to
After completing the formation of pure boron coating 180 and two-part anti-reflection coating 181, additional processing (block 140) is performed to complete the fabrication and packaging of image sensor 150. For example, as indicated by image sensor 150A, which is shown at the bottom of
Referring to the top of
A protective layer is then formed over the frontside surface 161A (block 102A). In one embodiment the protective layer (not shown) may be formed by depositing one or more protective materials on top of frontside circuit structures 171A. The one or more protective materials may comprise silicon dioxide, silicon nitride or other material.
In some embodiments, the wafer including membrane 160A is thinned from the backside to expose epitaxial layer 163A in, at least, the active sensor areas (block 104A). This step may involve polishing, etching, or both. In some embodiments, the entire wafer is back-thinned using known techniques. In other embodiments, only the active sensor areas are thinned all the way to the epitaxial layer.
Next, the backside surface is cleaned and prepared for boron deposition/formation (block 108A). During this cleaning process, the native oxide, and any contaminants, including organics and metals, should be removed from the exposed backside surface (i.e., the surface exposed during the thinning process of block 104A). In one embodiment, this cleaning can be performed using a dilute HF solution or using an RCA cleaning process. After cleaning, the wafer can be dried using the Marangoni drying technique or a similar technique to leave the surface dry and free of water marks. In preferred embodiments, the wafer is protected in a controlled environment during the thinning and cleaning/preparing processes (e.g. in a vacuum environment or in an environment purged with a dry, inert gas such as nitrogen) to minimize native oxide regrowth after the cleaning.
An amorphous layer of pure boron is then deposited on the exposed backside surface using a high-temperature boron deposition process (block 110A). In one preferred embodiment, this deposition can be performed using a mixture of diborane and hydrogen gases at a temperature of about 700-800° ° C. to create a high-purity amorphous boron layer. In preferred embodiments, the thickness of the amorphous boron layer is between 2-20 nm. The minimum thinness is generally limited by the compromise between the need for a pinhole-free uniform film and the absorption of the photons of interest by the boron. In preferred embodiments, prior to the boron deposition, the wafer can be held at a high temperature for a few minutes in a reducing environment, such as a dilute hydrogen gas or a low-pressure hydrogen gas, to remove any native oxide layer that might have regrown after the back-thinning process while the wafer is maintained in the same chamber used for boron deposition. In preferred embodiments, the temperature is approximately 800° C. for less than 5 minutes, and the boron deposition process is performed immediately in the same chamber.
A two-part AR coating 181A is then formed over pure boron coating 180A using any of the processes described above with reference to blocks 120 and 130 (
Next, processes associated with the completion of image sensors 150 are performed (block 141A). In one embodiment, these processes include removal or patterning of the front-side protective layer to facilitate the fabrication of metallic interconnects 172A on frontside circuit structures 171A. In some embodiments, this removal/patterning may include various wet and dry etching processes and photolithographic patterning steps. Metallic interconnects 172A may be formed using one or more of Al, Cu, or another metal. After interconnect fabrication is complete, a passivation layer may be deposited on the frontside surface to protect completed image sensors 150A.
In some embodiments, the protective layer formation process (block 120A) and completion of the image sensors (block 141A) may be performed in the order indicated in
After completing front-end circuit elements 170A, image sensor 150A is packaged. This packaging process may include flip-chip bonding or wire bonding of a chip to a substrate. The package may include a window that transmits wavelengths of interest or may comprise a flange or seal for interface to a vacuum seal.
Next, as indicated in block 101B, complete front-end circuit elements 170B (i.e., both front-end circuit structures 171B and metal interconnects 172B) are generated on frontside surface 161B of p− epi layer 163B using standard semiconductor processing steps such as lithography, deposition, ion implantation, annealing, and etching. Charge-coupled device (CCD) and/or CMOS sensor elements and devices may also be created during the fabrication of front-end circuit elements 170B. Note that both frontside circuit structures 171A (e.g., poly-silicon interconnects) and metal interconnects 172A may be formed at this time because the subsequent low-temperature processing will not damage the metal interconnects.
In some embodiments one or more protective layers (e.g., silicon dioxide, silicon nitride or other material) are then deposited on front-end circuit elements 170B (block 102B). This protection may include attaching the wafer including membrane 160B to a handling wafer (not shown), such as a silicon wafer, a quartz wafer, or a wafer made of other material. The handling wafer may include through-wafer vias for connecting to the circuit elements.
Next, the wafer is thinned from the backside to expose the epitaxial layer in, at least, the active sensor areas. This process may involve polishing, etching, or both. In some embodiments, the entire wafer is back-thinned. In other embodiments, only the active sensor areas are thinned all the way to the epitaxial layer.
Next, membrane 160B is cleaned and prepared for the formation of pure boron coating 180B. During this cleaning/preparing process, the native oxide, and any contaminants, including organics and metals, should be removed from the backside surface. In one embodiment, this cleaning can be performed using a dilute HF solution or using an RCA clean process. After cleaning, the wafer can be dried using the Marangoni drying technique or a similar technique to leave the surface dry and free of water marks. In preferred embodiments, the wafer is protected in a controlled environment (e.g. in a vacuum environment or in an environment purged with a dry, inert gas such as nitrogen) during the back-thinning and cleaning/preparing processes to minimize native oxide regrowth after the cleaning.
pure boron coating 180B is then formed by depositing an amorphous layer of pure boron on the exposed backside surface using a low-temperature boron deposition process (block 110B). In some embodiments, p+ (second) epi layer 165B is formed on p− epi layer 163B by growing epitaxial silicon on the exposed backside surface using a low-temperature epitaxial growth process while doping the second epitaxial silicon layer with boron during the epitaxial growth process. The amorphous layer of pure boron is then deposited over second epitaxial layer 165B. In preferred embodiments, second epitaxial silicon layer 165B and the amorphous boron layer are deposited at or below 450° C. so that the metal contacts on the front side are not damaged. In preferred embodiments, the thicknesses of the second boron doped epitaxial layer and amorphous boron layer are in the range of 2-20 nm.
A two-part AR coating 181B is then formed over pure boron coating 180B using any of the processes described above with reference to blocks 120 and 130 (
Referring to
Referring to
Referring to
Illumination source 902 is preferably configured to generate (emit) deep UV (DUV) and/or vacuum UV (VUV) incident light (radiation) LIN having a wavelength in the range of 100 nm to 300 nm but may be configured to generate light having wavelengths below 100 nm (e.g., 13.5 nm for future EUV lithography) or greater than 300 nm. In some embodiments illumination source 902 utilizes one or more light sources LS and one or more optical components (e.g., a frequency converter) to generate incident light LIN. In one embodiment, illumination source 902 may include a continuous light source, such as an arc lamp, a laser-pumped plasma light source, or a continuous wave (CW) laser. In another embodiment, illumination source 902 may include pulsed light source, such as a mode-locked laser, a Q-switched laser, or a plasma light source pumped by a mode-locked or Q-switched laser. Suitable light sources that may be included in illumination source 902 are described in U.S. Pat. No. 7,705,331, entitled “Methods and systems for providing illumination of a specimen for a process performed on the specimen”, to Kirk et al., U.S. Pat. No. 9,723,703, entitled “System and method for transverse pumping of laser-sustained plasma”, to Bezel et al., and U.S. Pat. No. 9,865,447, entitled “High brightness laser-sustained plasma broadband source”, to Chuang et al. These patents are incorporated by reference herein.
Stage 912 is configured to receive sample 908 and to facilitate movement of sample 908 relative to optical system 903 (i.e., such that optical system 903 focuses incident light LIN on different regions of sample 908 and directs reflected/scattered light from the different regions to detector assembly 904). Stage 912 may comprise an X-Y stage or an R-θ stage. In one embodiment, stage 912 can adjust the height of sample 908 during inspection to maintain focus. In another embodiment, optics 903 can be adjusted to maintain focus.
Optical system (optics) 903 comprises multiple optical components and other optical components that are configured to direct and focus incident light LIN onto sample 908, and to direct reflected (including scattered) light LR/S from the sample 908 to detector assembly 904. The exemplary optical components of optical system 903 illustrated in
Detector assembly 904 includes one or more of image sensor 150 that is fabricated using any of the methods described herein. In alternative embodiments, sensor 150 includes a back-illuminated CCD sensor, a back-illuminated CMOS sensor, and electron-bombarded image sensor incorporating a Back-thinned solid-state image sensor. Image sensor 150 may comprise a two-dimensional array sensor or a one-dimensional line sensor. In one embodiment, the output of detector assembly 904 is provided to a computing system 914, which analyzes the output. Computing system 914 can be configured by program instructions 918, which can be stored on a carrier medium 916. In some embodiments of inspection system 900 incorporating a Q-switched laser, image sensor 150 or sensors 150 within detector assembly 904 are synchronized with the laser pulses. In such embodiments, image sensor 150 may operate in a TDI mode during the laser pulse and then may readout the data through multiple outputs on both sides of the sensor in between laser pulses. Some embodiments of inspection system illuminate a line on sample and collect scattered and/or reflected light in one or more dark-field and/or bright-field collection channels. In such embodiments, image sensor 150 may be a line sensor or an electron-bombarded line sensor. Some embodiments of inspection system illuminate multiple spots on sample and collect scattered and/or reflected light in one or more dark-field and/or brightfield collection channels. In such embodiments, image sensor 150 may be a two-dimensional array sensor or an electron bombarded two-dimensional array sensor.
Additional details of various embodiments of inspection or metrology system 900 are described in U.S. Pat. No. 9,891,177, entitled “TDI Sensor in a Darkfield System”, to Vazhaeparambil et al., U.S. Pat. No. 9,279,774, entitled “Wafer inspection”, to Romanovsky et al., U.S. Pat. No. 7,957,066, entitled “Split field inspection system using small catadioptric objectives”, to Armstrong et al., U.S. Pat. No. 7,817,260, entitled “Beam delivery system for laser dark-field illumination in a catadioptric optical system”, to Chuang et al., U.S. Pat. No. 5,999,310, entitled “Ultra-broadband UV microscope imaging system with wide range zoom capability”, to Shafer et al., U.S. Pat. No. 7,525,649, entitled “Surface inspection system using laser line illumination with two dimensional imaging”, to Leong et al., U.S. Pat. No. 9,080,971, entitled “Metrology systems and methods”, to Kandel et al., U.S. Pat. No. 7,474,461, entitled “Broad band objective having improved lateral color performance”, to Chuang et al., U.S. Pat. No. 9,470,639, entitled “Optical metrology with reduced sensitivity to grating anomalies”, to Zhuang et al., U.S. Pat. No. 9,228,943, entitled “Dynamically Adjustable Semiconductor Metrology System”, to Wang et al., U.S. Pat. No. 5,608,526, entitled “Focused Beam Spectroscopic Ellipsometry Method and System”, to Piwonka-Corle et al., issued on Mar. 4, 1997, and U.S. Pat. No. 6,297,880, entitled “Apparatus for Analyzing Multi-Layer Thin Film Stacks on Semiconductors”, to Rosencwaig et al., issued on Oct. 2, 2001. All of these patents are incorporated by reference herein.
The various embodiments of the structures and methods of this invention that are described above are illustrative only of the principles of this invention and are not intended to limit the scope of the invention to the embodiments described. For example, additional steps may be added, or order of the steps may be changed from what is depicted in the flow charts in
This application claims priority to U.S. Provisional Patent Application No. 63/438,788 entitled “METHODS OF IMPLEMENTING FLUORIDE BASED ANTI-REFLECTION COATINGS ON BACK-ILLUMINATED SENSOR WITH BORON LAYER”, which was filed on Jan. 12, 2023.
Number | Date | Country | |
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63438788 | Jan 2023 | US |