Patent Description:
A photodetector that detects light incident on a semiconductor region includes a lens. In order to improve the detection efficiency by the photodetector, it is desirable that the focusing rate of the lens is higher.

<CIT> discloses features falling under the preamble of claim <NUM>. <CIT>, <CIT>, and <CIT> are further prior art.

According to one embodiment, a photomask includes a plurality of unit regions arranged in a first direction and a second direction crossing the first direction. Each of the plurality of unit regions includes a first region having a first light-shielding rate, and a second region having a second light-shielding rate different from the first light-shielding rate. The second region is provided around the first region. The unit regions include a first unit region and a second unit region having same size each other. A distance between the first unit region and a center of a range in which the unit regions are arranged is different from a distance between the second unit region and the center. A light-shielding rate of the first unit region is different from a light-shielding rate of the second unit region.

Various embodiments are described below with reference to the accompanying drawings. In the following description, components exhibiting the same or similar functions are designated by the same reference numerals throughout the drawings, and duplicate description will be omitted. It should be noted that each figure is a schematic diagram for describing the embodiment and promoting its understanding, and there are differences in its shape, dimensions, ratio, etc. from the actual device, but these are based on the following description and public known arts. The design can be changed as appropriate.

In the following description and drawings, the notations of n+, n-, p+, and p indicate relative levels of the impurity concentrations. In other words, a notation marked with "+" indicates that the impurity concentration is relatively greater than that of a notation not marked with either "+" or "-"; and a notation marked with "-" indicates that the impurity concentration is relatively less than that of a notation without any mark. When both a p-type impurity and an n-type impurity are included in each region, these notations indicate relative levels of the net impurity concentrations after the impurities are compensated.

In embodiments described below, each embodiment may be implemented by inverting the p-type and the n-type of the semiconductor regions.

According to a first embodiment, a photomask is provided. The photomask includes a plurality of unit regions arranged in a first direction and a second direction crossing the first direction. Each of the plurality of unit regions include a first region having a first light-shielding rate, and a second region having a second light-shielding rate different from the first light-shielding rate. The second region is provided around the first region. The plurality of unit regions include a first unit region and a second unit region having same size each other. A distance between the first unit region and a center of a range in which the plurality of unit regions are arranged is different from a distance between the second unit region and the center, and a light-shielding rate of the first unit region is different from a light-shielding rate of the second unit region.

<FIG> and <FIG> are schematic plan views of a photomask according to a first embodiment. <FIG> is a schematic plan view of an enlarged part of <FIG>. <FIG> is a schematic plan view of a further enlarged part of <FIG>. <FIG> is a schematic plan view of other example of a further enlarged part of <FIG>.

An axis that passes through the center of the photomask and is along the first direction and an axis that crosses the first direction and is along the second direction are defined. <FIG> is an enlarged view of the first quadrant when the photomask of <FIG> is divided into four quadrants by the axis along the first direction and the axis along the second direction. <FIG> is a schematic plan view in which a section of <FIG> is further enlarged, and shows an example in which the shape of a first region <NUM> is a substantially quadrangle. <FIG> is a schematic plan view of another example of <FIG>.

In <FIG>, the white portion (without filling) is the first region <NUM>. The black portion (filled) is a second region <NUM>. The first region <NUM> transmits light more than the second region <NUM>. The second region <NUM> has a lower light transmittance than the first region <NUM>. The light transmittance of the second region <NUM> may be zero, and it is not necessary to pass light. For example, the first region <NUM> is a non-light-shielding portion. The second region <NUM> is a light-shielding portion. In the example illustrated in <FIG>, a first light-shielding rate of the first region <NUM> is lower than a second light-shielding rate of the second region <NUM>. The light-shielding rate is the rate at which incident light is blocked in a certain region. The value obtained by subtracting the light-shielding rate from <NUM> is the transmittance in the region. The shape of the first region <NUM> may be a regular polygon or a substantially polygon.

A photomask 200a in <FIG> includes multiple non-light-shielding portions and a light-shielding portion. The multiple non-light-shielding portions are repeatedly arranged two-dimensionally. For example, the multiple non-light-shielding portions are arranged at equal intervals. One light-shielding portion is arranged around each non-light-shielding portion. The multiple non-light-shielding portions and the light-shielding portion are arranged so that the light-shielding rate at the central area of the photomask 200a is higher than the light-shielding rate at the peripheral area. One non-light-shielding portion corresponds to the first region <NUM>. A part of the light-shielding portion corresponds to the second region <NUM>.

The central area of the photomask 200a includes the center of the photomask 200a and is a region having a constant light-shielding rate. For example, in the central area of the photomask 200a, non-light-shielding portions having the same size are provided. The peripheral area is a region of the photomask 200a excluding the central area thereof. The size of the non-light-shielding portion provided in the peripheral area is different from the size of the non-light-shielding portion provided in the central area.

As shown in <FIG>, the photomask 200a includes multiple unit regions <NUM>. The multiple unit regions <NUM> are repeatedly arranged in the first direction and the second direction. Each unit region <NUM> includes the first region <NUM> and the second region <NUM> arranged around the first region <NUM>. In accordance with the invention, the second regions <NUM> adjacent to each other in the first direction or the second direction are in contact with each other.

In <FIG>, the distance connecting the centers of the adjacent first regions <NUM> is defined as a first distance a. That is, the first regions <NUM> are arranged at intervals of the first distance a, and are arranged at intervals of pitch a. When the shape of the first region <NUM> is a substantially quadrangle as shown in <FIG>, the center of the first region <NUM> is considered as follows in order to obtain the distance a. First, the approximate straight lines of the four sides of the first region <NUM> are obtained, and the four vertices of the quadrangle formed from the four approximate straight lines are determined. Then, diagonal lines of the four vertices are drawn, and the intersection thereof is set as the center of the first region <NUM>. In <FIG>, W<NUM> is the length of the first region <NUM> along the first direction or the second direction. W<NUM> is the length of the second region <NUM> along the first or second direction located between the adjacent first regions <NUM>. In <FIG>, W<NUM> is the length of the first region <NUM> in the second direction. W<NUM> is the length of the second region <NUM> located between the adjacent first regions <NUM> in the second direction. The sum of twice W<NUM> and W<NUM> is the distance a connecting the centers of the adjacent first regions <NUM>.

In the case where the unit region <NUM> is square and the lengths of the second regions <NUM> adjacent to each other on the up, down, left, and right of the unit region <NUM> are the same, the length of one unit region <NUM> along the first direction or the second direction is W1 + W2 + W1. Therefore, the length of the unit region <NUM> is 2W<NUM> + W<NUM>, which is a distance a.

Assuming that the light-shielding rate of the unit region <NUM> is a third light-shielding rate, the third light-shielding rate is a ratio of the area of the second region <NUM> in the unit region <NUM> to the area of the unit region <NUM>. That is, the third light-shielding rate is the area of the second region in the unit region / the area of the unit region (a<NUM>), and is represented by {(2W<NUM> + W<NUM>)<NUM>-W<NUM><NUM>} / (2W<NUM> + W<NUM>)<NUM>.

As shown in <FIG>, the multiple unit regions <NUM> include a first unit region 203a and a second unit region 203b. The multiple unit regions <NUM> for forming one lens are arranged in the range R. The center of the range R is defined as the center C. The center C is also, for example, the center of the photomask 200a in the first surface parallel to the first direction and the second direction. The distance between the center C and the first unit region 203a is longer than the distance between the center C and the second unit region 203b. The size of the second unit region 203b is the same as the size of the first unit region 203a. The light-shielding rate of the first unit region 203a is different from the light-shielding rate of the second unit region 203b.

The multiple unit regions <NUM> may further include a third unit region 203c. The distance between the center C and the first unit region 203a is shorter than the distance between the center C and the third unit region 203c. The size of the third unit region 203c is the same as the size of the first unit region 203a. The light-shielding rates of the first unit region 203a, the second unit region 203b, and the third unit region 203c are different from each other. In the photomask 200a according to the first embodiment, the second unit region 203b is closest to the center C among the multiple unit regions <NUM>. The light-shielding rate of the second unit region 203b is the largest among the multiple unit regions <NUM>. The third unit region 203c is the farthest from the center C among the multiple unit regions <NUM>. The light-shielding rate of the third unit region 203c is the smallest among the light-shielding rates of the multiple unit regions <NUM>.

The third light-shielding rate depends on the length W<NUM> along the first or second direction of the second region <NUM> located between the adjacent first regions <NUM>. The length W<NUM> is influenced by the area of the first region <NUM>. For example, in <FIG>, there is a gradient in the third light-shielding rates along the first direction or the second direction from the center C of the photomask 200a. In the vicinity of the center of the photomask 200a, the area of the first region <NUM> is small, so that the length W<NUM> is large. Light is less likely to pass through than in other regions, and the third light-shielding rate is high. In the peripheral area of the photomask 200a, the length W<NUM> is small because the area of the first region <NUM> is large. Light is more easily transmitted than in other regions, and the third light-shielding rate is low.

The sizes of the unit regions <NUM> are the same as each other. The third light-shielding rate of each unit region <NUM> is not less than <NUM>% and not more than <NUM>%. In other words, when the first light-shielding rate of the first region <NUM> is <NUM> and the second light-shielding rate of the second region <NUM> is <NUM>, the ratio of the area of the second region <NUM> in each unit region <NUM> is not less than <NUM>% and not more than <NUM>%. Each third light-shielding rate is inversely proportional to the square of the distance from the center C. Each third light-shielding rate may be in a relationship obtained by multiplying the ratio inversely proportional to the square of the distance from the center C by the conic constant or the square thereof. The conic constant is set in the range of -<NUM> to -<NUM>. In order to improve the characteristics of the manufactured lens, the conic constant is more preferably in the range of -<NUM> to -<NUM>.

When the third light-shielding rate is smaller than <NUM>%, the length of W<NUM> is small and the shielding property is insufficient. Therefore, the resolution as a gradation is lost. If W<NUM> + W<NUM> is increased to secure the length of W<NUM> and a new distance of a1 that is constant in the photomask is used, the distance between the opening holes (first region <NUM>) becomes too wide. It is not possible to prepare the gradation of the photomask so that the surface of the lens to be manufactured becomes sufficiently smooth. Further, when the third light-shielding rate is larger than <NUM>%, W<NUM> in <FIG> is smaller than <NUM>, which makes it difficult for light to pass through. If the third light-shielding rate is set to be larger than <NUM>% and W<NUM> is set to <NUM> or more so that light can pass through, W<NUM> becomes large and the distance between the opening holes becomes too wide. Therefore, it is not possible to prepare the gradation of the photomask so that the surface of the lens to be manufactured becomes sufficiently smooth.

Preferably, the third light-shielding rate of the unit region <NUM> of the photomask 200a is not less than <NUM>% and not more than <NUM>%. When the third light-shielding rate is <NUM>% or more, W<NUM> becomes <NUM> or more. It is possible to prepare the gradation of the photomask 200a so that the surface of the lens to be manufactured becomes sufficiently smooth. Further, when the third light-shielding rate is <NUM>% or less, W<NUM> can be made larger than <NUM>. Therefore, the phenomenon that although the first region <NUM> for transmitting light exists, light does not actually transmit can be suppressed. In order to reduce 2W<NUM> + W<NUM> and sufficiently smooth the surface of the lens, the third light-shielding rate is more preferably not less than <NUM>% and not more than <NUM>%.

By changing the area of the first region <NUM> while keeping the distance a, it is possible to make a gradient in the third light-shielding rates of the unit regions <NUM> from the center C toward the outer periphery of the photomask 200a. For example, the area of the first region <NUM> is the smallest near the center C of the photomask 200a and increases in proportion to the distance from the center C of the photomask 200a. That is, the area of the opening in <FIG> increases in proportion to the distance from the center C of the photomask 200a while keeping the distance a between the unit regions <NUM> constant. As a result, the third light-shielding rate of the unit region <NUM> becomes smaller as the distance from the center C of the photomask 200a increases, and the third light-shielding rates of the unit regions <NUM> can have a gradient.

As shown in <FIG>, and not in accordance with the terms of claim <NUM>, adjacent second regions <NUM> may be separated from each other. A separation region <NUM> is provided between the second regions <NUM> adjacent to each other in the first direction or the second direction. The light-shielding rate of the separation region <NUM> is different from the light-shielding rate of the second region <NUM>. The light-shielding rate of the separated region <NUM> per unit area may be the same as the light-shielding rate of the first region <NUM> per unit area. In this case, the distance between the separation regions <NUM> is narrow, and inherently no light may pass between the second regions <NUM>. In this case, the length of the unit region <NUM> is smaller than the distance a connecting the centers of the adjacent first regions <NUM>.

In the first direction or the second direction, it is preferable that the constant distance a is not less than <NUM> and not more than <NUM>. When the distance a is smaller than <NUM>, it means that, for example, the distance between the centers of the adjacent first regions <NUM> in <FIG> is smaller than <NUM>. For example, when an i-line exposure machine is used, the length W<NUM> of about <NUM> or more is required for light to pass through the first region <NUM>. When the length W<NUM> is <NUM> or more, the distance (2W<NUM>) between the adjacent first regions <NUM> is smaller than <NUM>. In the photomask 200a, if the length smaller than <NUM> is changed by, for example, <NUM>, only a maximum of <NUM> kinds of lengths can exist. As a result, when a lens is manufactured using this photomask 200a, the surface of the manufactured lens is not smooth. The distance a is preferably <NUM> or more in order to increase the kinds of distances between adjacent first regions <NUM>. Further, when the distance a is larger than <NUM>, the length W<NUM> is also sufficiently large, so that a sufficiently different kind of different length 2W<NUM> can be present in the photomask 200a. However, since the distance between the adjacent first regions <NUM> is large, the surface of the manufactured lens is not smooth. In order to sufficiently smooth the surface of the manufactured lens, the distance a is preferably <NUM> or less.

As shown in <FIG> and <FIG>, the diameter of the inscribed circle IC in the first region <NUM> is preferably larger than <NUM>. It is preferable that a distance d between at least a part of the first regions <NUM> adjacent to each other in the first direction or the second direction is smaller than <NUM>. The inscribed circle IC is a circle having the maximum area that is in contact with one or more sides of the first region <NUM> and can exist inside the first region <NUM>.

When the diameter of the inscribed circle IC in the first region <NUM> is larger than <NUM>, light can easily pass through the first region <NUM>. Further, as the distance d between the first regions <NUM> becomes longer, the light passing through the first region <NUM> is isolated, so that the reinforcing effect between the light passing through the adjacent first regions <NUM> is suppressed. Therefore, the resist has a shape that reacts with each light, and the surface of the lens has a bumpy uneven shape. When the distance d between the first regions <NUM> is smaller than <NUM>, the surface of the manufactured lens becomes smooth.

In <FIG>, <FIG> and <FIG>, the distance a is kept constant at any place of the photomask 200a, but there are multiple regions having different third light-shielding rates. For example, in <FIG>, the third light-shielding rate is constant in an arcuate region having a certain radius including the center C, and in the region from the arcuate region having a larger radius to the outside of the arcuate region including the center C described above, the third light-shielding rate is lower than the region in the arcuate region having a certain radius including the center C. Further, the distance a is constant in the multiple regions described above.

Of the multiple first regions <NUM>, the area of the first region <NUM> near the center C of the photomask 200a is the smallest. The area of each first region <NUM> increases in proportion to the distance from the center C of the photomask 200a. On the other hand, among the multiple second regions <NUM>, the area of the second region <NUM> near the center C of the photomask 200a is the largest. The area of each second region <NUM> becomes smaller in inverse proportion to the distance from the center C of the photomask 200a. Therefore, in the photomask 200a according to the first embodiment, the third light-shielding rates of the unit regions <NUM> have a gradient along the first direction or the second direction from the center C of the photomask 200a.

<FIG> is a schematic plan view of another photomask not falling under claim <NUM>. <FIG> is a schematic plan view of a section of <FIG> which is further enlarged. In the photomasks 200b shown in <FIG> and <FIG>, unlike <FIG>, the first light-shielding rate of the first region <NUM> is higher than the second light-shielding rate of the second region <NUM>. Therefore, in the photomasks 200b of <FIG> and <FIG>, there are a non-light-shielding portion (light transmitting portion) and multiple light-shielding portions. Non-light-shielding portions are arranged around each light-shielding portion.

In <FIG> and <FIG>, the distance connecting the centers of the adjacent first regions <NUM> is defined as a. When the shape of the first region <NUM> is, for example, a substantially quadrangle, the center of the first region <NUM> is obtained and the distance a is calculated in the same manner as in the explanation of <FIG>. The photomask 200b in <FIG> includes multiple light-shielding portions arranged two-dimensionally at equal intervals repeatedly, and non-light-shielding portions arranged around each light-shielding portion. The non-light-shielding portion and the light-shielding portion are arranged so that the light-shielding rate at the central area of the photomask 200b is higher than the light-shielding rate at the peripheral area. One light-shielding portion corresponds to the first region <NUM>. A part of the non-light-shielding portion corresponds to the second region <NUM>.

In the pattern of the photomask 200b of <FIG>, the third light-shielding rate is high (the transmittance is low) in the vicinity of the center C of the photomask 200b, similarly to the photomask 200a of <FIG>. This is because the area of the first region <NUM> is large in the vicinity of the center C, and the area of each first region <NUM> becomes smaller as the distance from the center C of the photomask 200b increases. That is, the third light-shielding rates of the unit region <NUM> have a gradient along the first direction or the second direction from the center C of the photomask 200b.

In <FIG>, the unit region <NUM> surrounded by the broken line includes the first region <NUM> and the second region <NUM>, similarly to the one illustrated in <FIG>. The length of the unit region <NUM> along the first direction or the second direction is <NUM> × W<NUM> + W<NUM>. The third light-shielding rate in <FIG> is represented by W<NUM><NUM> / (2W<NUM> + W<NUM>)<NUM> because it is the ratio of the area of the first region <NUM> in the unit region <NUM> to the area of the unit region <NUM>.

<FIG> is a schematic plan view of another photomask not falling under claim <NUM>.

As shown in <FIG>, in the photomask 200b, the multiple unit regions <NUM> include the first unit region 203a, the second unit region 203b, and the third unit region 203c. The distance between the center C and the first unit region 203a is longer than the distance between the center C and the second unit region 203b. The distance between the center C and the third unit region 203c is longer than the distance between the center C and the first unit region 203a. The light-shielding rate of the first unit region 203a is lower than the light-shielding rate of the second unit region 203b and higher than the light-shielding rate of the third unit region 203c. For example, the third light-shielding rate of each unit region <NUM> is not less than <NUM>% and not more than <NUM>%. Each third light-shielding rate is inversely proportional to the square of the distance from the center C. Each third light-shielding rate may be in a relationship obtained by multiplying the ratio inversely proportional to the square of the distance from the center C by the conic constant or the square thereof.

<FIG> is a schematic plan view of a method for manufacturing a lens using the photomask according to the first embodiment. <FIG> is a schematic plan view of the lens manufactured by the manufacturing method of <FIG>.

The method for manufacturing the photodetector will be described below. The photodetector includes a lens manufactured using the photomask 200a or 200b described above. First, as shown in <FIG>, the p--type semiconductor layer <NUM> is provided on the p+-type semiconductor layer <NUM>. The low-concentration silicon p--type semiconductor layer <NUM> may be epitaxially grown on the surface of the high-concentration p+-type semiconductor layer <NUM>. The surface of the high-concentration p+-type semiconductor layer <NUM> is located on the incident side of light. The insulating portion <NUM> is formed in the p--type semiconductor layer <NUM>. Next, ion implantation is performed to form a p-type semiconductor layer <NUM> (a first semiconductor region). An n+-type semiconductor region <NUM> (a second semiconductor region) is provided on the p-type semiconductor layer <NUM>. An element <NUM> of an avalanche photodiode (APD) is formed by the p-type semiconductor layer <NUM> and the n+-type semiconductor region <NUM>. In this way, the insulating portion <NUM> is provided around the element <NUM>. In addition, polysilicon (a quenching portion <NUM>) for quenching resistance is formed. The quench portion <NUM> is electrically connected to the n+-type semiconductor region <NUM> via an interconnect <NUM> and a contact plug. Here, it is possible to form an array in which the elements <NUM> for detecting light are arranged two-dimensionally. In this case, the element <NUM> for detecting light and the circuit for selecting the element <NUM> for detecting light can be provided on the same substrate. After that, the photodetector <NUM> is manufactured by repeating film formation and patterning of the metal layer to form an interconnect layer (not shown) in the insulating layer <NUM>.

The lens is manufactured, for example, by grating. First, as described above, the element <NUM> and the insulating layer <NUM> for detecting light are formed. An insulating layer <NUM> (for example, a non-photosensitive and flat resist) is formed on the insulating layer <NUM>. Next, <NUM> of a photosensitive positive resist <NUM> forming the lens <NUM> is applied thereto. The resist <NUM> is then baked at <NUM> for <NUM> seconds. Then, the resist <NUM> is exposed for <NUM> seconds using the photomask 200a or 200b of the first embodiment. The exposed resist <NUM> is developed. Then, bleaching is performed, and baking is performed again at <NUM> for <NUM> minutes and at <NUM> for <NUM> minutes. As a result, the lens <NUM> as shown in <FIG> can be manufactured. If a minute lens is manufactured by the method described above, a micro lens is formed. By forming multiple micro lenses by the same method, a micro lens array (MLA) in which micro lenses are arranged on a photodetection element array arranged in two dimensions can be obtained. The best conditions such as the exposure time differ depending on the type of the photosensitive resist, the thickness of the resist, and the interval of the mask pattern. Preferably, one lens <NUM> is provided corresponding to one element <NUM>. The center of the one element <NUM> in the first surface and the center of the one lens <NUM> in the first surface are on the same axis. The first surface is parallel to the first direction and the second direction. The same axis crosses the first surface. In other words, when viewed from the third direction crossing the first surface, the center of the element <NUM> in the first surface overlaps the center of the corresponding lens <NUM> in the first surface.

As described above, the photomask 200a or 200b according to the first embodiment includes multiple unit regions <NUM> arranged in the first direction and the second direction. Each unit region <NUM> includes the first region <NUM> and the second region <NUM> provided around the first region <NUM>. The first region <NUM> has the first light-shielding rate. The second region <NUM> has the second light-shielding rate different from the first light-shielding rate. The multiple unit regions <NUM> include the first unit region 203a and the second unit region 203b having the same size as each other. The distance between the first unit region 203a and the center C is different from the distance between the second unit region 203b and the center C. The third light-shielding rate of the first unit region 203a is different from the third light-shielding rate of the second unit region 203b.

In order to realize the first embodiment, first, the three-dimensional shape of the lens is calculated by using an optical simulation or the like. The height of the lens at each coordinate in the three-dimensional shape was converted into an aperture ratio corresponding to the height by spreadsheet software. Further, in order to obtain the aperture ratio, the size of the first region <NUM> converted into a ratio of the square of the distance a and the area of the first region <NUM> was calculated, and a numerical table of the coordinates and the size of the first region <NUM> was created. The created numerical table is read by a computer, and a light-shielding pattern with an opening hole for each coordinate is converted into mask data. The photomask of <FIG> was manufactured by forming a metal layer corresponding to the light-shielding pattern on the surface of a light-transmitting glass substrate. The metal layer includes, for example, chromium.

The distance a in the manufactured photomask is <NUM>, and there is a gradient of the third light-shielding rates in the direction away from the center of the photomask. Using this photomask, a lens group was manufactured by the above-mentioned lens manufacturing method.

<FIG> are images of an electron microscope in a cross section of the lens manufactured by using a photomask according to the example <NUM>.

The manufactured lens surface is sufficiently smooth. Therefore, the incident light is less scattered on the lens surface, and the light is refracted and incident. Using ZEMAX as analysis software, the transmittance was calculated by a ray tracing method when linear light was incident from directly above the lens. The transmittance is <NUM>%, and a photodetection element equipped with this lens is expected to have high photon detection efficiency.

In the photomask 200a or 200b, the shape of the manufactured lens can be arbitrarily adjusted by adjusting the gradient of the third light-shielding rates of the multiple unit regions <NUM>. For example, in the photomask 200a or 200b, the third light-shielding rate of each unit region <NUM> near the center C is the same as each other. By increasing the number of unit regions <NUM> having the same third light-shielding rate, as shown in <FIG>, a lens having a flat surface near the center can be manufactured.

In example <NUM>, the photomask 200b shown in <FIG> was manufactured. The photomask 200b has multiple island-shaped second regions <NUM>. The distance a between the centers of the second regions <NUM> adjacent to each other in the first direction or the second direction is <NUM>. The elements other than these in the example <NUM> are the same as those in the example <NUM>. Using this photomask 200b, a lens group was manufactured by the above-mentioned lens manufacturing method.

The surface of the manufactured lens was sufficiently smooth. Therefore, the incident light is less scattered on the lens surface, and the light is refracted and incident. As a result of analyzing the transmittance in the same manner as in example <NUM>, the transmittance was <NUM>%. A photodetector equipped with this lens is expected to have high photon detection efficiency.

<FIG> is a plan view of a photomask according to a comparative example <NUM>. In the comparative example <NUM>, a photomask 200r shown in <FIG> was manufactured. In the photomask 200r, each of the multiple regions arranged in two dimensions is either a non-light-shielding region or a light-shielding region. By changing the density of the non-light-shielding region and the density of the light-shielding region from the center of the photomask 200r to the outside, a gradient of the light-shielding rates is formed. Using this photomask 200r, a lens set was manufactured by the above-mentioned lens manufacturing method.

<FIG> is an image of an electron microscope in a cross section of a lens manufactured by using a photomask according to the comparative example <NUM>. The surface of the manufactured lens is not smooth and the surface roughness is large. Therefore, the incident light is scattered on the surface, which leads to a decrease in transmittance. Therefore, a photodetection element equipped with this lens is not expected to have high photon detection efficiency.

<FIG> is an image of an electron microscope of a cut cross section of a lens manufactured by using a photomask according to an example <NUM>, <FIG> is an image of the electron microscope of the cut cross section of the lens after processing the lens shown in <FIG>.

In the example shown in <FIG>, multiple convex lenses <NUM> are formed on the surface of the resist <NUM>. In <FIG>, a broken line is attached to a portion that functions as the lens <NUM>. As shown in <FIG>, there is a gap G between the lenses <NUM>. The light incident in the gap G is not refracted toward the element <NUM>. In order to improve the photon detection efficiency of the photodetector <NUM>, it is preferable that the gap G is small.

In the example <NUM>, the resist <NUM> shown in <FIG> is etched back by reactive ion etching (RIE). By etching back, a part of the resist <NUM> is removed while maintaining the shape of the lenses <NUM>. At this time, by increasing the straightness of the ions incident on the resist <NUM>, the gap G can be made smaller as shown in <FIG>. For example, the gap G can be made substantially zero, and the lenses <NUM> can be brought into contact with each other. This makes it possible to further improve the photon detection efficiency of the photodetector <NUM>.

<FIG> is a schematic plan view of another photomask according to the first embodiment. <FIG> is a schematic plan view of an enlarged part of <FIG>.

In <FIG> and <FIG>, the white portion is the first region <NUM>. The black portion is the second region <NUM>. For example, the first region <NUM> is more likely to transmit light than the second region <NUM>. The first region <NUM> is a non-light-shielding portion. The second region <NUM> is a light-shielding portion. That is, the relationship between the light-shielding rate of the first region <NUM> and the light-shielding rate of the second region <NUM> in a photomask 200c shown in <FIG> is the same as the relationship between the light-shielding rate of the light-shielding rate of the first region <NUM> and the light-shielding rate of the second region <NUM> in the photomask 200a shown in <FIG>.

As shown in <FIG>, in the photomask 200c, the third light-shielding rates of the multiple unit regions <NUM> have multiple gradients from the center C toward the outer circumference. Specifically, the range R in which the multiple unit regions <NUM> are arranged includes a first portion <NUM> and a second portion <NUM>. The first portion <NUM> includes the center C of the range R and its vicinity. The second portion <NUM> is provided around the first portion <NUM>.

As shown in <FIG>, multiple unit regions <NUM> are arranged two-dimensionally in each of the first portion <NUM> and the second portion <NUM>. In the first portion <NUM>, the third light-shielding rates of the multiple unit regions <NUM> have a gradient from the center toward the outer circumference of the first portion <NUM>. The gradient of the third light-shielding rates from the outer circumference of the first portion <NUM> toward the inner circumference of the second portion <NUM> is opposite toward the gradient of the third light-shielding rates from the center toward the outer circumference of the first portion <NUM>. The gradient in the third light-shielding rates from the outer circumference of the first portion <NUM> toward the inner circumference of the second portion <NUM> is steeper than the gradient of the third light-shielding rates from the center toward the outer circumference of the first portion <NUM>.

In the second portion <NUM>, the third light-shielding rates of the multiple unit regions <NUM> have a gradient toward the outer circumference of the second portion <NUM>. The gradient of the third light-shielding rates from the inner circumference toward the outer circumference of the second portion <NUM> is opposite toward the gradient of the third light-shielding rates from the outer circumference of the first portion <NUM> toward the inner circumference of the second portion <NUM>. The change in the third light-shielding rate from the inner circumference toward the outer circumference of the second portion <NUM> is more gradual than the change in the third light-shielding rate from the outer circumference of the first portion <NUM> toward the inner circumference of the second portion <NUM>.

In the photomask 200c shown in <FIG>, the third light-shielding rate of the unit region <NUM> decreases from the center toward the outer circumference of the first portion <NUM>. The third light-shielding rate of the unit region <NUM> increases from the outer circumference of the first portion <NUM> toward the inner circumference of the second portion <NUM>. The third light-shielding rate of the unit region <NUM> decreases from the inner circumference toward the outer circumference of the second portion <NUM>.

As shown in <FIG>, the size of the first region <NUM> of the unit region <NUM>-<NUM> provided in the central area of the first portion <NUM> is smaller than the size of the first region <NUM> of the unit region <NUM>-<NUM> provided on the outer circumference of the first portion <NUM>. The size of the first region <NUM> of the unit region <NUM>-<NUM> is larger than the size of the first region <NUM> of the unit region <NUM>-<NUM> provided on the inner circumference of the second portion <NUM>. The size of the first region <NUM> of a unit region <NUM>-<NUM> provided on the outer circumference of the second portion <NUM> is larger than the size of the first region <NUM> of the unit region <NUM>-<NUM>.

As shown in <FIG>, the photomask 200c may further include a third portion <NUM>. Multiple third portions <NUM> are provided around the second portion <NUM>. In the illustrated example, four third portions <NUM> are provided. The four third portions <NUM> are located at the corners of an imaginary square surrounding the first portion <NUM> and the second portion <NUM>.

The multiple unit regions <NUM> are also arranged two-dimensionally in the third portion <NUM>. In the third portion <NUM>, the third light-shielding rates of the multiple unit regions <NUM> have a gradient from the outer circumference to the center of the third portion <NUM>. For example, the light-shielding rate of the unit region <NUM> increases from the outer circumference of the second portion <NUM> toward the outer circumference of the third portion <NUM>. The light-shielding rate of the unit region <NUM> decreases from the outer circumference toward the center of the third portion <NUM>.

As shown in <FIG>, the size of the first region <NUM> of a unit region <NUM>-<NUM> provided on the outer circumference of the third portion <NUM> is smaller than the size of the first region <NUM> of the unit region <NUM>-<NUM>. The size of the first region <NUM> of the unit region <NUM>-<NUM> provided at the center of the third portion <NUM> is larger than the size of the first region <NUM> of the unit region <NUM>-<NUM>.

<FIG> is an image of an electron microscope in a cross section of a lens manufactured by using the photomask according to the example <NUM>.

In the example <NUM>, the lens <NUM> was manufactured using the photomask 200c shown in <FIG>. As shown in <FIG>, multiple convex portions are also formed on the surface of the manufactured lens <NUM> according to the multiple gradients of the light-shielding rates in the photomask 200c. In the illustrated example, the lens <NUM> includes convex portions p1 to p3. The convex portion p1 is formed corresponding to the first portion <NUM>. The convex portion p2 is formed corresponding to the second portion <NUM>. The convex portion p3 is formed corresponding to the third portion <NUM>.

<FIG> is a schematic plan view of a photomask according to a second embodiment, not falling under claim <NUM>.

A photomask 300a shown in <FIG> includes multiple unit regions arranged in a two-dimensional manner. Each unit region <NUM> includes a first region <NUM> and a second region <NUM>. The second region <NUM> is provided around the first region <NUM>. The light-shielding rate of the first region <NUM> is different from the light-shielding rate of the second region <NUM>. In the illustrated example, the light-shielding rate of the first region <NUM> is smaller than the light-shielding rate of the second region <NUM>.

The sizes of the first regions <NUM> are the same as each other. The distance between the first regions <NUM> adjacent to each other in the first direction or the second direction changes along the first direction or the second direction. In the photomask 300a, the distance between the first regions <NUM> becomes smaller as the distance from the center C of the photomask 300a increases. That is, in the photomask 300a, the distance between the adjacent first regions <NUM> changes from the center C toward the outer circumference, so that the third light-shielding rates of the multiple unit regions <NUM> have a gradient.

For example, the multiple unit regions <NUM> include unit regions 303a to 303d. Each of the distance between the unit region 303a and the center C and the distance between the unit region 303b and the center C is shorter than each of the distance between the unit region 303c and the center C and the distance between the unit region 303d and the center C. The distance d1 between the first region <NUM> of the unit region 303a and the first region <NUM> of the unit region 303b is longer than the distance d2 between the first region <NUM> of the unit region 303c and the first region <NUM> of the unit region 303d.

<FIG> is a schematic plan view of another photomask according to the second embodiment, not falling under claim <NUM>.

In a photomask 300b shown in <FIG>, the distance between the first regions <NUM> increases as the distance from the center C of the photomask <NUM> increases. For example, the distance d1 between the first region <NUM> of the unit region 303a and the first region <NUM> of the unit region 303b is shorter than the distance d2 between the first region <NUM> of the unit region 303c and the first region <NUM> of the unit region 303d.

As in the second embodiment, the gradient of the light-shielding rates may be controlled by adjusting the distance between the first regions <NUM> having the same size. Even when the photomask according to the second embodiment is used, a lens having a smooth surface shape can be manufactured as in the case where the photomask according to the first embodiment is used.

In the photomask specifically described above, the third light-shielding rate of the unit region <NUM> near the center C is higher than the third light-shielding rate of the unit region <NUM> far from the center C. These photomasks are used when exposing to positive photoresists. In the photomasks 200a, 200b, 300a, or 300b, the relationship between the first light-shielding rate and the second light-shielding rate in each unit region may be reversed while maintaining the size of each first region and the size of each second region. In that case, a photomask used when exposing to a negative photoresist is obtained.

<FIG> and <FIG> are schematic plan views of another photomask according to the embodiment, not falling under claim <NUM>.

<FIG> shows the photomask 200a that can be used for a negative photoresist. The first light-shielding rate of the first region <NUM> is higher than the second light-shielding rate of the second region <NUM>. The multiple unit regions <NUM> include the first unit region 203a, the second unit region 203b, and the third unit region 203c. The distance between the center C and the first unit region 203a is longer than the distance between the center C and the second unit region 203b. The distance between the center C and the third unit region 203c is longer than the distance between the center C and the first unit region 203a. The light-shielding rate of the first unit region 203a is higher than the light-shielding rate of the second unit region 203b and is lower than the light-shielding rate of the third unit region 203c. For example, the light-shielding rate of the second unit region 203b provided near the center C of the photomask 200a is <NUM>%, and the light-shielding rate of the third unit region 203c farthest from the center C of the photomask 200a is <NUM>%.

<FIG> shows the photomask 300a that can be used for a negative photoresist. The first light-shielding rate of the first region <NUM> is higher than the second light-shielding rate of the second region <NUM>. The multiple unit regions <NUM> include unit regions 303a to 303d. Each of the distance between the unit region 303a and the center C and the distance between the unit region 303b and the center C is shorter than each of the distance between the unit region 303c and the center C and the distance between the unit region 303d and the center C. The distance d1 between the first region <NUM> of the unit region 303a and the first region <NUM> of the unit region 303b is longer than the distance d2 between the first region <NUM> of the unit region 303c and the first region <NUM> of the unit region 303d.

Even when a negative type photoresist photomask as shown in <FIG> or <FIG> is used, a lens having a smooth surface can be manufactured as in the case of using the positive type photoresist photomask described above.

<FIG> is a schematic plan view of another photomask according to the embodiment, not falling under claim <NUM>.

As in a photomask <NUM> shown in <FIG>, multiple ranges R may be arranged two-dimensionally. In each range R, multiple unit regions <NUM> are arranged. Each range R has a light-shielding rate gradient for forming one lens, and corresponds to the photomask 200a shown in <FIG>, the photomask 200b shown in <FIG>, the photomask 300a shown in <FIG>, or the photomask 300b shown in <FIG>. MLA can be formed by exposing the resist <NUM> with the photomask <NUM>.

Hereinabove, exemplary embodiments of the invention are described with reference to specific examples. However, the embodiments of the invention are not limited to these specific examples. For example, one skilled in the art may similarly practice the invention by appropriately selecting specific configurations of components included in photodetectors such as elements, semiconductor regions, insulating portions, insulating layers, interconnects, contact plugs, lenses, etc., from known art. Such practice is included in the scope of the invention to the extent that similar effects thereto are obtained.

Further, any two or more components of the specific examples may be combined within the extent of technical feasibility and are included in the scope of the invention to the extent that the purport of the invention is included.

Moreover, all photodetectors, photodetection systems, lidar devices, and mobile bodies practicable by an appropriate design modification by one skilled in the art based on the photodetectors, the photodetection systems, the lidar devices, and the mobile bodies described above as embodiments of the invention also are within the scope of the invention to the extent that the purport of the invention is included, the invention being defined by the scope of the appended claims only.

Claim 1:
A photomask (200a, 200c), comprising:
a plurality of unit regions (<NUM>) arranged in a first direction and a second direction crossing the first direction,
each of the unit regions including
a first region (<NUM>) having a first light-shielding rate, and
a second region (<NUM>) having a second light-shielding rate higher than the first light-shielding rate, and provided around the first region,
characterized in that the unit regions includes a first unit region (203a) and a second unit region (203b) having same size each other,
a distance between the first unit region and a center (C) of a range (R) in which the unit regions are arranged being longer than a distance between the second unit region and the center, and
a light-shielding rate of the first unit region is lower than a light-shielding rate of the second unit region,
wherein the second regions (<NUM>) adjacent to each other in the first direction or the second direction are in contact with each other.