Improved precision greyscale photolithography process

Abstract

Title: Improved Precision Grayscale Photolithography Process The invention relates to a grayscale lithography process comprising supplying a substrate (10) coated with a first layer of photosensitive resin (20), exposing this layer to a first exposure (R1) through a first mask (100), and developing the image. The first mask is configured to form in the resin a set of first structures (25), each having a height within a first range of heights (Δ1). The process includes forming a second layer of photosensitive resin (30), exposing this layer to a second exposure (R2) through a second mask (200), and developing the image.The masks are configured to allow the formation of a set of second structures (35), each having a height within a second range of heights (Δ2) disjoint from the first range of heights (Δ1). Figure for the abbreviation: Fig.3E.

Description

Title of the invention: Improved precision grayscale photolithography process technical field

[0001] The present invention relates to the field of photolithography, more specifically to that of greyscale lithography. It relates in particular to the fabrication of structures comprising elements of different heights. STATE OF THE ART

[0002] Greyscale lithography is a photolithography technique that allows the creation of three-dimensional (3D) microstructures in a single lithographic and development step. It is particularly used in the fabrication of optical microelements, MEMS (microelectromechanical systems), MOEMS (microoptoelectromechanical systems), microfluidic devices, and textured surfaces.

[0003] This technique is based on varying the thickness along a dimension Z over which a photosensitive resin is exposed by modulating the dose of ultraviolet (UV) received by the resin in space. Once the exposed portions have developed, the resin exhibits a 3D structure (seen by scanning electron microscopy (SEM) and shown in Figures IB and ID) and can, for example, serve as a mold for the fabrication of 3D microstructures.

[0004] The ultraviolet dose received locally by the resin can be modulated, in particular, by adjusting the dimensions and positioning of opaque areas present on the lithography mask (Figures IA and IC). These opaque areas are typically created by depositing chromium on a glass or quartz mask.

[0005] Grayscale lithography thus makes it possible to obtain 3D microstructures with a characteristic height ranging from a few hundred nanometers to a few hundred micrometers. However, the accuracy of the height at each point of the microstructure is highly dependent on the variability of mask fabrication and the variability of the process. The variability of the final height of the microstructure is all the greater when the pattern density at the mask level is low, as illustrated in Figures 2A and 2B. These figures are experimental results obtained for a resin layer with an initial thickness of 1.6 pm, in which pillars of different sizes were formed. In [Fig. 2A], it is clear that the bars The error, proportional to three times the standard deviation (3°), is greater than for higher densities. Figure 2B, which records the value of 3° for different density values, also shows this trend. One explanation for this phenomenon is that regions of the resin exposed through areas of the mask with low pattern density receive a higher dose of sunlight than other regions. Consequently, these regions interact with the developing agent for a longer period, which may explain the significant inaccuracy in the height of the final structures formed in these areas.

[0006] The variability in the height of the formed structures is very unfavorable and limits the applications of grayscale photolithography that can be considered. For example, some optoelectronic devices can currently be manufactured with difficulty or not at all by a grayscale photolithography process. Applications requiring a spectral signature at different wavelengths, for example, such as Fabry-Perot sensors, require very precise control of the height of the cavities associated with each wavelength. If we consider the dimensioning of the different channels associated with as many wavelengths within a Fabry-Perot cavity (see [Fig.2C]): For a desired spectral range R and a given number N of channels, the difference AX between the wavelengths of one channel to another is AX = R / N, and the difference AH between the channel heights of one channel to another is AH = AX / 2n, where n is the reflection index of the cavity material. For example, if we want a spectral range from 400 nm to 1000 nm (AX = 700 nm) with N = 32 and n = 1.5, this leads to AH = 7.3 nm. The height of the smallest cavity (for X = 400 nm) would be approximately 133 nm, and that of the largest cavity (for X = 1100 nm) would be 366 nm. It appears that such precision over such varied cavity heights, especially starting from a typical initial resin thickness of approximately 1.5 pm, is unattainable with current greyscale photolithography methods.

[0007] There is therefore a need for a solution enabling the production of structures of different sizes with good precision by grayscale photolithography. ABSTRACT

[0008] To achieve this objective, according to a first aspect of the invention, a greyscale lithography process comprising: a. the supply of a substrate having a top surface extending mainly in a horizontal plane, the substrate being covered with a first layer of photosensitive resin, b. exposure of the first layer of photosensitive resin to initial exposure to sunlight through a first mask, followed by development of the first layer of photosensitive resin, the first the mask being configured so that once the first layer of resin is developed, it presents a set of initial structures, each with a height within a first range of heights, the height of the initial structures being measured along a so-called vertical direction perpendicular to the horizontal plane, c. the formation of a second layer of photosensitive resin on the upper surface of the substrate, d. an exposure of the second layer of photosensitive resin to a second insolation radiation through a second mask and a development of the second layer of photosensitive resin, the first mask and the second mask being configured so that the development of the second layer of resin allows the formation of a set of second structures each having a height within a second range of heights disjoint from the first range of heights, the height of the second structures being measured along the vertical direction, the first structures and the second structures being in distinct areas in projection in the horizontal plane.

[0009] By proceeding as proposed by the invention, it is possible, when it is desired to form a set of structures having varying heights, as is the case in many optoelectronic devices, to first form, using the first mask, a first set of structures having the smallest heights. Then, using the second layer of resin and the second mask, structures with greater heights can be formed. It is understood that it is not mandatory to form the structures from smallest to largest; the groups of structures of different sizes can be formed in any order.

[0010] By proceeding sequentially, using two photolithography steps, it is avoided to have to simultaneously form structures with very different heights. In the case of a single resin layer dedicated to forming all the structures, it is necessary to remove material to a very great depth to form the smallest structures. The process according to the invention eliminates this need, since the smallest structures can be formed from a resin layer whose height does not necessarily have to allow for the creation of the larger structures.

[0011] Thus, the process according to the invention makes it possible to release the constraints on the lithography mask as well as on the process conditions. Consequently, the errors related to these two parameters are limited. The effective heights of the structures obtained by the process according to the invention are much closer to the target values ​​compared to the processes of the prior art.

[0012] Furthermore, according to one embodiment of the invention, it is possible to retain a residual layer of the first resin layer, which will form part of the second structures created during the second lithography step. By using the thickness of the residual layer to form the second structures, the required thickness of the second resin layer is reduced. This allows the lithography step to be performed in a thinner resin layer, which is again beneficial to the accuracy of the height of the structures.

[0013] The process according to the invention thus makes it possible to simultaneously limit the impact of variability on the mask and variability of the process. This results in a significant improvement in manufacturing accuracy compared to existing photolithography processes.

[0014] A second object of the present invention relates to a use of the process according to the first aspect of the invention in the manufacture of a photonic device taken from among: a multispectral filter, a phase grating, an imager, a coupler.

[0015] The advantages provided by the process according to the first aspect of the invention apply mutatis mutandis to this use. BRIEF DESCRIPTION OF THE FIGURES

[0016] The aims, objects, features and advantages of the invention will become clearer from the detailed description of an embodiment thereof, which is illustrated by the following accompanying drawings in which:

[0017] [Fig. 1A] [Fig. 1B] [Fig. 1C] [Fig. 1D] Figures IA to 1D represent grayscale lithography masks and scanning electron microscope views of resins exposed through these masks and then developed. Figures IA and IB relate to the fabrication of 3D structures with platforms at different heights. Figures IC and 1D relate to the fabrication of 3D microlenses with a dome shape.

[0018] [Fig.2A] Fig.2A is a graph illustrating the height of pillars obtained by greyscale photolithography from different pattern densities on the mask, as well as the error on this height.

[0019] [Fig.2B] Fig.2B illustrates the error in the height of pillars obtained by greyscale photolithography from different pattern densities on the mask.

[0020] [Fig.2C] Fig.2C illustrates the dimensioning of the different channels associated with as many wavelengths within a Fabry-Perot cavity.

[0021] [Fig.3A] [Fig.3B] [Fig.3C] [Fig.3D] [Fig.3E] Figures 3A to 3E illustrate a first embodiment of the process according to the invention, in which the second structures are entirely formed in the second layer of resin.

[0022] [Fig.4A] [Fig.4B] [Fig.4C] [Fig.4D] [Fig.4E] Figures 4A to 4E illustrate a second embodiment of the process according to the invention, in which the second structures are formed by a residual layer of the first resin layer and by the second resin layer.

[0023] [Fig.5A] [Fig.5B] [Fig.5C] [Fig.5D] [Fig.5E] [Fig.5F] Figures 5A to 5F illustrate a third embodiment of the process according to the invention, in which the second insolation step is carried out through the first mask having undergone a translation relative to the substrate with respect to the first insolation step.

[0024] [Fig.6A] [Fig.6B] [Fig.6C] [Fig.6D] Figures 6A to 6D illustrate the possibility of using the present invention to manufacture a photonic coupler.

[0025] The drawings are given by way of example and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate understanding of the invention and are not necessarily to scale with practical applications. In particular, the dimensions are not representative of reality. DETAILED DESCRIPTION

[0026] Before proceeding with a detailed review of embodiments of the invention, optional features that may be used in combination or alternatively are listed below:

[0027] According to a preferred example, the first and second masks are configured so that the first and second height ranges each extend over a range less than or equal to 300 nm, preferably less than or equal to 200 nm, and preferably less than or equal to 100 nm. Thus, each lithography step is dedicated to producing structures whose various heights fall within a restricted range. This improves the accuracy of the structure heights.

[0028] According to a preferred example, the first mask and the second mask are configured so that the heights of the first structures are less than the heights of the second structures.

[0029] According to a preferred embodiment, the process further comprises, after the exposure and development of the first layer of photosensitive resin and before the formation of the second layer of photosensitive resin, a hardening of the first layer of photosensitive resin.

[0030] According to one embodiment, the first mask has areas transparent to the first insolation radiation, so that the development of the The first layer of resin causes, apart from the initial structures, its shrinkage along its entire height in the vertical direction.

[0031] According to one embodiment, the first mask is configured so that once the first resin layer is developed, it has, outside of the initial structures, a residual layer, and each of the second structures includes a portion of this residual layer. By retaining a residual layer, the surface topography created by the first resin layer is minimized. The second resin layer is therefore deposited onto a surface with a less pronounced surface topography. The roughness transferred to the upper surface of the second resin layer is thus limited. This embodiment therefore minimizes the surface roughness of the second structures.

[0032] According to one example, the first mask has opaque areas to the first insolation radiation, so that the residual layer has at least locally a height equal to the height of the first resin layer before insolation and development, the height of the residual layer and the height of the first resin layer being measured along the vertical direction.

[0033] According to one example, the second mask is the first mask having undergone a translation relative to the substrate.

[0034] According to one embodiment, the second photosensitive layer is deposited on the first structures, and the second mask has areas transparent to the second insolation radiation, so that the development of the second resin layer causes the first structures to be revealed.

[0035] In the context of the present invention, a resin is defined as an organic or organo-mineral material that can be shaped by exposure to a beam of electrons, photons, X-rays, a beam of light in the ultraviolet, extreme ultraviolet (EUV) or deep ultraviolet (Deep UV) range, typically in the wavelength range of 193 nm to 248 nm, the emission lines of a mercury lamp, namely: 365 nm for the I line, 435 nm for the G line and 404 nm for the H line.

[0036] The invention applies equally to positive resins, i.e., those whose exposed part becomes soluble in the developer and where the unexposed part remains insoluble, and to negative resins, i.e., those whose unexposed part becomes soluble in the developer and where the exposed part remains insoluble.

[0037] The contrast of a resin, commonly denoted y, reflects the effectiveness of the behavior referred to in the literature as the "threshold" behavior of the resin. The greater the contrast, the smaller the dose variation required for the resin to transition from a state in which it cannot be developed to a state in which it can be developed (or vice versa for a negative resin). The value of the contrast y of a resin, Whether it is positive or negative in tone, it is generally determined by the slope of the curve according to the following equation: e _ j, where e is the thickness of the resin film after exposure and development, e0 is the initial thickness of the resin film, D is the applied exposure dose and Do is the dose at which the entire thickness of the film is developed.

[0038] The "nature" of a material such as a resin means its chemical composition, that is to say, the nature and proportion of the species constituting the material. Two layers are considered to be made of the same resin if they have the same chemical composition.

[0039] In this description, the dose is defined as the amount of energy received by a resin per unit area. This energy can be in the form of photons (photolithography) for a photosensitive resin. It is then the product of the intensity of the incident light radiation (generally expressed in Watts / m²) and the exposure time (expressed in seconds). The dose is then usually expressed in Joules per m², or more often in millijoules (mJ) per cm² (10⁴ m²) or even in mJ / m². This energy can also be in the form of electrons (electron lithography) for an electrosensitive resin. The dose is then usually expressed in coulombs per m², or more often in microcoulombs (pC) per cm² (10² m²), or in pC / m².

[0040] The pattern density D to be imposed at the mask level to obtain a given structure height can be obtained from the following equations:

[0041] [Math.l] - [e^^^ + e^dose]

[0042] [Math.2]

[0043] [Math.3] dose = Dsr^ 1 - D)2

[0044] Where: hresin is the desired structure height, hddark is the resin erosion height without exposure, "dose" is the exposure dose, and h0 is the initial resin thickness after deposition and before exposure / development. eb, e2, e3, and e4 are parameters that can be obtained by extraction from the contrast curve. C is the Dill coefficient of the resin, generally provided by the resin manufacturer.

[0045] A parameter "approximately equal to / greater than / less than" a given value means that this parameter is equal to / greater than / less than the given value, to within 20%, or even 10%, or even 5% of that value. A parameter "approximately between" two given values ​​means that this parameter is at least equal to the smallest given value, plus or minus 20%, or even 10%, close to that value, and at most equal to the largest given value, plus or minus 20%, or even 10%, close to that value.

[0046] In this patent application, the terms thickness for a layer and height for a structure or device will be preferred. The thickness is measured in a direction normal to the principal plane of extension of the layer, and the height is measured perpendicular to the horizontal XY plane. Thus, a layer typically has a thickness in the so-called vertical Z direction when it extends primarily along the horizontal XY plane. The relative terms "on," "under," and "below" preferentially refer to positions measured in the vertical Z direction.

[0047] The method according to the invention will now be described in more detail with reference to the figures.

[0048] A first step of the process includes providing a substrate 10. This substrate has a top face 11 extending mainly along the horizontal XY plane. The substrate 10 can, for example, be taken from a glass substrate, a silicon substrate or a substrate with CMOS (Complementary Metal Oxide Semiconductor) components.

[0049] The substrate 10 is coated with a first layer of photosensitive resin 20. Advantageously, the first layer of resin 20 is in contact with the upper face 11 of the substrate 10. The first layer of resin 20 has a thickness e20 measured along the vertical direction Z, perpendicular to the horizontal plane XY. e20 is typically less than or equal to Ipm, for example between 200 nm and Ipm, for example approximately equal to 500 nm.

[0050] A second step of the process includes exposing the first layer of resin 20 through a first mask 100. This exposure step is carried out by exposing the first layer of resin 20 to a first incident radiation Ri through the first mask 100. The development of the first layer of photosensitive resin 20 is then carried out. The characteristics of the first mask 100 will be described later, depending on the embodiments.

[0051] This second step allows the formation of first structures 25 in the first layer of resin 20. These first structures 25 each have a height h251, h252, h253, h254 measured along the vertical direction Z. When the first layer of resin 20 is in contact with the substrate 10, the height of the first structures 25 is measured from the upper face 11 of the substrate 10.

[0052] The height of each first structure 25 is within a first range of heights denoted Ab. Thus, the first mask 100 is dedicated to the manufacture of structures whose height lies within this first range Ab.

[0053] Advantageously, once the first structures 25 have been formed, the first layer of resin 20 is hardened. This includes, in particular, hardening the first structures 25.

[0054] A third step of the process involves forming a second layer of photosensitive resin 30 on the substrate 10, more specifically on the upper surface 11 of the substrate 10. Typically, the second layer of resin 30 covers the portions of the first layer of resin 20 remaining after its development. The second layer of resin 30 has a thickness e30 measured along the vertical direction Z, perpendicular to the horizontal plane XY. e30 corresponds to the maximum height of the second layer of resin. e30 is chosen according to the height of the maximum height of the second structure 35 that one wishes to fabricate.

[0055] The second layer of photosensitive resin 30 can be of the same nature as the first layer of photosensitive resin 20. It is also possible to use resins of different natures.

[0056] A fourth step of the process comprises exposing the second layer of resin 30 through a second mask 200. This exposure step is carried out by exposing the second layer of resin 30 to a second incident radiation R2 through the second mask 200. The development of the second layer of photosensitive resin 30 is then carried out. The characteristics of the second mask 200 will be described later, depending on the embodiments.

[0057] This fourth step allows the formation of second structures 35. The second structures 35 can each be formed entirely by the second layer of resin 30, or partly by the second layer of resin 30 and partly by the first layer of resin 20. These two possibilities will be detailed further.

[0058] The second structures 35 each have a height h35i, h352, h353 measured along the vertical direction Z.

[0059] The height of each second structure 35 is included in a second range of heights denoted A2. Thus, the second mask 200 (possibly in combination with the first mask 100) is dedicated to the manufacture of structures whose height lies within this second range A2. The first range of heights Ai and the second range of heights A2 are disjoint. In other words, their intersection is zero.

[0060] Advantageously, once the second structures 35 are formed, the second layer of resin 30 is hardened. This includes, in particular, hardening the second structures 35.

[0061] The first radiation Ri and the second radiation R2 to which the resin layers 20, 30 are subjected each have a principal direction substantially perpendicular to the horizontal XY plane. These Rb R2 radiations are typically UV (ultraviolet) radiations; they can thus be radiations emitted in a wavelength range from approximately 100 nm to approximately 400 nm, for example, 365 nm. However, they can also be radiations with wavelengths outside this range. Generally, but not limited to, radiations emitted in a wavelength range from approximately 90 nm to approximately 500 nm can be considered. The first radiation Ri and the second radiation R2 may be similar or different.

[0062] Ideally, the resin(s) used shall have at least one of the following characteristics: a. A substantially linear response between the radiation dose to which it is exposed and the thickness over which it is exposed. b. A sufficiently low contrast, for example less than 2, to allow the implementation of greyscale lithography, but sufficiently high, for example greater than 1, to avoid excessively long exposure times. Advantageously, the contrast is between 1.1 and 1.5. c. Good film-forming properties, guaranteed for example by the presence of a film-forming agent in its composition. d. Weak inhibition of dissolution.

[0063] Examples of resins that can be used in the context of the invention include resins produced by Micro Resist Technology bearing the commercial references ma-P 1215G, ma-P 1225G and ma-P 1275G.

[0064] The hardening of resins can be achieved by thermal or chemical means.

[0065] The first mask 100 and the second mask 200 have transparent areas and opaque areas.

[0066] The transparent zones correspond to regions of the mask 100, 200 whose composition is transparent to insolation radiation RB R2, while the opaque zones correspond to regions of the mask 100, 200 whose composition is opaque to insolation radiation RH R2. A zone is considered opaque, for example, when it blocks at least 90% of the incident Rb R2 radiation. A zone is considered transparent when it transmits at least 60% of the incident RB R2 radiation.

[0067] For example, the mask can be a glass mask with chromium deposits. The opaque areas then correspond to the areas of the mask 100, 200 where chromium has been deposited, while the transparent areas correspond to the areas that remained free of chromium.

[0068] According to the principle of grayscale lithography, for a given region of the mask 100, 200, the surface density D of the opaque areas within this region determines the radiation dose received by the region of the underlying resin layer 20, 30 and therefore, consequently, the thickness e over which this region of the resin layer 20, 30 is exposed by the R2 radiation. This density D is typically modulated from one region to another of the mask 100, 200 so as to spatially modulate the exposed thickness in the resin layer 20, 30.

[0069] For a given region of the mask, the surface density D of the opaque areas is the ratio between the surface area of ​​the region occupied by the opaque areas and the total surface area of ​​the region. These areas can, for example, be evaluated at the level of a lower face of the mask 100, 200 located opposite the resin layer 20, 30 during exposure. It is typically on this face that the material deposits (for example, chromium) forming the opaque areas are made.

[0070] A first embodiment of the process according to the invention will now be described with reference to figures 3A to 3E.

[0071] Fig. 3A illustrates the supply of the substrate 10 surmounted by the first layer of resin 20 and the exposure of the first layer of resin 20 by the first radiation Ri through the first mask 100.

[0072] The transition from [Fig.3A] to [Fig.3B] illustrates the development of the first layer of resin 20. This results in a plurality of first structures 25 formed in the first layer of resin 20.

[0073] As illustrated, the height h25i, h252, h253, h254 of each first structure 25 is found in the first range Ab

[0074] Advantageously, the first mask 100 used in this embodiment comprises transparent areas 110. These transparent areas 110 allow the first layer of resin 20 to be locally removed along its entire height. The upper surface 11 of the substrate 10 is thus preferably locally exposed between the first structures 25 (see [Fig. 3B]).

[0075] The first mask 100 also includes areas 120 that are neither entirely transparent nor entirely opaque, called intermediate areas 120, dedicated to the formation of the first structures 25 with distinct heights, according to the principle of greyscale lithography.

[0076] Advantageously, at this stage of the process, the first layer of resin 20 is hardened, that is to say here, the first structures 25 are hardened.

[0077] Fig. 3C illustrates the deposition of the second layer of photosensitive resin 30 on the substrate 10. As illustrated, the second layer of resin 30 extends over the first structures 25 and directly into contact with the upper face 11 of the substrate 10 in the areas where the latter has been exposed by insolation and development of the first layer of resin 20.

[0078] The second layer of resin 30 thus forms an encapsulation of the first structures 25.

[0079] Fig. 3D illustrates the exposure of the second layer of resin 30 by the second radiation R2 through the second mask 200.

[0080] The transition from [Fig.3D] to [Fig.3E] illustrates the development of the second layer of resin 30. This results in a plurality of second structures 35 formed in the second layer of resin 30.

[0081] As illustrated, the heights h35i, h352, h353, h354 of each second structure 35 are found in the second range A2. In the illustrated example, the second range A2 includes values ​​greater than those in the first range Ab

[0082] Advantageously, the second mask 200 used in this embodiment includes transparent areas 210. These transparent areas 210 allow the second layer of resin 30 to be locally removed along its entire height. Advantageously, during the exposure of the second layer of resin 30, these transparent areas 210 are positioned directly above the first structures 25. This allows the first structures 25 to be exposed (see [Fig. 3E]). Since the first structures 25 preferably undergo prior hardening, they are not altered by the exposure and development step undergone by the overlying second layer of resin 30.

[0083] The second mask 200 also includes areas 220 that are neither entirely transparent nor entirely opaque, called intermediate areas 220, dedicated to the formation of the second structures 35 with distinct heights, according to the principle of greyscale lithography.

[0084] Preferably, in this embodiment, when considering each of the first mask 100 and the second mask 200 in the same position of exposure of the resin layers 20, 30, the transparent areas 110 of the first mask 100 are located in the same place as the intermediate areas 220 of the second mask 200. Furthermore, preferably, the transparent areas 210 of the second mask 200 are located in the same place as the intermediate areas 120 of the first mask 100.

[0085] In other words, by superimposing the first mask 100 and the second mask 200, we then preferably have, in projection onto the horizontal XY plane: a. an overlap of the transparent areas 110 of the first mask 100 and the intermediate areas 220 of the second mask 200, and / or b. a superposition of the transparent areas 210 of the second mask 200 and the intermediate areas 120 of the first mask 100.

[0086] It is understood that the intermediate zones 120, 220 of each mask 100, 200 may in some cases correspond to opaque zones, if it is desired that the structure formed after development 25, 35 has a height equal to the layer of resin 20, 30 deposited.

[0087] Thus, in this embodiment, each mask 100, 200 is entirely dedicated to the formation of structures 25, 35 in a range of heights Ai, A2.

[0088] A second embodiment of the process according to the invention will now be described with reference to Figures 4A to 4E.

[0089] Fig. 4A illustrates the supply of the substrate 10 surmounted by the first layer of resin 20 and the exposure of the first layer of resin 20 by the first radiation Ri through the first mask 100.

[0090] The transition from [Fig. 4A] to [Fig. 4B] illustrates the development of the first resin layer 20. As in the first embodiment, this results in a plurality of first structures 25 formed in the first resin layer 20. Here too, the height h25i, h252, h253, h254 of each first structure 25 falls within the first range Ab

[0091] Advantageously, the first mask 100 used in this embodiment includes opaque areas 130. These opaque areas 130 make it possible to locally retain the entire height of the first layer of resin 20. After development of the first layer of resin 20, a residual layer 23 is thus obtained having a thickness e23 equal to e20.

[0092] According to another example, instead of completely opaque areas 130, it is possible to use areas that are neither entirely opaque nor entirely transparent. These areas are then configured so that exposure to sunlight and the development of the first resin layer 20 removes the resin to only a portion of its height. After the development of the first resin layer 20, a residual layer 23 is thus obtained with a thickness e23 between 0 (excluded, or else the thickness is taken locally as zero) and e20.

[0093] In both cases, a residual layer 23 is formed which will be used for the formation of the second structures 35 (see above).

[0094] It is understood that the residual layer 23 does not necessarily have a uniform thickness. In particular, it is possible to locally omit the residual layer 23. The residual layer 23 can be formed using areas in the first mask 100 with different opaque area densities. Forming a residual layer 23 with a non-uniform height can allow for more varied heights of the second structures 35.

[0095] As in the first embodiment, the first mask 100 also includes areas 120 that are neither entirely transparent nor entirely opaque, called intermediate areas 120, dedicated to the formation of the first structures 25 with distinct heights, according to the principle of greyscale lithography.

[0096] Advantageously, at this stage of the process, the first layer of resin 20 is hardened, that is to say here, the first structures 25 and the residual layer 23 are hardened.

[0097] Fig. 4C illustrates the deposition of the second layer of photosensitive resin 30 on the substrate 10. As illustrated, the second layer of resin 30 extends over the first structures 25 and over the residual layer 23.

[0098] Fig. 4D illustrates the exposure of the second layer of resin 30 by the second radiation R2 through the second mask 200.

[0099] The transition from [Fig.4D] to [Fig.4E] illustrates the development of the second resin layer 30. This results in a plurality of second structures 35, each formed by a portion of the second resin layer 30 and a portion of the residual layer 23. The height h35i, h352, h353, h354 of each second structure 35 is greater than or equal to the local thickness e23 of the residual layer 23.

[0100] As illustrated, the heights h35i, h352, h353, h354 of each second structure 35 are found in the second range A2. In the illustrated example, the second range A2 includes values ​​greater than those in the first range Ab

[0101] Advantageously, the second mask 200 used in this embodiment includes transparent areas 210. These transparent areas 210 allow the second layer of resin 30 to be locally removed along its entire height. Advantageously, during the exposure of the second layer of resin 30, these transparent areas 210 are positioned directly above the first structures 25. This allows the first structures 25 to be exposed (see [Fig. 4E]). Since the first structures 25 preferably undergo prior hardening, they are not altered by the exposure and development stage experienced by the overlying second layer of resin 30.

[0102] The second mask 200 also includes areas 220 that are neither entirely transparent nor entirely opaque, called intermediate areas 220, dedicated to the formation of the second structures 35 with distinct heights, according to the principle of greyscale lithography.

[0103] Preferably, in this embodiment, when considering each of the first mask 100 and the second mask 200 in the same position of exposure of the resin layers 20, 30, the opaque areas 130 of the first mask 100 are located in the same place as the intermediate areas 220 of the second mask 200. Furthermore, preferably, the transparent areas 210 of the second mask 200 are located in the same place as the intermediate areas 120 of the first mask 100.

[0104] In other words, by superimposing the first mask 100 and the second mask 200, we then preferably have, in projection onto the horizontal XY plane: a. an overlap of the opaque areas 130 of the first mask 100 and the intermediate areas 220 of the second mask 200, and / or b. a superposition of the transparent areas 210 of the second mask 200 and the intermediate areas 120 of the first mask 100.

[0105] It is understood that the intermediate zones 120, 220 of each mask 100, 200 may in some cases correspond to opaque zones, if it is desired that the structure formed after development 25, 35 has a height equal to the layer of resin 20, 30 deposited.

[0106] Thus, in this embodiment, the first mask 100 is dedicated not only to the formation of the first structures 25 in the first range of heights Ai, but also participates in the formation of the second structures 35 in the second range of heights A2.

[0107] A third embodiment of the process according to the invention will now be described with reference to Figures 5A to 5F.

[0108] This embodiment combines the principles of the first two embodiments.

[0109] Fig. 5A illustrates the supply of the substrate 10 surmounted by the first layer of resin 20 and the exposure of the first layer of resin 20 by the first radiation Ri through the first mask 100.

[0110] The transition from [Fig. 5A] to [Fig. 5B] illustrates the development of the first resin layer 20. As in other embodiments, this results in a plurality of first structures 25 formed in the first resin layer 20. Here too, as illustrated, the height h25i, h252, h253, h254 of each first structure 25 lies within the first range Ab

[0111] In this embodiment, the first mask 100 comprises a continuous opaque zone 130. The opaque zone 23 preferably extends over a major part of the first resin layer 20, for example over at least the half the surface area of ​​the first resin layer 20. This opaque zone 130 allows the entire height of the first resin layer 20 to be retained locally. After the development of the first resin layer 20, a residual layer 23 is obtained with a thickness e23 equal to e20. As before, it is possible to configure the first mask 100 so that the residual layer 23 has a thickness e23 between 0 (excluded, or else the thickness is locally set to zero) and e20. Furthermore, the residual layer 23 can have a non-uniform thickness.

[0112] As in other embodiments, the first mask 100 also includes areas 120 that are neither entirely transparent nor entirely opaque, called intermediate areas 120, dedicated to the formation of the first structures 25 with distinct heights, according to the principle of greyscale lithography.

[0113] Preferably, the first mask 100 used in this embodiment comprises transparent areas 110. These transparent areas 110 allow the first layer of resin 20 to be locally removed along its entire height. The upper face 11 of the substrate 10 is thus preferably locally exposed between the first structures 25 (see [Fig. 5B]).

[0114] Advantageously, at this stage of the process, the first layer of resin 20 is hardened, that is to say here, the first structures 25 and the residual layer 23 are hardened.

[0115] Fig. 5C illustrates the deposition of the second layer of photosensitive resin 30 on the substrate 10. As illustrated, the second layer of resin 30 extends over the first structures 25 and over the residual layer 23. It also extends directly into contact with the upper face 11 of the substrate 10 in the areas where the latter has been exposed by insolation and development of the first layer of resin 20.

[0116] Fig. 5D illustrates the exposure of the second layer of resin 30 by the second radiation R2 through the second mask 200. In the present embodiment, the second mask 200 corresponds to the first mask 100 having undergone a translation relative to the substrate 10. To do this, it is possible to move the first mask 100, the substrate 10 or both.

[0117] The transition from [Fig.5D] to [Fig.5E] illustrates the development of the second resin layer 30. This results in a plurality of second structures 35, each formed by a portion of the second resin layer 30 and a portion of the residual layer 23.

[0118] As illustrated, the height h35i, h352, h353, h354 of each second structure 35 is within the second range A2. Due to the use of the same mask 100 for both stages of exposure to insolation radiation, the height h35i, h352, h353, h354 of each second structure 35 is equal to the sum of the local thickness e23 of the residual layer 23 and the height h25i, h252, h253, h254 of the first structure 25 formed by the same zone in the mask 100: h35i=e23+ h25i, h352=e23+ h252, h352=e23+ h252, h352=e23+ h252.

[0119] In the typical case where the residual layer 23 has a uniform thickness (constant e23), this embodiment makes it possible to form sets of structures 25, 35 exhibiting the same height differences between them. With appropriate dimensioning of e23, it is possible for the structures 25, 35 to all exhibit the same height difference from one to the next.

[0120] As in other embodiments, advantageously, exposure and development of the second resin layer allows the first structures 25 to be revealed. In this embodiment, as illustrated in [Fig. 5D], simply moving the mask 100 relative to the substrate 10 can suffice to directly expose the first structures 25 and the portion of the second resin layer 30 covering them to the second radiation R2. It is therefore not necessary to provide transparent areas in the mask 100 specifically for direct exposure of the region of the first structures 25.

[0121] Thus, in this embodiment, a single mask 100 allows the formation of the first structures 25 in the first range of heights Ai and the second structures 35 in the second range of heights A2.

[0122] It is understood that the three embodiments described above have been described for a positive photosensitive resin. However, the invention also applies to negative resins. The principles of the two main embodiments (second structures made without or with a residual layer of the first resin layer) can be implemented by using opaque areas instead of transparent areas on the first mask 100 and on the second mask 200, and vice versa.

[0123] It is further understood that the process can be continued with the deposition of one or more additional layers of photosensitive resin and the formation of further structures, based on the same principles as those described for two sets of structures 25, 35. Figure 5F, in particular, illustrates the result that can be obtained after depositing a third layer of resin and exposing it through the first mask 100, which has undergone another translation relative to the substrate 10. These steps enabled the formation of third structures 45, the height of each of which falls within a third range of heights A3. This third range of heights A3 is disjoint from both Ai and A2.

[0124] Figures 6A to 6D illustrate the possibility of using the present invention for the manufacture of various devices, in this case a photonic coupler.

[0125] As illustrated in Figures 6A (top view) and 6B (cross-sectional view), a photonic coupler is formed of several structures 61, 62, 63, 64 based on a dielectric material. The process according to the invention can be used to manufacture a mold for producing such a coupler. In particular, it is possible to first form resin structures corresponding to a portion of the coupler structures (for example, structures 61 and 62, see [Fig. 6C]), and then resin structures corresponding to the remainder of these structures (for example, structures 63 and 64, see [Fig. 6C]).

[0126] In view of the different embodiments described above, it appears that the present invention provides an effective solution for improving the accuracy of the height of structures formed by photolithography.

[0127] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention.

Claims

Demands

1. A greyscale lithography process comprising: • supplying a substrate (10) having a top face (11) extending mainly in a horizontal plane (XY), the substrate (10) being coated with a first layer of photosensitive resin (20), • exposing the first layer of photosensitive resin (20) to a first insolation radiation (Ri) through a first mask (100) and then developing the first layer of photosensitive resin (20), the first mask (100) being configured so that once the first layer of resin (20) is developed, it has a set of first structures (25) each having a height within a first range of heights (AJ), the height of the first structures (25) being measured along a so-called vertical direction (Z) perpendicular to the horizontal plane (XY),• the formation on the upper face (11) of the substrate (10) of a second layer of photosensitive resin (30), • the exposure of the second layer of photosensitive resin (30) to a second insolation radiation (R2) through a second mask (200) and the development of the second layer of photosensitive resin (30), the first mask (100) and the second mask (200) being configured so that the development of the second layer of resin (30) allows the formation of a set of second structures (35) each having a height within a second range of heights (A2) disjoint from the first range of heights (Ai), the height of the second structures (35) being measured along the vertical direction (Z), the first structures (25) and the second structures (35) being located in distinct areas in projection onto the horizontal plane (XY).

2. A method according to the preceding claim wherein the first mask (100) and the second mask (200) are configured to that the first range of heights (AJ) and the second range of heights (A2) each extend over a range less than or equal to 300 nm, preferably less than or equal to 200 nm, preferably less than or equal to 100 nm.

3. A method according to any one of the preceding claims wherein the first mask (100) and the second mask (200) are configured so that the heights of the first structures (25) are less than the heights of the second structures (35).

4. A method according to any one of the preceding claims further comprising, after exposure and development of the first layer of photosensitive resin (20) and before formation of the second layer of photosensitive resin (30), a hardening of the first layer of photosensitive resin.

5. A method according to any one of the preceding claims in which the first mask (100) has areas transparent to the first insolation radiation (RJ, so that the development of the first layer of resin (20) causes, apart from the first structures (25), its shrinkage over its entire height in the vertical direction (Z).

6. A method according to any one of the preceding claims wherein the first mask (100) is configured so that once the first layer of resin (20) has been developed, it has, apart from the first structures (25), a residual layer (23), and wherein each of the second structures (35) includes a part of the residual layer (23).

7. A method according to the preceding claim wherein the first mask (100) has opaque areas to the first insolation radiation (Ri), such that the residual layer (23) has at least locally a height equal to a height of the first resin layer (20) before insolation and development, the height of the residual layer (23) and the height of the first resin layer (20) being measured along the vertical direction (Z).

8. A method according to any one of the two preceding claims wherein the second mask (200) is the first mask (100) having undergone a translation relative to the substrate (10).

9. A method according to any one of the preceding claims, wherein the second photosensitive layer (30) is deposited on

10. the first structures (25), and in which the second mask (200) has areas transparent to the second insolation radiation (R2), so that the development of the second layer of resin (20) causes the exposure of the first structures (25). Use of the process according to any one of the preceding claims in the manufacture of a photonic device taken from: a multispectral filter, a phase grating, an imager, a coupler.

Images