Microstructured transfer mask

Abstract

Microstructured transfer mask (300), (400) for locally differentiated transfer of materials (140) onto a substrate, comprising a carrier material (110), a light-absorbing layer (320), (330), (420), (430) and a light-reflecting layer (130), characterized in that the lateral heat conduction on the coated side of the transfer mask (300), (400) is reduced by recesses (310), (440), (450) in the carrier material (110) with a depth of 1 to 200 micrometers.

Description

[0001] The invention relates to a transfer mask for the local vapor deposition of substrates for the production of structures with a minimum width of less than ten micrometers. This mask is used, for example, in the production of OLED smartphones with UHD resolution, or 3840 x 2160 pixels for a 5.5-inch screen. The subpixels required for this have an edge length of approximately ten micrometers. OLED microdisplays are another possible application.

[0002] Transfer masks are used to transfer structures to a substrate by imaging the structures to be produced on the mask and then transferring them to a substrate using the mask (DE 10 2013 108 315 A1). In semiconductor technology, photolithography is used extensively to image the mask structures on the substrate by exposing masked substrates. The structures on the substrate are usually created using subtractive processes, i.e. processes in which material is removed. The transfer mask described in the aforementioned publication for the structured evaporation of material is suitable for evaporation using flash lamps because it is designed to be resistant to the thermally induced mechanical stresses that occur during this process.

[0003] DE 10 2009 041 324 A1 discloses a method for the local vapor deposition of a substrate using a transfer mask. In this method, a transfer mask is used to locally vaporize organic coating material from the mask onto the substrate. The structures on the substrate are formed using additive processes, i.e. processes that add material. For vapor deposition, the coating material is deposited over the entire surface of the transfer mask, but is then vaporized only in the desired locations. For this purpose, the transfer mask has reflective or absorbing regions, or both, in a required structure. If the transfer mask is positioned above or on the substrate, energy is introduced by light radiation, and thus vaporization only occurs in the regions where the coating material absorbs sufficient energy to vaporize.Due to the layer thicknesses to be deposited in this process in the range of ten to several hundred nanometers, a pulsed energy input, e.g. by a flash lamp, is sufficient for the evaporation of organic materials.

[0004] DE 10 2013 206 484 A1 also discloses a transfer mask suitable for vapor deposition of organic materials. Its structure includes spacers for adjusting the distance and thus the free volume between the vapor deposition layer and the substrate to prevent layer defects.

[0005] Fig. Figure 1 shows a cross-section of a typical transfer mask structure (100) consisting of a light-transparent carrier material (110) such as quartz glass. A light-absorbing material (120) – hereinafter referred to as absorber – such as chromium nitride is first applied over the entire surface of the carrier material (110). A preferred coating method is magnetron sputtering due to the high adhesive strength and film density that can be achieved with this method. The absorber is then microstructured using photolithography and an etching step, following common process steps in semiconductor technology. A light-reflecting material (130) – hereinafter referred to as reflector – such as silver is deposited over the entire surface of the microstructured absorber. Here, too, a magnetron sputtering process is preferred for deposition of the reflector. Typical layer thicknesses for reflectors or absorbers are 50 to 150 nanometers.With the last-mentioned process step, the production of the transfer mask (100) is complete. The layer to be locally transferred (140), e.g., an organic material such as aluminum tris(8-hydroxyquinoline), is then applied over the entire surface of the reflector using a thermal vapor deposition process in a vacuum. Magnetron sputtering is neither possible nor necessary with organic layers due to their low chemical stability. Typical layer thicknesses of the material to be transferred (140) are ten to a few hundred nanometers. If the back of the transfer mask is now illuminated with pulsed light (150), e.g., light from a flash lamp or a laser, the absorber (120) heats up so strongly that the material to be transferred (140) evaporates only in the area of ​​the absorber. The evaporated material condenses on a substrate (not shown), which is attached in close proximity to the front of the transfer mask (100).The distance between the transfer mask (100) and the substrate can be adjusted to less than one micrometer by means of spacers applied to the substrate or transfer mask (100).

[0006] Depending on the chemical and thermal properties of the material or the application, the absorber and reflector can be interchanged, e.g., due to microstructuring capabilities. To prevent chemical reactions, diffusion barriers or protective layers can be applied between the reflector and absorber, or between the transfer mask and the layer to be transferred, such as a 30 nm thick layer of aluminum oxide or silicon nitride.

[0007] However, the process is problematic due to the minimal possible resolution of the structures transferred onto the substrate. The minimal possible resolution is largely determined by the pulse duration of the radiation source as well as by the heat conduction between the absorber and adjacent areas from which no material is to be transferred. In DE 10 2011 082 939 A1, the resolution of the transfer mask is increased by an anisotropic thermal conductivity of the light-absorbing material, e.g., by microstructuring. In this case, the absorber and reflector are interchanged, in contrast to Fig. 1. Nevertheless, the minimum possible resolution remains limited to approximately ten micrometers due to thermal conduction. This applies even if there is no direct contact between the absorber and reflector. In the latter case, the thermal conduction of the carrier material (110) primarily limits the resolution. The object of the invention described below is to achieve a minimum resolution of less than ten micrometers.

[0008] In Fig. Figure 1 illustrates the heat transfer (160) or heat conduction from the absorber (120) into the carrier material (110). Although a carrier material (110) made of quartz glass already has a relatively low thermal conductivity compared to a reflector (130) made of silver, the heat conduction in the glass leads to undesired heating or evaporation of the material (140) in the regions (170). Thus, the total evaporation area is larger than the area of ​​the absorber (160). As previously mentioned, this also applies if there is no direct contact or direct thermal connection between the absorber (120) and the thin layers (130) or (140) in the region (170). The heat transfer in the carrier material (110) is illustrated in a thermal simulation in Fig. 2.

[0009] In Fig. 2, a 0.5 micrometer thick absorber layer applied over the entire surface of the quartz glass transfer mask was simulated for the sake of simplicity. Layers (130) and (140) were neglected as a first approximation. The solid curve (substrate depth = + 0.5 micrometers) shows the temperature curve on the surface of the absorber as a function of time. The exposure of the absorber starts at time t = 0 milliseconds and ends at t = 0.3 milliseconds. The dotted curve (substrate depth = -1.5 micrometers) represents the temperature curve in a plane in the glass that is 1.5 micrometers away from the absorber layer. The dashed or triangular curve shows the temperature curve in another plane in the glass that is parallel to the absorber layer and at a distance of 3.0 micrometers or 7.0 micrometers, respectively. In the case shown, the assumed evaporation temperature is in a range between 450 degrees Celsius and 500 degrees Celsius.To put it simply, a glass layer with a thickness of significantly more than 1.5 micrometers would have to be used between the absorber and reflector as a thermal resistor in order to keep the temperature of the reflector below the minimum evaporation temperature of 450 degrees Celsius throughout the entire process step. In practice, the geometries are much more complex and three-dimensional, and so are the temperature distributions. The corners and edges of the absorber play a particularly important role. Nevertheless, the simulation shows that even with an optimized material selection or structuring of the absorber and the reflector, the thermal conduction of the carrier material does not allow microstructures of just a few micrometers due to the areas (170) (see . Fig. 1).

[0010] For evaporation temperatures significantly above 500 degrees Celsius, thermal radiation plays a dominant role. This temperature range is not considered here because it is unusual in practice for transfer masks and the lifetime of the masks is severely limited due to thermomechanical stresses in the deposited layers. Thermal convection between the transfer mask and the substrate can be neglected to a first approximation for all temperature ranges, since the material transfer is only carried out under vacuum conditions.

[0011] Fig. 3 shows a structure (300) according to the invention. The production of the transfer mask includes at least one additional, but relatively simple process step, as is usual with MEMS (micro electro mechanical systems): First, as in the previously mentioned production method, the absorber is applied over the entire surface of the carrier material (110) via magnetron sputtering. Subsequently, the absorber is microstructured into regions (320) and (330) using photolithography and an etching step. With a suitable choice of photoresist, e.g. SU-8, the photoresist can be used not only to microstructure the absorber layer, but also subsequently, with the aid of anisotropic DRIE (deep reactive ion etching), the same structure can be transferred several micrometers deep into the carrier material, creating a depression (310). The subsequent process steps for the deposition of the reflector (130) and the material to be evaporated (140), however, do not change.Consequently, no additional lithography step is required for the recess (310) in the carrier material (110). Compared to magnetron sputtering, the sidewalls of the recess (310) remain virtually uncoated during thermal evaporation of the organic material onto the transfer mask (300). This is due to the lower process pressure of the thermal evaporation process and thus the lower collision probability of the vapor particles compared to magnetron sputtering. During the exposure process, however, the heat transport (160) in the carrier material (110) leads to undesirably high heating only in the region (370), so that no organic material is evaporated except at the locations of the absorbers (320) and (330).The recess (310) in the carrier material (110) increases the distance between the absorbers (320) and (330) and areas of the reflector (130) coated with the organic material (140) - i.e. areas at the bottom of the recess (310). The size of the absorbers (320) or (330) or the distance between the absorbers does not change. An important aspect here is the heat capacity of the volume in the carrier material (110), which results from the recess (310) below the absorbers (320) or (330). It is calculated from the area of ​​the respective absorber and the recess in the carrier material. As a first approximation, the energy introduced by the exposure of the respective absorber is distributed in this volume by the heat flow (160). The resulting temperature increase should remain below the evaporation temperature.As an alternative to DRIE, the depressions can also be created by laser ablation or other methods.

[0012] If during the production of the transfer mask (300) in Fig. 3 If the light absorber (320, 330) is swapped with the light reflector (130), the material to be transferred (140) ideally evaporates only in the area of ​​the bottom of the depressions (310). Due to the depressions, the volume in which the material that has already evaporated but has not yet condensed on the substrate (not sketched) is located during transfer increases. The volume increase achieved by the depression leads to a reduction in the maximum vapor pressure according to the law for ideal gases. Depending on the selected process parameters and the type of material to be transferred, the reduction in the maximum vapor pressure can lead to a change in the density and / or degree of crystallization of the layer condensed on the substrate.

[0013] In the Fig.4 shows an optimized layer structure of a transfer mask (400), which, however, requires greater effort to produce the mask. After the microstructuring of the absorber applied over the entire area into regions (420) and (430), the recesses are defined in a second lithography step and then etched with DRIE. The recess is no longer extended to the entire area between two adjacent absorbers (420) and (430), but is replaced by two recesses (440) and (450). The recesses (440) and (450) have a constant width, designated by “B” in the figure, which is independent of the distance between the absorbers. The structure etched with DRIE is therefore linked to a significantly smaller tolerance with regard to etching depth and etching profile, i.e. deviation of the recess (410) from the box shape. The reason for this is the exchange of etching and etching elements that is known with DRIE and is dependent on the aspect ratio of the recess (440) or (450).Deposition gases. A box shape without pronounced rounding at the edges or corners can only be adjusted for a constant width of the recess (440) or (450). Rounding the edges would also have different effects on the regions (420) and (430) due to their different sizes. The volume in the carrier material (110) between the recesses (440) and (450) represents additional heat capacity, which serves to lower the temperature.

[0014] In all cases described above, it is possible to swap the reflector with the absorber depending on the materials used or their applications.

[0015] In principle, instead of the depressions (310) or (440), (450) in the carrier material (110), a layer several micrometers thick of another material with the lowest possible thermal conductivity and the highest possible heat capacity can be applied, which is then structured in place of the carrier material (110). However, this is difficult to achieve in practice due to the numerous requirements for the additional material. Examples of the requirements include similar thermal expansion coefficients to the carrier material, a temperature resistance of well over 500 degrees Celsius, a thickness of several micrometers, or compatibility with the cleaning process of the transfer mask after exposure. List of reference symbols 100 Typical structure of a transfer mask 110 Carrier material of the transfer mask 120 light absorbers 130 light reflector 140 material to be transferred 150 Pulsed Light 160 Heat transport 170 areas with undesirably high heating 300 Structure of the transfer mask according to the invention 310 Recess in the transfer mask 320 light absorbers 330 light absorbers 370 areas with undesirably high heating 400 Optimized structure of the transfer mask according to the invention 420 large area light absorbers 430 small area light absorbers 440 Optimized deepening 450 Optimized deepening

Claims

[1] Microstructured transfer mask (300), (400) for the locally differentiated transfer of materials (140) onto a substrate, which contains a carrier material (110), a light-absorbing layer (320), (330), (420), (430) and a light-reflecting layer (130), characterized by that the lateral heat conduction on the coated side of the transfer mask (300), (400) is reduced by depressions (310), (440), (450) in the carrier material (110) with a depth of 1 to 200 micrometers. [2] Microstructured transfer mask according to claim 1, wherein at least the light-absorbing layer (320), (330), (420), (430) is microstructured. [3] Microstructured transfer mask according to claim 1, wherein at least the light-reflecting layer (130) is microstructured. [4] Microstructured transfer mask according to claim 1 to 3, wherein the recesses (440), (450) have a constant width. [5] Microstructured transfer mask according to claim 1, wherein a protective layer is applied to the transfer mask directly under the material to be transferred (140). [6] Microstructured transfer mask according to claim 1, wherein a diffusion barrier is applied between the light-reflecting layer (130) and the light-absorbing layer (320), (330), (420), (430).

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