METHOD FOR FORMING A LIFT-OFF MASK STRUCTURE

Abstract

A method for forming a lift-off mask structure includes providing a substrate body, depositing a layer of bottom anti-reflective coating, BARC, over a surface of the substrate body, and depositing a layer of photosensitive resist over the BARC layer. The method further includes exposing the resist layer to electromagnetic radiation through a photomask, and forming the lift-off mask structure by applying a developer for selectively removing a portion of the BARC layer and of the resist layer such that an underlying portion of the surface of the substrate body is exposed.

Claims

1. A method for forming a lift-off mask structure, the method comprising: providing a substrate body; depositing a layer of bottom anti-reflective coating, BARC, over a surface of the substrate body; depositing a layer of photosensitive resist over the BARC layer; exposing the resist layer to electromagnetic radiation through a photomask; and forming the lift-off mask structure by applying a developer for selectively removing a portion of the BARC layer and of the resist layer such that an underlying portion of the surface of the substrate body is exposed.

2. The method according to claim 1, wherein the BARC layer, after forming the lift-off mask structure, is characterized by an undercut profile with negative sidewall slopes.

3. The method according to claim 1, wherein the resist layer, after forming the lift-off mask structure, is characterized by an overcut profile with positive sidewall slopes.

4. The method according to claim 1, wherein a material of the BARC layer is not light-sensitive.

5. The method according to one of claim 1, wherein a material of the BARC layer is absorbent, in particular highly absorbent, at a wavelength of the electromagnetic radiation.

6. The method according to one of claim 1, wherein a material of the BARC layer is an organic material.

7. The method according to claim 1, wherein a material of the BARC layer and a material of the photosensitive resist layer are characterized by refractive indices at a wavelength of the electromagnetic radiation that differ by less than 10%, in particular less than 5%, from each other.

8. The method according to claim 1, wherein a material of the BARC layer is characterized by a refractive index that causes destructive interference within the resist layer during the exposure to the electromagnetic radiation.

9. The method according to claim 1, wherein depositing the BARC layer comprises depositing a BARC material with a thickness of less than 500 nm, in particular less than 200 nm, over the surface of the substrate body.

10. The method according to claim 1, wherein depositing the photosensitive resist layer comprises depositing a positive photoresist.

11. The method according to claim 1, further comprising a step of baking the BARC layer before depositing the resist layer.

12. The method according to claim 1, wherein a material of the BARC layer is soluble in the developer, in particular in an isotropic manner.

13. A device that is manufactured following a process that comprises forming a lift-off mask structure according to claim 1.

14. The device according to claim 13, wherein for manufacturing the device, the method according to claim 1 is applied repeatedly.

15. The device according to claim 13, wherein the method is applied for manufacturing a multi-layered interference filter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In the figures:

[0043] FIG. 1A to 1D show intermediate products of a lift-off mask according to the improved concept;

[0044] FIG. 1E shows a lift-off mask according to the improved concept;

[0045] FIG. 2 shows an intermediate product of a device manufactured following a process that includes a lift-off mask according to the improved concept after deposition of a target material; and

[0046] FIGS. 3 to 5 show finalized devices manufactured following a process that includes a lift-off mask according to the improved concept after lift-off.

DETAILED DESCRIPTION

[0047] FIG. 1A shows an intermediate product of a lift-off mask according to the improved concept after deposition of a bottom anti-reflective coating, BARC, layer 11 over a surface of the substrate body 10.

[0048] The substrate body 10 is, for example, a semiconductor substrate, such as a silicon wafer or a part of a silicon wafer, such as a chip. Alternatively, the substrate body 10 is a glass substrate, e.g. a mirror substrate. The substrate body 10 can further comprise functional layers, such as CMOS layers, that are deposited on a substrate.

[0049] The BARC layer 11 is deposited onto a top surface of the substrate body 10 as a wet BARC via spin coating, for instance. A thickness of the BARC layer 11 is in the order of 200 nm to 500 nm, for example. The BARC layer 11 is of an organic material, such as a polyvinylphenol derivate. Alternatively, the BARC layer 11 can be a single thin layer of a transparent material such as a silica, magnesium fluoride and fluoropolymers, or the BARC layer 11 can comprise alternating layers of a low-index material like silica and a higher-index material. Optionally, after depositing the BARC layer 11, the intermediate product, in particular the BARC 11, can be temperature treated, for example during a baking process, for adjusting its response to a specific developer recipe. The BARC layer 11 is light-insensitive. This means that its response to a developer is unaffected by light at least at a wavelength used during an exposure, e.g. UV light at a wavelength of 365 nm corresponding to the i-line lithography.

[0050] FIG. 1B shows the intermediate product of the lift-off mask of FIG. 1A after depositing a layer of photosensitive resist 12. The resist layer 12 is a typical positive resist, for example based on a mixture of diazonaphthoquinone, DNQ, and novolac resin, which is a phenol formaldehyde resin. Therein, a positive photoresist is understood as a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The unexposed portion of the photoresist remains insoluble to the photoresist developer. The resist layer 12 is deposited via spin coating, for instance with a typical thickness between 450-1500 nm.

[0051] FIG. 1C shows the intermediate product of the lift-off mask of FIG. 1B during a lithographic exposure step through a photomask 20. Using photolithography, a pattern of the photomask 20 is transferred to the photoresist layer 12. In other words, portions of the resist layer 12 that are covered by opaque portions of the photomask 20 regarding a wavelength of the exposing radiation 21 remain unexposed while portions of the resist layer 12 that are not covered by the opaque portions are exposed to the radiation 21. In the shown example of the resist layer 12 being of a positive resist, the chemistry of the exposed portions of the resist layer 12 is altered by the radiation 21, such that these portions are soluble in a developer solution.

[0052] During the exposure, the BARC layer 11 suppresses unwanted lithography effects, such as reflective notching and the formation of standing wave patterns within the resist layer 12, for example via absorption and/or destructive interference. To this end, a refractive index of the BARC layer 11 is adjusted according to a refractive index of the resist layer 12 and/or the substrate body 10. For example, the refractive indices of the aforementioned elements differ from each other by less than 5%.

[0053] Alternatively to employing a positive resist as described above, the employment of a negative resist as the resist layer 12 is likewise possible according to the improved concept. For negative resists, the exposed portions remain after the developing while the non-exposed portions are dissolved and thus removed.

[0054] FIG. 1D shows the intermediate product of the lift-off mask of FIG. 1C after a first part of the developing. As described above, previously exposed portions of the positive resist layer 12 are dissolved and thus removed using a developer solution. Therein, a positive sidewall profile 12a, i.e. an overcut profile, can be formed. The slope angle of the positive side walls can be predetermined by parameters of the exposure with the exposing radiation 21. For example, an adjustable focal point of the radiation 21 can be set to a specific depth within the resist layer 12.

[0055] FIG. 1E shows a finalized lift-off mask 1 according to the improved concept after a second part of the developing. During the second part, the portion of the BARC layer 11 that is exposed after removing the aforementioned portions of the resist layer 12 is likewise removed by the developer solution used to remove the portions of the resist layer 12. Therein, the BARC layer 11 reacts to the developer solution and an isotropic manner. This way, a negative sidewall profile 11a, i.e. an undercut profile, can be formed. The slope angle of the negative side walls can be predetermined by parameters of the aforementioned temperature treatment of the BARC layer 11 before deposition of the resist layer 12, for instance. For example, the slope angle is in the order of 45, thus creating an undercut that corresponds to or is in the order of a thickness of the BARC layer 11, for instance in the order of 200 nm.

[0056] As can be seen in FIG. 1E, the finalized lift-off mask 1 on the substrate body 10 is characterized by a BARC layer 11 with a negative sidewall profile 11a and by a resist layer 12 with a positive sidewall profile 12a. This ensures that a significantly smaller feature spacing, i.e. neighboring openings in the lift-off mask, can be achieved compared to conventional approaches that employ purely negative sidewall profiles of the entire lift-off mask. Furthermore, unwanted liftoff effects such as shadowing are inhibited by the positive sidewall profile 12a of the resist layer 12. It will be understood that the developer solution can perform the removal of the resist layer 12 and the BARC layer 11 in a simultaneous manner instead of the subsequent manner illustrated in FIGS. 1D and 1E, which mainly serves for illustration purposes.

[0057] FIG. 2 shows an intermediate product of a device manufactured following a process that includes a lift-off mask according to the improved concept after deposition of a target material 13. For example, the target material 13 is a metal or dielectric. The target material 13 is deposited in a uniform manner on the liftoff mask 1, i.e. remaining portions of the resist layer 12, and in openings created after the developing of the lift-off mask 1. The edges of the structured material 13 are illustrated with a positive sidewall profile. These are typical due to the deposition process not being perfectly anisotropic, resulting in a slight deposition below the roof of the lift-off mask. Perfectly vertical edges of the target material 13 are only achievable via an etching process and not with a lift-off process.

[0058] FIG. 3 shows the intermediate product of FIG. 2 after stripping the lift-off mask 1, i.e. the resist layer 12 and the BARC layer 11, completely from the substrate body 10. Therein, the negative sidewall profile 11a of the BARC layer 11 ensures an unhindered access to the mask-substrate interface for the lift-off solution.

[0059] FIGS. 4 and 5 show further exemplary embodiments of a device manufactured following a process that comprises a lift-off mask according to the improved concept. FIG. 4 illustrates that due to the sidewall profiles of the lift-off mask, a significantly smaller feature spacing can be achieved. For example, the target material 13 forms optical elements such as photodiodes of a high-resolution CMOS image sensor.

[0060] FIG. 5 illustrates how a stack of target materials 13 can be formed via multiple lift-off processes according to the improved concept. Therein, one layer of target material 13 is deposited for each lift-off step. This can be used to form multi-layered optical interference filters on a glass substrate, for instance. Such filters can comprise between 20 and 100 alternating layers of two different target materials 13, for example. For illustration purposes, the sidewall profile of the target material 13 are kept vertical here.

[0061] Exact methods to deposit BARC and resist layers and to perform the actual lift-off are well-known concepts and thus are not further detailed in this disclosure.

[0062] It is further pointed out that a lift-off mask according to the improved concept is not limited to manufacturing optical devices but can also be used for defining micro- or nano-sized structures of various types, e.g. electrodes of a CMOS circuit.

[0063] The embodiments of the lift-off mask and the device manufactured using such a lift-off mask disclosed herein have been discussed for the purpose of familiarizing the reader with novel aspects of the idea. Although preferred embodiments have been shown and described, many changes, modifications, equivalents and substitutions of the disclosed concepts may be made by one having skill in the art without unnecessarily departing from the scope of the claims. In particular, the disclosure is not limited to the disclosed embodiments, and gives examples of many alternatives as possible for the features included in the embodiments discussed. However, it is intended that any modifications, equivalents and substitutions of the disclosed concepts be included within the scope of the claims which are appended hereto.

[0064] Features recited in separate dependent claims may be advantageously combined. Moreover, reference signs used in the claims are not limited to be construed as limiting the scope of the claims.

[0065] Furthermore, as used herein, the term comprising does not exclude other elements. In addition, as used herein, the article a is intended to include one or more than one component or element, and is not limited to be construed as meaning only one.

[0066] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that any particular order be inferred.