# Patent

## Title
DUV photoresist process

## Abstract

DUV lithography process that eliminates post exposure baking of a photoresist. Thick photoresist may be processed to obtain enhanced sidewall profiles for microelectronic devices.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority to U.S. Provisional Patent Application No. 61/808,160 entitled “Post Exposure Bake Removal for DUV Photoresist Process” filed on Apr. 3, 2013 for Xianzhong Zeng, et al. which is incorporated herein by reference.


BACKGROUND
FIG. 1 illustrates a conventional process 100 for preparing a photoresist. In process 100 a photoresist is provided on a substrate to form a coating in block 12, and then the photoresist coating is soft baked via block 14. Exposure of the coating through a mask is then performed via block 16. Process 100 proceeds to bake the resist coating after exposure via block 18, a procedure known as post exposure bake. A post exposure bake procedure is considered necessary to forman image in the resist. The conventional process concludes with development block 19, in which the pattern on the mask is transferred to the resist coating.
Features patterned with photoresist masks prepared in accordance with process 100 have demonstrated deformed sidewalls in certain photolithography processes. FIG. 2A depicts a photoresist 30 on substrate 32. Photoresist 30 has been baked after exposure in accordance with the method of FIG. 1. As a result of performing a post exposure bake, the top and bottom resist critical dimension (CD) shrink to form the curved side walls 35 of FIG. 2A. After development, sidewall 35 of photoresist 30 has a deformed profile that deviates from the substantially vertical sidewalls that are desirable for fabricating microelectronic structures. Thus, process 100 generates photoresists that are unsuitable for forming microelectronic structures with predetermined critical dimensions.
A photoresist processed in accordance with the method of FIG. 1 was used to define the side shields 20 of FIG. 2B. On substrate 22, side shields 20 are shown flanking cavity 25 in an intermediate structure. Intermediate structure may be used to fabricate, for example, a magnetic recording head. Cavity 25 has a narrower opening f near top 23, and a much wider opening g near base 28. Specifically, FIG. 2B illustrates an intermediate structure having a width f near the top 23 of cavity 25 and a width g near base 28 of cavity 25. Since width f is significantly dissimilar to width g, the sidewall profile of features patterned with photoresists prepared by process 100 adversely impacts the critical dimensions of patterned devices.
It is believed that post exposure baking can deform photoresist sidewalls prepared in accordance with process 100. Therefore, photoresists prepared in accordance with process 100 impartan undesirable curvature to side shields 20 in FIG. 2B.
In light of the aforementioned problems, there is a need for improving the methods for processing photoresists for high resolution lithography.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flowchart of a conventional method for producing a photoresist.
FIG. 2A depicts a photoresist produced in accordance with the prior method of FIG. 1.
FIG. 2B depicts a side shield produced in accordance with the prior method of FIG. 1.
FIG. 3 is a flowchart of a method for processing a photoresist in accordance with one embodiment of the disclosure.
FIG. 4A depicts a photoresist produced in accordance with the process of FIG. 3.
FIG. 4B depicts a side shield produced with a photoresist processed in accordance with an embodiment of the method of FIG. 3.
FIG. 5 illustrates a trench formed using a photoresist mask prepared in accordance with an embodiment of the method of FIG. 3.
FIGS. 6A-6B illustrate a sequence of figures representing a process for forming a side shield in accordance with another embodiment of the disclosure.



DETAILED DESCRIPTION
Several embodiments of the invention will be described in reference to FIGS. 3-6. The figures are not drawn to the scale of an actual device or system, and are merely illustrative of the embodiments described herein.
One embodiment of the invention is directed to forming a pattern in a deep ultraviolet (DUV) lithography process. The flowchart of FIG. 3 summarizes a process 300 of an embodiment of the disclosure. Process 300 begins at block 315 in which a thick photoresist is placed on a substrate to form a coating. Then the coating is soft baked or heated in block 325 to remove residual solvent. After soft baking, the coating can have a thickness of about 0.25 to about 5 micrometers in some embodiments. In other embodiments, the coating will have a thickness of about 1 to about 3 micrometers after block 325.
Following the process of FIG. 3, the photoresist is then exposed via block 335. After exposure, the photoresist is developed in accordance with block 345, without any intervening baking performed between the exposure and development blocks. In certain embodiments, by adopting the process of FIG. 3, approximately 10-20% of processing time can be saved.
The process of FIG. 3 may be implemented in several different ways. In one embodiment, a resist is coated onto a substrate in block 315. Then the resist is baked as shown in block 325. Thereafter, the resist is exposed at a high dosage in block 335. During exposure, a ranging dosage ranged from 160-200 mJ/cm2 is provided by a laser to expose portions of the positive photoresist. Since the resist can be irradiated with wavelengths of less than 300 nm, suitable light sources encompass DUV with wavelengths of 248 nm. Specifically, the light source may be a laser such as a krypton fluoride laser. After exposure, the resist is developed in block 345. In this embodiment, post exposure baking is not performed. Following development, the exposed regions of the resist are removed in block 345. Acid diffusion of the photoresist may occur at room temperature. Despite the absence of post exposure baking from the process of FIG. 3, the contrast and sensitivity of the resulting photoresists was acceptable. It is believed that a combination of a high dosage exposure and omission of post exposure baking forms smooth patterned features in certain embodiments.
Yet another embodiment of FIG. 3 is directed to forming a patterned feature in a DUV lithography process. This embodiment comprises providing a photoresist on a substrate; heating the photoresist prior to exposure; exposing a portion of the photoresist to an illumination dose greater than or equal to 160 mJ/cm2; and then developing the photoresist to form a pattern. In this embodiment, a post exposure bake (PEB) prior to block 245 is not performed.
FIG. 4A illustrates a photoresist 40, on substrate 42, produced in accordance with one embodiment of the process of FIG. 3. When processed in accordance with this embodiment, a substantially straight sidewall profile 35 for photoresist 40 is obtained. By removing PEB in the process flow and/or controlling exposure, substantially straight photoresist side walls 35 were obtained, as shown in FIG. 4A.
In certain embodiments, the photoresist is a positive photoresist. A suitable photoresist is Shinetsu I051, available from Shin-etsu, MicroSi, Inc. Although, in certain embodiments a positive photoresist is used, other embodiments of the process could employ a negative photoresist instead.
The positive photoresist is a combination of a suitable polymer with a photoactive compound (PAC). The PAC absorbs light energy during block 335, resulting in the generation of an acid. The acid reacts with the polymer to break some of the polymeric bonds. In certain embodiments, the absence of PEB does not preclude PAC from absorbing sufficient amount of light energy to cleave chemical groups from the polymer. Thus chemical amplification occurs in the deep UV process in some embodiments of the present disclosure. The exposed photoresist film is developed with a basic chemical developer in block 345 to transfer the pattern from a mask to the photoresist.
FIG. 4B depicts a structure such as a side shield 45 on a substrate 43 that is patterned with a photoresist produced in accordance with an embodiment of FIG. 3. FIG. 4B illustrates side shields 45 in an intermediate structure that may be used to fabricate a magnetic recording head. Side shields 45 flank cavity 44. Cavity 44 has an upper portion 41 with a width j and a lower portion 47 with a width k. Width j of cavity 44 substantially equals width k of cavity 44. The consistent widths for cavity 44 are an improvement over the side shield profile obtained in FIG. 2B. Thus, by using a photoresist processed in accordance with embodiments of the disclosure, substantially straight side shields 45 may be obtained.
Features patterned with a photoresist 50 produced in accordance with FIG. 3 will now be discussed in association with FIG. 5. After forming a photoresist 50 with straight side walls 58 in accordance with an embodiment of the disclosure, further processing can be performed. For example, FIG. 5 illustrates a trench 55 patterned in intermediate layer 57 using photoresist 50 as a mask. The resulting trench 55 is produced with a smooth sidewall profile 53.
Yet another example of patterning with a photoresist produced in accordance with an embodiment of FIG. 3 is shown in FIGS. 6A and 6B, where common elements have been omitted from for clarity. FIG. 6A illustrates an intermediate structure that includes a write pole 70 that has been ion milled and planarized to a predetermined width. Layer 66 is provided adjacent to sidewalls 61 of write pole 70 to separate pole 70 from intermediate layer (interm layer) 64. On an upper surface of intermediate layer 64 and write pole 70 lies a remnant of mask 60 which was used to define write pole 70. Seed layer 68 can be deposited across the structure of FIG. 6A in preparation for depositing shield material in a manner that is known to the skilled artisan. The deposition of seed layer 68 creates channels 65.
After the structure in FIG. 6A is formed, a photoresist mask 67, prepared in accordance with an embodiment of FIG. 3, is disposed on seed layer 68. FIG. 6B illustrates the substantially straight side walls 77 of photoresist mask 67. Photoresist mask 67 is used to define side shield 75 when side shield material is deposited onto seed layer 68 to extend within channels 65. Due to the substantially straight profile of photoresist mask 67, side shields 75 are formed with substantially straight side walls 72.
Example 1
A solution of Shinetsu I051 was spun onto a wafer, and then heated at 110° C. After baking, the photoresist had a thickness of between about 1-4 microns. Using an ArF laser, the resist coating was exposed at a wavelength of 248 nm and an energy of 170 mJ/cm2. Then, the coating was developed to completely transfer the pattern from the mask to the substrate. A PEB was not performed prior to development.
The above detailed description is provided to enable any person skilled in the art to practice the various embodiments described herein. While several embodiments have been described, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention.
Various modifications to these embodiments will be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other embodiments. Thus, many changes and modifications may be made to the embodiments, by one having ordinary skill in the art, without departing from the spirit and scope of the invention.

## Claims

1. A method of forming a pattern in a DUV lithography process, the method comprising:
providing a photoresist on a substrate to form a coating;
heating the coating prior to exposure;
exposing a portion of the coating at a wavelength between 248 nm and 300 nm while irradiating the coating at a dosage from 160 mJ/cm2 to 200 mJ/cm2, inclusive; and
developing the coating to form a pattern, without performing a post exposure bake prior to developing the coating.

2. The method of claim 1, wherein the coating has a thickness of about 0.25 micrometers to about 5 micrometers after the coating is heated.

3. The method of claim 1, wherein the coating has a thickness of about 1 micrometers to about 3 micrometers after the coating is heated.

4. The method of claim 1, wherein the coating remaining after development comprises a structure having substantially straight sidewalls.

5. The method of claim 1, wherein the coating is exposed at a wavelength of 248 nm.

6. The method of claim 1, wherein the exposing a portion of the coating further comprises irradiating the coating with either a krypton fluoride laser or an argon fluoride laser.

7. A method of forming a pattern in a DUV lithography process, the method comprising:
providing a photoresist on a substrate to form a coating;
heating the coating prior to exposure;
exposing a portion of the coating to an illumination dose from 160 mJ/cm2 to 200 mJ/cm2, inclusive; and
developing the coating to form a pattern, without performing a post exposure bake prior to developing the coating.

8. The method of claim 7, wherein the coating has a thickness of about 0.25 micrometers to about 5 micrometers after the coating is heated.

9. The method of claim 7, wherein the coating has a thickness of about 1 micrometers to about 3 micrometers after the coating is heated.

10. The method of claim 7, wherein the coating remaining after development comprises a structure having substantially straight sidewalls.

11. The method of claim 7, wherein the coating is exposed with either a krypton fluoride laser or an argon fluoride laser.

12. A method of forming a pattern in a DUV lithography process, the method comprising:
providing a chemically-amplified photoresist on a substrate to form a coating;
heating the coating prior to exposure;
exposing a portion of the coating at a wavelength between 248 nm and 300 nm while irradiating the coating with an illumination dose from 160 mJ/cm2 to 200 mJ/cm2, inclusive; and
developing the coating to form a pattern, without performing a post exposure bake prior to developing the coating.

13. The method of claim 12, wherein the coating has a thickness of about 0.25 micrometers to about 5 micrometers after the coating is heated.

14. The method of claim 12, wherein the coating has a thickness of about 1 micrometers to about 3 micrometers after the coating is heated.

15. The method of claim 12, wherein the coating remaining after development comprises a structure having substantially straight sidewalls.

16. The method of claim 12, wherein the coating is exposed at a wavelength of 248 nm.

17. The method of claim 12, wherein the coating is exposed with a krypton fluoride laser or an argon fluoride laser.

18. The method of claim 12, wherein the chemically-amplified photoresist is catalyzed with an acid.