Photolithography - Oscar Zabala, Yalin Liu

Overview

Fig 1: Photo-lithographic process overview

Photolithography is the process of transferring a template from a photomask onto a substrate that is coated with a light-sensitive chemical, termed photoresists. The substrate is generally a thin silicon wafer and the photoresist is a polymer. The overall process consists of the following steps: substrate preparation, photoresist spin coat, prebake, exposure, post-exposure bake, development, and pattern transfer^[1]. It should be noted that photolithography requires flat surfaces to work properly, and it can be difficult to create complicated 3-dimensional structures.

There are two types of photoresists that may be used:

In positive-tone photoresist, the exposed region is made more soluble. This photoresist will give a higher resolution and the ability to modify the sidewall profile.
In contrast, exposure of negative-tone photoresist causes the exposed region to crosslink and be less soluble. These photoresists will give high aspect ratio features and can be used to pattern thick resists.

Optical Lithography Process

Fig 2: SEM images of 30 nm, 40 nm, 50 nm, and 70 nm gratings fabricated in a 65-nm thick PMMA layer on a silicon substrate, with 10 keV electrons, at the various area doses. The gratings were developed for 5 sec. in a 1:3 MIBK:IPA solution at room temperature^[8].

Substrate Preparation

The substrate preparation step is necessary to successfully adhere the photoresistive material onto the substrate. Typically, a wafer will be baked at 200°C-400°C for 30-60 minutes to evaporate any water present on the surface. The surface may also be pre-treated to improve adhesion, and wet etching can be used to remove organic material and/or native oxide. This baking process will not fully remove the surface silanol groups formed when silicon atoms bond with water. The hydroxide group of the silanol is replaced by subjecting the surface to a reactive vapor, usually a silane such as hexamethyldisilazane (HMDS). The dehydration bake and subsequent priming steps may be performed in the same oven^[1].

Photoresist Spin Coating

Once the substrate is prepared, the photoresist is applied via spin coating. The coating may be poured before spinning, or as the wafer is spinning. The desired resist thickness is obtained by varying the spin speed, based on the resist’s viscosity^[1]. The acceptable spin speeds are limited by the onset of turbulent airflow at the edge of the wafer (determined by Reynold’s number) on the high end, and lack of uniformity at speeds lower than 1000rpm, usually speeds around 3000rpm for one minute are used ^[2].

Retrieved from Orbital Tube The video above shows the spin coating process in slow motion.

Post-Apply Bake

This step serves to reduce the resist's film thickness, improve its adhesion, get rid of excess solvent, and make the film less vulnerable to particulate contamination. One bake method is termed proximity baking and consists of bringing the wafer very close (~100μm) to a hot metal plate. The wafer is then removed and brought close to a cool plate to ensure consistent and thorough cooling^[1].

Alignment and Exposure

The mask pattern may be printed through direct contact, proximity, or projection. In the case of direct contact, the mask is placed directly onto the photoresist. Although this method offers high resolution, the mask may be damaged and yields are not favorable for high throughput production. Proximity printing keeps the mask a short distance above the wafer but increases the minimum resolution limit. Finally, projection printing uses an optical system to project an image of the mask onto the wafer.^[1]

The exposure process involves several variables such as exposure time and intensity. Figure 2 ^[8] shows an example of differently sized gratings made using electron beam lithography, and the effect of beam intensity has on effectively creating specific grating spacing.

Post-Exposure Bake

The post-exposure bake aims to reduce undesirable ridges formed on the sidewalls of the resist material after exposure. These ridges are caused by interference of incoming and reflected light forming a pattern of varying light intensity based on depth within the photoresist layer. Baking takes place at around 100°C-130°C, and the ridges are smoothed out by the diffusion of photoproducts within the resist.^[1]

Development

The development process dissolves the areas left unexposed during the exposure step. Tetramethylammonium hydroxide (TMAH) can be used, and surfactants may be included to increase wafer wetting.^[1]

Pattern Transfer

The photoresist material may be subjected to a hard baking process to strengthen the final resist image. The goal is to harden the photoresist material so that it may withstand the harsh pattern transfer steps. Temperatures during post-bake must be carefully controlled in order to avoid softening the resist material, degrading the final image quality.

At this point, the photolithographic process is complete, and the wafer is ready for etching ( wet or dry), selective deposition, ion implantation^[1] or used directly for replica molding.

Applications of Photolithography

Fig 3: Rapid prototyping^[4]

Fig 4: Microcontact printing, μCP^[3]

Fig 5: High resolution printing and contact photolithography^[5]

Photolithography techniques have been used in making a variety of devices, like electronic devices, biosensors, and microfluidic devices. In recent reports, fabrication techniques have emerged from development efforts in photolithography. ^[3] Photolithography, in general, has three applications: fabricating stamps and molds, solid objective printing, and three-dimensional nanofabrication using a conformable photomask.^[3] In the application of fabrication techniques, people have been using rapid prototyping for a device in a computer-aided design (CAD) program.^[4] The process contains producing a positive relief of photoresist on a silicon wafer. Figure 3 ^[4], obtained from McDonald's work, illustrates how rapid prototyping works.^[4]

To be more specific, microcontact printing has been used with the principle of photolithography. Microcontact printing (μCP) is a flexible, non-photolithographic method that routinely forms patterned self-assembled monolayers (SAMs) containing regions terminated by different chemical functionalities with submicron lateral dimensions.^[4] The procedure is in Figure 4.^[1]

As mentioned in the previous session, microfluidic devices have long been known to be made by a photolithography technique. There are several reports of microfluidic systems based on PDMS.^[4] One of the systems is called the CE system.^[5] The channels in PDMS were made by casting the polymer against a commercially obtained master that was composed of a positive relief structure of silicon. However, the devices were not sealed tightly.^[5] In the following work, another method using a combination of high-resolution printing and contact photolithography process was created.^[5] This technique has been described previously and has been used to generate a variety of microstructures with dimensions >20 μm.^[6],[7] The process is shown in Figure 5.^[5]

References

1. C. A. Mack, Field Guide to Optical Lithography; SPIE Press, 2006. ISBN: 9780819462077

2. C. Mack, Fundamental Principles of Optical Lithography: The Science of Microfabrication; Wiley, 2007. ISBN: 9780470727300

3. Rogers, J. A.; Nuzzo, R. G. materialstoday 2005, 1–7. DOI: 10.1016/S1369-7021(05)00702-9

4. McDonald, J. C.; Duffy, D. C.; Anderson, J. R.; Chiu, D. T.; Wu, H.; Schueller, O. J. A.; Whitesides, G. M.: Fabrication of microfluidic systems in poly(dimethylsiloxane). Electrophoresis 2000 Jan;21(1):27-40. DOI: 10.1002/(SICI)1522-2683(20000101)21:1<27::AID-ELPS27>3.0.CO;2-C

5. Xia, Y.; Whitesides, G. M.: Soft Lithography. Angew Chem Int Ed Engl. 1998 March 16; 37(5):550-575. DOI: 10.1002/(SICI)1521-3773(19980316)37:5<550::AID-ANIE550>3.0.CO;2-G. PMID: 29711088.

6. Duffy, D. C.; McDonald, J. C.; Schueller, O. J. A.; Whitesides, G. M. Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane). Analytical Chemistry 1998, 70 (23), 4974–4984. DOI: 10.1021/ac980656z

7. Qin, D.; Xia, Y.; Whitesides, G. M. Rapid prototyping of complex structures with feature sizes larger than20 μm. Adv. Mater. 1996, 8, 917. DOI: 10.1002/adma.19960081110

8. M. A. Mohammad; S. K. Dew; K. Westra; P. Li; M. Aktary; Y. Lauw; A. Kovalenko; M. Stepanova. Nanoscale resist morphologies of dense gratings using electron-beam lithography. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena 25..2007,745. DOI: 10.1116/1.2731330