Learn about every aspect of photoresist (1)

Photoresist is an etching-resistant film material whose solubility changes through irradiation or radiation of ultraviolet light, excimer laser, electron beam, ion beam, X-ray and other light sources. Major applications include the production of integrated circuits and discrete devices in semiconductor field, flat panel displays, LEDs, flip-chip packaging, magnetic heads and precision sensors.

In the earliest period, photoresist was used in the printing industry. It was only in the 1920s that photoresist was gradually used in the field of printed circuit boards. In the 1950s, photoresist began to be used in semiconductor industry. In the late 1950s, Eastman Kodak and Shipley designed positive and negative photoresists for the semiconductor industry, respectively.
Photoresist utilizes the difference of dissolution rates between exposure and non-exposure areas to transfer images. Specifically explained from the process, photoresist with photochemical sensitivity can coat semiconductor, conductor and insulator by photochemistry reaction. After exposure and development, the part left can protect the bottom layer. Then the required micro-patterns can be transferred from mask to the substrate to be processed by using etching with etchant. Therefore, photoresist is a key chemical material in micro-fabrication technology.
1. Ten-step lithography process
Surface preparation: cleaning and drying wafer surface.
Coating: Appling a thin photoresist to the surface by spin coating.
Soft baking: Partial evaporation of photoresist solvents by heating.
Alignment and exposure: The mask is precisely aligned with wafer, and then the photoresist is exposed.
Development: Removing non-polymeric photoresist .
Hard Baking: Continuous evaporation of solvents.
Developmental examination: checking surface alignment and defects
Etching: Removing the top layer of the wafer through the opening part of photoresist.
Stripping: Removing the photoresist from wafer.
Final inspection: Surface inspection for etching irregularities and other problems.
In fact, photoresist is the core of lithography. In the manufacturing process of large-scale integrated circuits, lithography and etching technology is the most important technology in fine line graphics processing, which determines the minimum feature size of the chip, accounting for 40-50% of chip manufacturing time and 30% of manufacturing cost.
With the increasing resolution of semiconductor manufacturing, the demand for advanced photoresist is becoming more and more urgent. The innovation of materials fundamentally supports the development of chip manufacturing technology.
The preparation, baking, exposure, etching and removal processes are fine-tuned according to the specific properties of photoresist and the desired effect. The selection of photoresist and the development of photoresist technology are a very long and complex process. Photoresist needs to match with many process steps such as lithography machine, mask and semiconductor, so once a lithography process is established, it is rarely changed.
However, the breakthrough of research and development in photoresist is more difficult. For semiconductor manufacturers, it takes a long test cycle to replace the photoresist used. At the same time, the cost of developing photoresist is also very huge. For manufacturers, production line cooperation is needed in mass production testing, and the cost of testing is also huge. For the R&D team, the investment of a single lithographic machine is more than tens of millions of dollars, so it is difficult for small enterprises to face sustained huge R&D investment.