When the ≤ 2um resolution was demanded from the semiconductor industry, the KTFR material is no longer a viable option for many IC processes. The search for second generation of photoresist started and fortunately many ground work around resins and photo-active compounds had been started decades ago. The material suppliers from Germany had played critical roles for the second generation Novolak/Diazonnaphthaquinone (DNQ) photoresist development.
Chemists from Kalle Company (Germany) produced a series of photo-active compounds including DNQ for their copy business for “blueprints” and other engineering drawings in 1930’s. They tested their new photo active compounds with Novolak resin, which was manufactured by their next-door-neighbor company, Albert Chemical. During the course of optimizing the formulation, the Kalle’s team found that the Novolak resin dissolution rate was suppressed in presence of DNQ but much more increased upon exposure to light. Oskar Suss, who led the research team at Kalle, understood what happened and also knew the significance of this phenomenon. The first Novolak/DNQ formulation based on dissolution rate was therefore discovered. The Novolak/DNQ formulation was a great success as the material for positive-tone printing plates at Kalle.
The Novolak/DNQ formulation was supplied to the US market as the material for the business of "print plates" by Azoplate, which was associated with Hoechst AG (also the parent company for Kalle Company). The high resolution and other great material properties made the Novolak/DNQ formulation such an advanced imaging material at the time. It was no doubt that it will pave the way into semiconductor industry someday. The question was just when and how.
The history was again made between two companies with geolocation vicinity. For this time, it was in a more interesting way through a family tie. At the time, the Azoplate was located at Murray Hill, New Jersey, the same city that Bell labs was also suited. A technician from Bell labs complained the poor resolution of KTFR to his son, who happened to work at Azoplate. The son believed that he had a solution and therefore provided a bottle of Novolak/DNQ formulation to his father. We can imagine what happened thereafter. The most important photoresist material for semiconductor industry in the century (or even in human history, in my personal opinion) found its destiny in the semiconductor industry. For the next quarter of century, Novolak/DNQ led the way for the semiconductor industry for features sizing from 2um to 200nm. The Novolak/DNQ photoresist, as the enabling material, experienced the great age of personal computer and internet. All these forever changed the way that our human beings live from every fundamental aspect.
The success for the Novolak/DNQ formulation was imminent. The new Novolak/DNQ photoresist was perfectly matched to the new projection exposure tools at the time. Within a very short period of time, the new resist completely supplanted the KTFR material. As a matter of fact, the market takeover caused quite turmoil for mask makers and material vendors. But the marching of the semiconductor industry went on and this new resist material occupied over 90% photoresist market share over a quarter of century.
Below in the Figure 1, the basic phenol unit from novolak resin is shown. DNQ reaction upon exposure is also shown in the figure. For the Novolak/DNQ formulation, a large solubility contrast between exposed and dark area is the key for the success for the material.
Figure 1. Phenol-formaldehyde polymer (novolak) and Diazonnaphthaquinone (DNQ) reaction upon exposure.
The etch resistance from novolak/DNQ was excellent and it had been a key attribute for the success of the material. In addition, aqueous developers, as more environment-friendly reagents, soon became the new industry standard from the perspective of Environment/Health/Safety (EHS) once they were implemented for IC processes .
The absorption spectra for DNQ and novolak are shown in the Figure 2 below. The absorption band for DNQ covers quite a broad UV range and therefore the novolak/DNQ formulation can be used for g-line, I-line, or broad band photoresists.
Figure 2. Absorption spectra for DNQ and Novolak.
In summary, Novolak/DNQ photoresist is positive-tone, non-chemically amplified, and aqueous developable resist. It had been used as the major imaging material to form patterns with dimensions from a few hundred nanometers to 2 microns. Many Novolak/DNQ products are still being used in different semiconductor processes nowadays. They are still the dominant material for numerous processs where the resolution can be satisfied. It is expected that this trend will continue for many more years because it is such a great and easy-to-use material.
Something interesting to point out: many significant scientific discoveries are often associated with coincidence. If Albert was not the neighbor to Kalle nor Bell’s lab was close to Azoplate, we might see these material commercialization delayed for years. Can you imagine what the human society would be like if the Novolak/DNQ material was delayed? As I has always believed, technology has become the king for everything in our human society in this information age. No one or any historic events could affect our society as much as new technologies.