Resist for near infrared (NIR)
With the development of photoresists for an exposure in the wavelength range of 500 to 1100 nm using laser light, new procedures became possible. Lithographic processes at an exposure wavelength > 480 nm are not possible with standard photoresists. (See „Resists for 488 nm“ ).
A new resist series was recently developed which is sensitive for laser exposure in this spectral range. The operation principle of these resists is again based on chemical enhancement. The addition of dyes which absorbe at a particular wavelength increases the sensitivity for this laser-wavelength.
A first proof of principle confirmed that it is possible to induce a cross-linking at exposure wavelengths of 532 nm and 1064 nm leading to complete layer build-up. The initial film thickness of 6 µm was fully reached during the writing of spots in a particular energy interval. Below this interval, no structures remained while the resist film was destroyed and removed above this interval due to the high input power of the laser beam. The development was carried out with generally used aqueous-alkaline developers.
In a second step, lines were written. The writing speed of the laser beam is of high importance. The faster the writing speed, the lower is the local energy input.
Contrariwise, a low writing speed leads to a high energy input, which destroys and removes the resist layers above a certain limit value.
Fig. 1: Lines written with NIR-lasers
These new negative resists are in addition particularly sensitive in the range of 200 – 500 nm for all lasers and can thus be used for according applications. These include not only applications in micro electronics, but also for e.g. the engraving of rollers or the production of carpets or CNC encoding disks during which generally no yellow light is used. For these applications, a NIR-resist for white-light conditions is available whose absorptions spectrum shows no absorption above 300 nm. This resist can be processed without yellow light.
By addition of suitable dyes to the negative CAR resists, structuring is also possible at wavelenghts beyond the normal wavelength range, if pulsed laser beams are used in sufficient intensity. The sensitivity for the different laser wavelengths is considerably increased if dyes absorbing at the respective wavelength are added. A structuring is in this case possible with pulsed laser light both at 532 nm and at 1064 nm. The exposure leads to a heating of the dye which intensively absorbs in the respective wavelength range, thus causing 2-photon absorption processes in the area of the very high energy densities of the laser beam. This induces crosslinking of the resist layer. Development was carried out with standard aqueous-alkaline developers.
In addition to the laser energy applied, also the writing speed of the laser beam is of major importance. The faster the writing speed, the lower is the local energy input. A low writing speed vice versa results in a high energy input which destroys or removes the resist films if a certain threshold value is exceeded.
By writing parallel lines, surfaces with strictly periodic wave structures can be produced which are of interest for the manufacture of adaptive optics, e.g. for microwave applications. The periodicity can in this case be adjusted by the choice of the beam overlap. At a beam overlap of ≤ 20 %, parallel lines are obtained. With a slightly higher overlap, periodic wave structures result, and finally closed, almost even surfaces are obtained during writing with a beam overlap of ≥ 75 %.
Fig. 2 and 3: Lines written with pulsed laser (532 nm) with different overlaps. Surface scan at the Dektak 150 after development with AR 300-475. Left: beam diameter 100 µm; right: beam diameter 50 µm