Blue Diode Lasers Mature For Mask Writing
The recent availability of GaN-based diode sources has opened a host of new applications in various fields. Optical storage - practically the raison d'être for blue laser diode technology - is leading the way with a new generation of writers/players that implement the recently standardised Blu-ray Disc format. Sony already has drives out in the consumer market. These laser diodes are also used for mastering discs in the older DVD format.
Importantly, several other application areas have been able to make use of the technology well before Blu-ray Disc was finalised as a standard. The graphic arts and printing industries has enthusiastically adopted blue laser technology in Computer-to-Plate setting systems, where the shorter wavelength laser light primarily allows the development of more rapid exposure systems. In BioPhotonics too - specifically confocal microscopy and similar fluorescence-tagging applications in the Life Sciences - one can see rapid adoption of this new solid-state source.
Another field that can immediately benefit from the new blue-violet sources is microlithography. The fabrication of masks by a laser pattern generator is often an effective alternative to the traditional electron-beam technique, primarily when the cost of the equipment is an important factor.
The physical linewidth achievable by laser writing is directly related to the ratio between the laser wavelength and the numerical aperture of the beam focusing optics. Presently, most laser pattern generators are based on He-Cd sources operating at 442 or 325nm. The new blue diode laser has wavelengths generally lying between 400 to 410nm with future power outputs upwards of 30mW, with lower power versions being available around 375 and 445nm. Although not wholly suitable for replacing the He-Cd laser in the high-resolution systems operating at 325nm, it can immediately improve the performance of traditional 442nm pattern generators.
In a joint effort by TOPTICA Photonics (Germany) and Microtech (Italy), the real possibility of using a blue diode source for high-resolution patterning has been recently demonstrated. Clearly defined, sub-micron features have been produced on standard chrome and iron oxide masks, as well as by direct writing on final substrates (Si, GaAs, glass, etc.).
In order to successfully adopt the new source, a number of problems had to be initially solved. First, a quality circular beam had to be made by reshaping the emission of the slightly astigmatic and strongly elliptical diode source. This was achieved by passing the beam through precision anamorphic optics, resulting in a fairly gaussian-shaped beam well suited for focusing down to the diffraction limit. Second, a reliable intensity level control had to be developed. This automatically regulates the diode current in order to keep the emitted power at the level necessary for pattern generation, thereby taking into account temperature fluctuations and laser diode aging. Third, the pattern generator had to integrate the new source into a system optically optimised for 442nm, requiring a revision of the existing optical setup in order to minimise the optical losses with the new shorter wavelength.
Advantageous in terms of power efficiency and compactness, the new blue source can effectively combat the efficiency and associated thermal issues normally detrimental to effective pattern generation. The use of He-Cd lasers involves using an inefficient laser head, typically requiring powers greater than 500W for producing an optical beam with only a few tens of mW. Aside from the overall efficiency, a far more critical point is the transport of this excess power away from the laser as heat. This can be achieved by natural convection/conduction cooling, thereby often allowing the laser head to become excessively warm, or by forced fan cooling. In both cases, cooling contributes to the complexity of the pattern generator. Natural cooling requires an effective thermal screening of the laser head from the rest of the system, while forced cooling requires accurate control of the vibrations induced by the fan or the fast air flow in the laser head. Thermal stability is a critical requirement for precision patterning and in this aspect the use of the blue laser source is considerably beneficial.
A further interesting feature of the new source is the possibility of direct beam modulation via appropriate and precise modulation of the diode current. Typical pulse repetition rates can be better than 20Mbit/s. The acousto-optic modulators (AOM) used in traditional pattern generators can therefore be removed. In the new design, the source is driven directly by TTL signals, with no intermediate RF drives as needed with the AOM. The fast on/off switching capabilities of the diode through its driving electronics allows the generation of grey-level patterns by simple digital pulse width modulation (PWM) of the laser beam, whereas the original AO modulator system required an analogue control synchronous with the digital on/off drive.
From these preliminary results, it appears that the new GaN-based sources are already mature enough for integration into pattern generators. This may lead to a new generation of compact systems that resemble a sort of personal desktop mask-making tool.
AngelTech Live III: Join us on 12 April 2021!
AngelTech Live III will be broadcast on 12 April 2021, 10am BST, rebroadcast on 14 April (10am CTT) and 16 April (10am PST) and will feature online versions of the market-leading physical events: CS International and PIC International PLUS a brand new Silicon Semiconductor International Track!
Thanks to the great diversity of the semiconductor industry, we are always chasing new markets and developing a range of exciting technologies.
2021 is no different. Over the last few months interest in deep-UV LEDs has rocketed, due to its capability to disinfect and sanitise areas and combat Covid-19. We shall consider a roadmap for this device, along with technologies for boosting its output.
We shall also look at microLEDs, a display with many wonderful attributes, identifying processes for handling the mass transfer of tiny emitters that hold the key to commercialisation of this technology.
We shall also discuss electrification of transportation, underpinned by wide bandgap power electronics and supported by blue lasers that are ideal for processing copper.
Additional areas we will cover include the development of GaN ICs, to improve the reach of power electronics; the great strides that have been made with gallium oxide; and a look at new materials, such as cubic GaN and AlScN.
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