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Figure 1: SMI optical system.
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Figure 2: 10 µm deep structures in BTX. Design courtesy of Amkor.
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Figure 3: Multiple depth structures in polyimide.
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Figure 3: Multiple depth structures in polyimide.
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Figure 3: Multiple depth structures in polyimide.
Scanned Mask Imaging: The Economical Approach to High Resolution Micro Machining Using UV Solid-State Lasers
David Milne and Dave Myles
Micro structuring of materials by direct laser ablation can offer a faster, more cost effective and environmentally friendly method for MEMS manufacture in many applications compared to conventional semiconductor technologies. ‘Mask Projection’ and ‘Direct Write’ are the two methods most widely used for micro structuring of materials by laser. In mask projection systems, excimer lasers are typically used. They can achieve high quality ablation at high resolution, but the machines have a high capital cost and regular laser maintenance is required. For the direct write method, solid state lasers are better suited due to their high coherence, allowing the beam to be focused to a small spot. These systems are flexible to operate, and usually much cheaper to own and maintain than an excimer system. However with direct write, it is very difficult to ablate arbitrary patterns with both fine and large-scale features, and to maintain depth control in complex 3D structures. This article explores a new method for micro structuring materials where a UV multi-mode solid state laser and scanner are used to illuminate a photo-mask. An image of the mask is subsequently projected and de-magnified onto the substrate through a projection lens. This method is called Scanned Mask Imaging. This process effectively ablates arbitrary features down to a resolution of a few microns, a process which direct write methods are unable to achieve. SMI can achieve ablation quality comparable to that of excimer laser systems, at a fraction of the cost with greater ease of installation and maintenance of the hardware.
Laser Mask Projection with Excimer Lasers
Characteristics of Excimer Lasers
Excimer lasers are the highest average power pulsed sources available in the ultraviolet region. Systems with outputs up to many hundreds of watts at wavelengths of 248 nm and 308 nm are in use for a wide range of micro machining applications. The outputs are characterised by high (up to 1 J) energy pulses at modest (up to a few 100 Hz) repetition rates. With pulse lengths in the 20 ns range pulse powers over 50 MW are readily achievable. Excimer lasers emit beams with poor temporal and spatial coherence having large bandwidth (200 to 300 pm) but a very high number of modes (M2 > 50), which make them ideal for mask imaging. Since the output beams are large (e.g. 10 x 30 mm), with asymmetric and highly divergent properties, complex beam shaping and homogenising optics are essential to transform the beams to the required shape and uniformity. The illuminated area of the mask is projected onto the substrate by a lens of suitable resolution and reduction factor (typically 3x to 5x) to give a fluence suitable for ablation of organic or thin film materials. Such a complex optical system can have relatively poor transmission, so the maximum area that can be exposed at the substrate with a single pulse at an appropriate (around 1J/cm2) fluence is a few tens of mm2. Hence schemes have been devised to increase the processable area at the substrate.
Excimer Laser Approach
The excimer laser based approaches can achieve excellent micro machining results with feature resolutions down to a few microns, with good depth uniformity and depth control. However, the cost of ownership of excimer laser based production systems is very high due to the high capital cost of the lasers, the complexity and short lifetime of the homogenisation and projection optics at UV wavelengths, and the frequent replacement of laser parts. There is a burden of limited gas lifetime and the need to renew the gas in the cavity to maintain laser output. This has limited the adoption of high resolution micro machining using mask projection for many manufacturing applications, particularly those related to sub-component manufacturing for mobile consumer electronics.
Scanned Mask Imaging with UV multi-mode solid state laser
SMI Optical System
Scanned Mask Imaging is based on the use of high power frequency tripled (355nm), Q-switched, multimode, diode pumped solid state (DPSS) YAG lasers. These lasers have beam output characteristics that differ to excimer lasers, operating with pulse repetition rates generally in the few kHz to few tens of kHz range with pulse energies up to a few mJ. Highly robust commercial units are available with powers up to 100W from a single cavity, with typical pulse lengths in the range 50 to 100ns. Output beams are entirely suitable for mask imaging having poor spatial coherence (M2 in 10 to 25 range) and are axisymmetric in shape and divergence so require only simple optics to propagate. Unlike excimer lasers the DPSS laser temporal coherence is high as the bandwidth is only a few pm, which simplifies the manufacture of high resolution imaging optics. As the DPSS laser single pulse energy is much lower than that from an excimer laser, in order to obtain the same fluence at the substrate a much smaller spot is required. The SMI beam is reshaped to create a square spot with quasi-top hat distribution, which is then raster scanned in 2D over the mask area. The image of the mask is reduction imaged onto the substrate to give an appropriate fluence. SMI technology gives imaging resolution and ablation rates similar to those of an excimer system, but with significantly simpler optics and dramatically lower cost of ownership.
Figure 1 shows a schematic diagram of the optical system used for SMI. The laser beam is expanded using a simple telescope, and shaped and homogenized by a Diffractive Optical Element (DOE) to form a square (approx. 1 x 1mm), flat top beam at the beam waist of a plano-convex lens. The plane of homogenization is imaged onto the mask using an infinity imaging system consisting of a second singlet and the telecentric f-theta scan lens following a 2D galvo scanner.
SMI System Design
The design considerations for the SMI mask imaging system are exactly the same as for an excimer mask imaging system. The magnification of the projection lens is chosen to be the lowest possible (to minimize mask size) but at the same time give the desired fluence at the substrate for a spot size at the mask giving a fluence well below the damage threshold of the mask. The lens used to ablate the structures shown in the results section had a demagnification of 3.5, offering a good compromise between high resolution and fluence (up to 2 J/cm2) at the substrate, whilst limiting the mask size which limits the maximum mask scan speed requirements. The lens is of double telecentric type with field of 20 x 20 mm and a numerical aperture of 0.11 giving a theoretical limiting resolution of 3 µm. The mask is mounted on a stage to allow precise overlay of multiple images at the substrate to create 3D structures in materials.
The SMI system has been used with a 20W average power DPSS laser to micro-machine structures in a range of organic materials including polyimide and polyimide type materials widely used for micro-electronics devices (e.g. proprietary materials such as ABF, Ultimax, BTX etc. Typical ablation rates of these materials are in the range 0.5 to 0.7 µm/shot at a fluence of 1 J/cm2. Figure 2 shows a series of SEM images of structures micro machined to a single depth of 10 µm in BTX by SMI using a single mask. The inter-digitated structure, appropriate for micro fluidic applications has line/space dimensions of 3 µm/3 µm and an aspect ratio of 3.
Multiple depth structures can be formed by mask exchange. Figure 3a shows a multi-depth structure formed in polyimide by SMI using a sequence of 4 different masks. Each layer is 5 µm deep.
When 3D structures with features that vary gradually in depth such as ramps, slopes, depressions or domes are required, SMI has considerably more flexibility than excimer laser mask projection systems. Because SMI scans the mask area in 2D with a beam that can be shaped with an aperture at the homogenous plane, it is possible to laser structure a feature at the mask plane in novel ways to form grooves, channels or textures within a pattern defined by the mask. Figures 3b and 3c shows some of the novel, textured features and ramps it is possible to create using such techniques.
Scanned mask imaging using a UV DPSS laser has been introduced as an alternative to excimer laser mask projection systems for high-resolution ablative micro machining. The resulting quality is comparable to that of excimer laser systems, but delivered at a fraction of the cost and burden of ownership, with structures down to 3 µm line width and spacing demonstrated. The SMI method is proving to have unlocked the potential for development of novel micro-machining techniques via programmable illumination patterns and dual plane imaging optics.
M-Solv