Typically laser process observation in done through scan head and scanning lens themselves (1). If there is a difference between observation and working wavelength, a lateral and longitudinal focus shift, caused by monochromatically corrected laser lenses, arises. The scanner aperture could limit the maximum usable aperture of the imaging lens which decreases the resolution.
Alternatively a transparent working plane can be observed from the opposite side (2).
For applications with high resolution specifications and an opaque working plane the observation can be installed next to the f-theta lens (3). A tilt between the optical axis and the orientation of the object plane results. Depending on the depth of field of the vision lens there is only one stripe in focus which can be imagined with high resolution. This lack of definition can be compensated by a suitable adaptor which is mounted between camera and observing lens.
Optical coherence tomography
Conventional observation lanses do not suffice for three dimensional scan field measuring. Optical coherence tomography is a contactless method and the solution for this kind of process observation.
The basic idea is the superposition of the wavelength of two beam paths. The generated interferogram helps to calculate a distance difference between reference and measuring beam path. If reference distance and distance difference are known the distance of the measuring beam path can be calculated. Other measurements, which are displaced in x- and y direction create a three dimensional map of the scan field. Accuracies in the range of some micrometers can be reached.
Standard galvanometer scanners are often not fast enough for lasers with extremely high repetition rates. Ultrafast polygon scanners are a highly suited alternative in this case. They are line scanning systems and much faster than galvanometer scanners, which highly decreases the laser processing time. Many existing f-theta lenses are suitable without significant impact on the performance. However, Sill Optics offers custom lenses to exploit the full advantage of the possibilities of polygon scanners.
There are two methods usually used for micro-structuring with lasers, maskless or direct write and mask projection.
Flexibility, ease of use and cost effectiveness are the features of the direct laser process with DPSS laser for feature sizes on a micrometer scale.
Mask projection systems usually use excimer lasers where a mask pattern is de magnified by a certain factor to reach feature sizes on a sub-micrometer scale. Additional benefits are good depth uniformity and distortion free features.
A combination of both techniques is the so-called “Scanned Mask Imaging (SMI)”. An expanded, homogenized and shaped beam scans a mask using a 2D scanner and a telecentric f-theta lens. The illuminated area of the mask is de-magnified with a double side telecentric low distortion lens onto the substrate area. Sill Optics designs telecentric lenses for scanning and distortion free imaging on customers’ requirements.
Long term exposure of black anodized aluminium housing parts to ultraviolett light can lead to bleaching. Eventually released particles might contaminate the lens surfaces and result in a decreased lifetime on the optical components. Therefore it is possible to buy f-thtea lenses and beam expanders for the ultraviolet wavelength band with a resistant colorless anodized housing (no extra costs). For scan lenses a special order of such a housing is necessary, but there is an extra catalog series for UV beam expanders ("S6EXN").
Besides the well-known f-theta lenses for galvanometer scanners where the scanner is prior to the scan lens, lens systems with adjustable focal length could be used for processing flat fields. These lens systems incorporate a moving lens and a focusing system. The position of the moving lens in respect to the focusing lens has to be synchronized with the scanner movement to avoid a spherical scan field. So it is possible to create an even scan field, but the beam spot diameter depends on its position and changes more over the field in respect to f-theta lenses. Nevertheless there are some applications which utilize the big advantage of working in three dimensions with z shifting optics.
Alignment turning is a high precise production technique which enables a minimum tilt between lens axis and optical axis. Production accuracy is about one micrometer with this tool. Before starting the process it is necessary to fi x the lens into its housing by curling, sticking or using knurled rings. After that the lens becomes positioned inside the machine so that optical and mechanical axis are exactly in line with each other. With the help of ceramic tools, the outer surfaces as well as the front and rear bearing surfaces are centered. Outside diameter tolerances of just a few microns are producible. If there are high specifications on lens centering, Sill Optics uses this technique for producing modern high precision optics.
It is useful to check the position and geometry of individual lenses after assembly for projects with a high centering sensitivity. The centering error of each surface within an optical system can be measured with a sub-micron resolution. If required the design can be adjusted after assembly which decreases the centering error extremely.
The measuring device can also be used for cementing pairs of lenses. Specifically it can be checked whether the center thickness, air gaps and radii are within the tolerance for systems with up to 20 surfaces.
Due to the constantly growing requirements for throughput and precision, in many industrial applications beam splitters and beam shapers are in use.
Beam splitters are optical components that split a beam into two or more beams with a certain angular spacing. To realize this, so-called DOEs, diffractive optical elements are often used. DOEs can generate 1D or 2D patterns. The angles between neighboring orders depend on the order numbers and are not linear. This nonlinearity results in non-equidistant spacing of focal points if standard lenses are used. To compensate this scan lenses can be designed in a special way.
Beam shaper transform a single mode Gaussian beam into a beam with uniform energy distribution. Diffractive or aspherical optical elements are used to achieve this. It should be noted that in subsequent optical systems such as beam expanders or f-Theta objectives, more than twice the beam diameter is often required as a free aperture. In addition, the imaging power must be diffraction limited to obtain the beam shape.