Laser photoplotters are the first step in creating circuit boards. These plotters make masks that several industries use for printed circuit boards or integrated circuits. The masks, sometimes called photoplots or photomasks, can have features limited to 20 micrometers, as is the case in printed circuit boards, or be in measurements with features sub-micrometer in size, such as photomasks for integrated circuit boards. Therefore, accuracy and resolution are of primary importance since it is the blueprint for the circuit or device.
Photolithography
The photomask, once created, are the patterns used in photolithography. Photolithography is the process used to transfer the images from the photomask onto the surface of semiconductor silicon wafers. Just like photography, the photomask is a similar to a negative that create multiple copies. The photomask creates duplicate patters on thousands of the silicon wafers if desired.
In photography, if the negative isn’t of good quality, then the pictures won’t be either. The same is true for the photomask. If these are inaccurate or of poor quality, the LCD panels or semiconductor chips they create are also of poor quality.
The photoplotter uses light to write the program from the computer. It transfers the information to a photosensitive medium by exposing it to the light according to the computer program. At one time, those who made photoplots did the task by hand and it was an arduous process. The data fed to the photoplotter generates the “artwork.”
Once the creators of photomasks complete data they transfer the image to a quarts substrate that’s been prepared with a Cr/CrO2 layer and a photoresistant coating on top in many cases, although there are other substrate and coatings used. Then E-Beam or laser lithography equipment writes a pattern on the photoresist layer. The use of developers finalizes the design on the area exposed.
Another process then etches the Cr/CrO2 layer, either with wet or dry etching, and reveals the quartz without affecting the photoresist area remaining. A strip process removes the photoresist area and then they clean and dry the mask area. There are normally dark and clear areas that remain. The dark area is still covered in Cr/CrO2 and the clear area is the naked quarts that transmit the incoming light source. All that remains is to take measurements for accuracy, clean up loose particles and repair any minute damage done in the process.
Gerber Scientific Instrument Company
The first photoplotter came from the Gerber Scientific Instrument Company in the 1960′s. While previous photoplots came from artwork drawn and then photographed, the Gerber Scientific Instrument Company created used numerically controlled data, called Gerber data, as a language to drive the machine.
They used the photosensitive film surface or glass plate to receive the image projected by the use of a xenon lamp. The head of the machine crossed above the surface of the film and produced flashes and draws. The flashes are similar to rubber-stamping a specific area with a simple design and the draws are arcs created by the continuous light from the moving head of the machine. This form of photoplotting was a vector photoplotter and while it dominated the industry for almost 20 was eventually replaced with laser photoplotting.
Laser photoplotting works in an entirely different way. It doesn’t use the vector system of X-Y movements or Gerber data that the vector photoplotter uses. Instead of a set of commands used, as in the vector, the laser photoplotters are in a raster format.
In raster format is made from pixels or a bitmap. The bitmap switches the laser off and on to create lines or designs. If there’s a line the laser flashes, if not, it shuts off. These machines function much faster than the original vector machines. Complex designs that once took as long as 10 hours on vector machines now take as little as 10 minutes on laser photoplotters.
Photoplotters are expensive
The price for laser photoplotters isn’t an amount normally found in the average person’s expendable cash budget. Some machines cost as much as six digits. Most of the machines are purchased by large companies in the printed circuit board (PCB) industry or to shops that outsource for these industries.
The top laser photoplotters are those with fine-line capability, speed and accuracy. Since one error in the photomasks can make all the duplications from it useless, accuracy is of utmost importance. Being able to integrate with CAM systems is also important.
Sometimes transferring the information to plotter ready format causes errors and takes extra time. Most laser photoplotters have recognizable formats such as Gerber. Normally, besides being in the appropriate format the data needs to be panelized and positioned so it’s possible to feed it directly into the plotter.
Printing silk screens
Some of the applications for the laser photoplotters include not only PCB, printed circuit board manufacturing, but also screens for printing silk screens, chemically etched metal components, microwave devices, large feature masks for semiconductor processing and microfluidics.
Look for photoplotters with a wide range of resolutions, often from 3 000 dpi to 128 000 dpi. The machine often runs slower at the higher-resolution spectrum than the lower but it does give you the capability of plotting very fine lines.
You need positioning accuracy in your photoplotter also. A closed loop system can ensure the correct pattern positioning of the entire plot area. If you work in an environment with an unstable temperature, you’ll need a laser plotter with a built in sensor to improve the dimensional stability at your environmental high or low temperatures.
Different industries have different requirements for their laser photoplotters. People that use the plotter for standard photo-chemical milling applications require a high degree of back-to-back registration accuracy, particularly when you consider the film envelopes that image both sides of the product simultaneously.
For electroforming, or other single sided applications, you need size control, excellent edge quality and the ability to use glass plates required for specific applications. Before you purchase your photoplotter, list all the important specification required by your industry and compare each type of laser photoplotter to find the one with the features you require.