Greyscale Photomask

CUSTOM MADE GREYSCALE PHOTOMASK
FOR PRICE, PLEASE REQUEST QUOTE

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GreyPM
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A greyscale image is one where each pixel in the image represents a different rate of intensity. These can be represented in many ways, but a general consensus is to use a range where 0% - totally clear and 100% is totally black. Our process is a binary one, whereby we can only image light on, light off,  so we cannot create a normal greyscale image as defined by "printers" or "photography". However, we have a solution that works in the vast majority of cases, and that is pixelation or dithering.

There are 2 different types of customers who want greyscale masks. One of these is optical - where the customer wants a section of the photomask to appear grey to an eye or camera / lens / video. This is the part that we can certainly help with, because we use a differing - or halftone -  process and the trick is to make the pixel that creates the "grey" smaller than the lens can resolve (normally 50% smaller) - so the lens is tricked into thinking it is looking at a grey tone. This generally works very well.

The second type of customer needs a transmission grey - where different amount of light is transmitted through the photomask to expose a photoresist underneath, usually building up 3D exposures. This method is more difficult to overcome with the dithering process, because it depends upon the type and thickness of resist used. It can partially work, but generally needs lots of experimentation with resist speeds, resist thickness and pixel size by the customer.

 

DITHERED GREYSCALE OPTICAL MASKS

Dithering is a process of representing grey levels to the human eye by placing certain sized dots or patterns close to each other, varying them in size and shape to represent a grey tone. This method has been used for many years in the printing and newspaper industry; however we have the ability to take this graphics pattern to a whole new level due to the small pixel sizes that we can generate.

We take an existing greyscale or colour image and run it through some graphics software (such as Corel Draw or Adobe Illustrator) to apply a dithering process, sometimes called a halftone process. The end result is a binary mask that appears grey to a lens or video due it being unable to resolve the pixels individually.  Generally we recommend using a pixel that is at least 20% smaller than the smallest pixel that the lens can process. 

 

DITHERED PATTERNS

There are various methods that we can use to create the dithered pattern. The most common is the "jarvis" method because that is less structured, and the tolerance accuracy of the greytone is smaller due to the fact that there are fewer internal corners that can be rounded.

Jarvis dithered pattern on chrome greyscale photomask Halftone dithered pattern on greyscale photomask
Jarvis dithered pattern Checkerboard dithered pattern

 

 

SIZE OF GLASS

Please enter the final plate size (length and width) that you need , along with any tolerances if the dimensions are critical. if this is not important, please let us know and we will offer you the best cost effective solution.

PIXEL AND PATTERN SIZE

The smallest possible pixel is 2um , although this leads to innacruate greyscale values and can only be processed over very small areas. We recomend keeping the pixel size to 4um minimum. Please select a size as big as possible - unrealistically small pixels can translate into files of many GB in size. The maximum number of pixels that we can allow is 20,000 in both X and Y, which translates to 400 million pixels in a dithered file. However, this is the limit and we would suggest keeping pixel counts smaller than 10,000 dpi . Smaller pixel count will result in cost savings and time savings.

For example, your pattern is 50mm x 50mm and your optical system can "see" 10um pixels. We would recommend therefore to use an 8um pixel, which works out ( 50mm / 0.010) = 5000 pixels in X and Y (25 million pixels in total)

So, given the example above, you could enter the information "50mm pattern using 8um pixels" into this field.

 

BASE MATERIAL

Soda Lime float glass, available in various thickness with various options for chrome density and reflectivity

White diffuser glass , only available in 1.25mm thickness and black chrome

Quartz / Fused silica , only available 2.2mm thick with either brown or black  chrome

 

CAD FORMAT

Please tell us what format you will be sending. Data formats listed above are only a small section of what is available. If your chosen format is not listed, please contact us for further information and clarification. We are also able to provide a full drafting service whereby we can create your designs from drawings and descriptions.

 

Glass Photomask Polarity POLARITY

Masks are normally referred to as Clearfield (positive) &  Darkfield (negative)

Positive means that the data you have drawn in CAD will be Chrome on the mask, with the background (the field) being clear glass.

Negative is the opposite of this, where the items drawn on CAD will be clear and the background will be chrome. 

We also need to know which way round the mask has been designed on your screen. The easiest way to select this is to add some reference text (anything you like) to the design somewhere - maybe in the corner - and then specify if the text needs to be right reading or wrong reading.

 

PRODUCTION SCHEDULE

Our standard production turnaround is 7 days from receipt of the artwork, purchase order and related documents. We generally send checkplots with every order, and it is vital that these are replied to within a short delay in order that we can meet the schedule required. The production schedule assumes the original data package is correct and does not need to undergo revisions or changes. For large volume orders, or large photomasks on high resolutions, please enquire before ordering premium services.

Standard (10 days) :   This is our standard service, and whilst we get 99% of orders shipped within this period, these are guidelines only and may change in periods of high demand. Where data packages are good, and all paperwork is completed promptly, we can often ship early.  

Premium (4 Days ) :   A premium service. A complete manufacturing package (data/instructions/order) must be with us by no later than 11.00am.  Adds 25% to the total cost.

 

YOUR DRAWING NUMBER

Please let us know the CAD file name / number so that we can tie together the uploaded data and the order.

 

NOTES

Here you can inform us of any other requirements or specifications regarding the order.

 

Please see the TECHNICAL sections, DOWNLOAD sections and FAQ for further information. If you still require clarification, please either email or click for LIVE SUPPORT.

MATERIAL SPECIFICATION

 

Soda Lime Glass is the most common substrate used for masks, due to its good quality/price ratio. The glass is optically good, very flat and has no imperfections. If used in a photo-lithographic process, glass can be used with a very wide variety of light sources from 350nm upwards, with an excellent UV transparency. Soda Lime glass has a thermal co-efficient expansion of 93 (10-7) which is approximately 10 ppm (parts per million) , and a transmittance value of 88% at 375-450nm, with a glass flatness class of 5um for the smaller plates and 10um for the larger plates. Some of the very big plates of 14" and above, may have a bigger flatness tolerance of upto 40um. We would recommend always using Soda Lime glass where it is suitable due to its relative inexpense. It can be combined with Anti Glare coatings.

 

Quartz (Fused Silica) is not as common due to the expense of the material, but quartz brings the very best stabilised material thermally, along with a very clear base that allows a wide variety of wavelengths to pass through at very good transmission rates. The thermal co-efficient expansion is 5(10-7) which is approximately 0.5 ppm (parts per million). Flatness of this material is best-in-class, with 2um flatness for small plates, and 5um for mid sized plates. It can be combined with Anti Glare coatings.

 

B270 or BOROFLOAT® are made out of Borosilicate, and are ideal for high temperature and harsh environment applications. Unlike common borosilicate that is drawn flat, BOROFLOAT® is produced by a float technique that yields superior surface accuracy. BOROFLOAT® is about three times more resistant to thermal shock than standard soda lime glass, and has a thermal co-efficient expansion is 35 (10-7) which is approximately 4 ppm

 

Translucent White Glass White Diffusing Glass is a semi-opaque diffuser designed to create even illumination profiles. Unlike alternative materials, which create their diffusive properties through a coating or lamination process, the White Diffusing Glass is a solid diffuse material, allowing it to be used in any orientation. Because the surfaces are polished, the reflected light will be somewhat specular, while the transmitted light will be near-Lambertian. White Diffusing Glass is ideal for creating even illumination as an attenuator or as a viewing screen. Transmissison wavelengths is similar to Soda Lime Glass (350nm <>850nm) and the transmission amount is approx @ 30%. It can be combined with Anti Glare coatings.

 

Nextrema WHITE is a specialist ceramic "white" glass that offers a white opaque surface (in the visible spectrum) , and is ideal for high temperature due to its very low thermal co-efficient which has a near zero linear thermal expansion/heat expansion coefficient. It can be combined with Anti Glare coatings.

 

 

 

COATING MATERIALS

 

Aluminium coatings are very soft and can easily be damaged during use and even cleaning. However, their conductive/resistive properties make them useful for certain applications, as well as the Aluminium being a better conductor of heat than chrome, so the material does not heat up as much during laser ablation processes.

 

Chrome Coatings are much more robust materials and can be cleaned fairly easily without fear of damage. It is coated onto the plates in very thin layers, which thickness may vary depending upon the OD (Optical Density) required - but generally ranges in the 0.1um to 0.15um thick.

 

We can coat with other materials, such as  Gold but specialist coatings means that we have to run a complete chamber full so there will be minimum order quantities required, varying between 5 and 30 plates depending upon the plate size.

 

 

 

COATING REFLECTIVITY

 

 

Low Reflective Chrome:  The chrome that is coated on the plate would normally have a bright mirror-like finish to it, reflecting at about 75%. We need to protect this surface with an oxide coating (chrome oxide) , and this oxide is applied to not only protect the surface but also to change the reflectivity of the plate. Our standard product is a LRC (Low reflective Chrome) which reflects around 12% in the UV wavelengths and therefore is most suitable for applications using Mask Aligners and other UV exposure equipment. Low Reflective refers to the oxide coating which is applied to the top surface of the chrome plate. This oxide makes the surface appear “yellow/gold” to our eyes when we hold the plate in a way that the light reflects off it. Because this oxide is applied after coating, the underneath surface of the chrome – the surface that sits against the glass – will remain bright/shiny with a mirror like finish.

 

High Reflective Chrome: We also have a small range of High Reflective chrome. This chrome still has an oxide coating over the surface, so is not as reflective as the "underneath" side of the plate, but still reflects high at around 50%

 

Blue chrome: We stock a small amount of “blue” reflective chrome. This is low reflective when using the plate in the visible spectrum, so is the best solution for components that are used optically.  With this material, our default is to have the oxide applied to both surface of the chrome so it will appear identical when looked from the front (chrome side) or the back (glass side), meaning internal reflections are kept to a minimum.

 

 

 

OPTICAL DENSITY

 

OD3 - Optical Density is a measurement given to the chrome optical characteristics, and will change depending upon the wavelength used. OD3 means that the chrome surface will transmit only 0.15% of light in the visible spectrum, and equates to a chrome thickness of approximately 0.1um

 

OD5 - The chrome is coated to a thicker layer of approximately 0.15um thick, allowing even less light to transmit through (0.015% in the visible spectrum)

 

OD1 - Some customers actually want a large amount of light to transmit, therefore bringing the contrast ration down. Our OD1 plates are coated very thinly, and appear "see through" to the human eye. They can be sued in specialist graphics applications or to replicate an Iron-Oxide plate which allows for easy registration of the plate to sub layers.

 

OD1.8 - Another thin chrome coating giving a transparent basis.

 

WRITE AREA

The way that the masks are manufactured means that we have a "keep out" area that we cannot write critical features to -  this equates to a 10mm border around the edge of the plate.  This is because the resist is slightly thicker in the corners and edges, where it builds up during the spinning process. We use ‘fringeless’ blanks wherever possible, so can image outside of this area if necassary, but recommend only  text and  references marks in this outer area.

 

DEFECT SPECIFICATIONS

The specification of the raw materials and the photo-resist, mean that there may be minor defects that appear randomly across the image. Usually these defects are cosmetic only, and most often the defect actually causes no working problem with the design. A defect is defined as having a size greater than the required rate, and can take many forms such as a "pinhole" in what should be the chrome/emulsion part of the mask, or it can be a chrome / emulsion spot in the clear part of the mask (also referred to as 'shorts' or 'breaks'). A defect is any flaw affecting the geometry that passes the authorised specification for the order. 

The customer may indicate the size of defects that will affect their process (defect spec). All defects which break these rules must be repaired, or if they cannot be repaired, the mask must be rejected and rewritten. Where no defect spec is given by the customer, we use our own internal standards.

There is also a class of defects known as cosmetic defects. These are defects that may not affect the geometry but still may not be acceptable to the customer. Cosmetic defects include scratches on the chrome outside the array, damaged or partially removed AR coating, contamination on the chrome, glass chips on the edge of the mask, etc.

The Customer Defect Specification Form is to be used by customers to inform us of their defect specifications. It may be supplied to cover every order within a given time frame, or on a per order basis. If the form is not submitted, our own internal specifications take over.  These specifications will be used by our front end engineers to asses if masks can be written before going into the write phase, and customers will be informed where specifications are deemed to be unachievable.

 

 

FAQ's

 

Q: What is the smallest feature that you can do ? 
A: This depends upon the resolution that we image at, the material that we use, the tone of the mask, and most importantly the feature type (Spot, square, line etc). This all makes it very hard to give a figure, but as a very rough guide we can make 2-3um on an "everyday" basis and down to 1um and possibly below if we "hold it's hand through production" .

Q: How accurate are the features, what tolerances do you work to ? 
A: This is dependant upon the resolution that we image the mask at. The QUICK REFERENCE brochure in the DOWNLOADS section will give you a chart, specifying both feature and dimensional tolerances.

Q: What resolution should i pick ? 
A: The higher the resolution, then the sharper the edges of the features, the smaller the features possible, the more accurate the features will be, the sharper the corners in of the features, and also the smoother the circles. Some customers will notice this difference straight away, some will hardly notice any difference. Every customer has different requirements and different priorities. If in doubt, chose a higher resolution to ensure your requirements are met, but we are happy to advise if you send us your database.

Q: What does the term POLARITY  mean ?
A: Polarity is the tone of the photomask, and in layman's terms it is what you might call Positive and Negative. When you draw your photomask design on your computer, you need to decide if the objects that you draw are going to be opaque on the photomask- and therefore to block the light - or transparent areas on the photomask. It's best to always think of the data that you draw either being positive data, or negative data.  If used in contact lithography, remember, this is polarity or tone of the mask that we manufacture, and NOT the polarity of the wafer you are using. 

Q: I've designed my mask with the wrong polarity - what do i do ? 
A: We can use the original design file and simply reverse the tone of the mask. This is much easier than supplying new data.

Q: What is Right Reading, Wrong Reading and Mirrored.
A: Some people use the photomask in contact exposure, and others use it as an optical component. Making sure that the design is the corect way round is important. To inform us exactly what you need, it is best to add a title or reference text somewhere on the design and then we can talk in terms of "right reading" or "wrong reading ". Another way of thinking about "wrong reading" is to think of the text being mirrored. Right Reading Chrome side Down is the most common way to describe a photomask that will be used for contact lithography. When you hold the mask in your hands with the chrome side facing down closest to the wafer, the image on the mask will be correct or how it should appear on the wafer. Right Reading Chrome Up is maybe the most common way to describe optical components. However, every customer has different requirements from the next, so you need to inform us via the order form.

Q: So what would Negative Data - Right Reading Chrome Down actualy mean ? 
A: Using the above Q&A's as a reference, we can now say that when you are holding the photomask plate in your hand, and the text is reading correctly, then the chrome will be on the underneath surface and the glass support is uppermost. The design will be an exact opposite - in tone - of what you had designed on the computer.

 

 

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How To Order Photomasks

 

 

 

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Designing Your Photomask - a Guide

 

 

 

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Material Specifications for standard photomasks

 

 

 

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Dimensional Stability of Film v Glass photomasks

 

 

 

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Photomasks - The Complete Guide

 

 

 

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Quality Assurance Certificate