FAQs

Do You Have a Question?

Can your software handle inverting curved features?

Within AutoCAD the shape are likely to be defined with a curve primitive, such as a ‘polyline bulge’. AutoCAD may still draw these as a ‘rough’ approximation, even though the curves are well defined within its database.

When the AutoCAD is converted to GDSII, there is no curve primitive, everything has to be represented as polygons. The software which converts the DXF to GDSII will usually have some options to control how smooth any curves will be after the data has been converted.

If you are inputting the DXF to L-Edit then saving this as GDSII then you will need to look at your L-Edit configuration options available when inputting DXF data. If you are using a separate program, such as DXF2GDS produced by Artwork Conversion Software, then it is an option when this conversion is performed.

One thing to note is that: the smoother the curves, the larger the GDS data file, and this may result in the masks needing to be written on more advanced machines which can handle the large data volumes or longer mask write times. (None of our mask writing tools have a curve primitive in their data formats, so you need to represent the curve as a polygon for writing the mask anyway).

You need to decide how smooth the curves need to be to suit your process requirements.

Can your software handle inverting curved features?

Within AutoCAD this shape is likely to be defined with a curve primitive, such as a ‘polyline bulge’. AutoCAD may still draw these as a ‘rough’ approximation, even though the curves are well defined within its database.

When the AutoCAD is converted to GDSII there is no curve primitive, everything has to be represented as polygons. The software which converts the DXF to GDSII will usually have some options to control how smooth any curves will be after the data has been converted.

If you are inputting the DXF to L-Edit and then saving this as GDSII then you will need to look at your L-Edit configuration options available when inputting DXF data. If you are using a separate program, such as DXF2GDS produced by Artwork Conversion Software, then it is an option when this conversion is performed.

One thing to note is that: the smoother the curves the larger the GDS data file, and this may result in the masks being required to be written on more advanced machines which can handle the large data volumes or longer mask write times. (None of our mask writing tools have a curve primitive in their data formats, so you need to represent the curve as a polygon for writing the mask anyway).

You need to decide how smooth the curves need to be to suit your process requirements.

Is your chrome layer ARC (anti-reflective chrome)?

Yes, we would normally use an anti-reflective chrome (ARC) layer on our masks as standard. HRC (High reflective chrome), also known as bright chrome finish, is available on request.

Semiconductor applications generally use ARC. HRC is used in specific applications such as optical targets.

Do you keep archives?

We now have a policy of retaining all the data that was used to write masks, unless specifically requested otherwise.

We possess the data for some orders that date back as far as 1992.

If you wish to ask about data for an old order, it is helpful if you know your Sales Order or Run number (4 or 5 digits).

Can I send my data in AutoCAD Format?

It is possible to convert AutoCAD DXF (ASCII) data into other formats that can be used for mask manufacture.

The main requirement for mask manufacture is the specification of areas. The problem is that AutoCAD also allows the specification of points, lines, arcs which, on their own, have no area, therefore are unsuitable for mask manufacture.

If the correct AutoCAD constructs are used then the conversion process is straightforward.

Statistically, jobs specified as DXF are twice as likely to have challenges during data preparation the data compared to other data formats such as MEBES or Calma GDSII Stream. As a result, a conversion charge may be applicable.

Can you perform Boolean operations?

 If you are creating data which is a combination of other layers within a database, logical operations such as; OR/XOR/AND, can be performed during the conversion of the data.

At Compugraphics we have a wealth of experience in this area. We can advise you about any Boolean operations that you may need to ensure the highest performance of your mask and implement them where required.

What if I want a mask with curved or circular features?

All of our mask exposure systems work on the principle of exposing an array of pixels to transfer the image from the data onto the mask.

The smaller the size of the pixel, the better the resolution of any curves or circular features, however this increases the size of the mask data which can result in longer exposure times and a higher cost of the mask.

The mask exposure tools are optimized to write rectangular data with 45-degree angles, so if it is possible to specify your design using squares or octagons, then the data will write faster than using circles.

Whilst this becomes very important for masks with thousands of circular or curved features, it is not an issue for a small numbers of curves or circular features on a mask, containing mostly rectangular features.

 

Writing Tool Writing Mode Min Pixel Size (μm)  Minimum Features
Core 2564 2-Pass 0.0625 or 0.05 1.5μm lines and 2.1μm contacts
Core 2564 4-Pass 0.03125 or 0.025 0.8μm lines and  1.6μm contacts
ALTA 3900 2,4,8-Pass 0.005 0.6μm curves and 1.0μm contacts

 

How can you clean a photomask?

How to clean photomasks manually

If the masks are not too heavily contaminated, acceptable results can be achieved with the following technique:

  • First soak the mask in a solution of Decon 90 for approximately 10-15 minutes
  • Wipe carefully with a clean, pre-wetted, PVA sponge
  • Rinse the mask in running filtered D.I. water (starting by continuing the wiping process under running D.I. water if possible)
  • Leave to dry in a clean air environment

You can also use our photomask cleaning service,  particularly recommended for heavily contaminated masks

  • Our cleaning processes are based on the use of a sulphuric acid / hydrogen peroxide mix
  • Our UK and Fremont facilities use purpose designed automatic single substrate mask cleaning tools such as the HMR900 system, manufactured by HamaTech, now part of SUSS MicroTec
  • HMR900 tools clean the mask by dispensing a small quantity of sulphuric acid and hydrogen peroxide onto one surface (facing upward) while it is slowly rotated in the process chamber. The mask is then rinsed free of chemicals, sprayed with a high pressure ionised (with ammonia) water spray, brushed with a rotating PVA sponge brush, rinsed again and finally spun dry.
Why would I want to use a copy mask?

If the mask is used in a Stepper, Scanner or Projection Aligner, then it does not make physical contact with the wafer. The mask should, therefore, last a lifetime if it is kept free from physical damage caused by handling or static electricity.

If the mask is used to contact print onto the wafer or other substrate, it will become worn over time, eventually reaching a point where it needs to be replaced.

If a Master Mask was used, then the cost of the replacement mask would be the same as making a new mask. If the dimensions are large enough (>=2μm) and the specifications for tolerance, registration and defectivity are less critical, then a copy of the mask can be made using a contact print process, which is considerably cheaper than making a new master mask.

A copy mask produced by contact printing is not an identical copy; the reading sense of the mask is reversed as part of the contact printing, which is also known as a mirror image copy.

What does Critical Dimension mean?

After a mask has been manufactured, a measurement is made of a feature on the mask to check that the exposure and processing of the mask are within the specified tolerances.

The feature checked is known as the Critical Dimension. This should reflect the smallest feature size present on the mask and also the type of features (Lines or Contacts). Users are encouraged to include suitable features in their data, preferably at defined standard locations.

The maximum size of Critical Dimension for our CD tools is 60um, and it must be less than 21um if the mask is to be measured on the more modern automated CD tools (LWM + M5K).

If the user does not include any suitable measurement points, we can measure one of the features that we add at the edge of the mask for our SPC system.

Data Files: Sending large data files

Generally it is a good idea to compress or zip your data (with winzip, 7zip, winrar etc.) prior to sending it.

If your data is less than 20 Mb you can send this by email to:

customer-support-uk@compugraphics.com ( for European and other international customers outside USA)

us@compugraphics.com (customers in USA)

If it is larger than this please contact the above email addresses to request an ftp account . Alternatively we can use other file sharing systems.

Which Mask Data Formats can you use for mask making?

The mask writing tools at Compugraphics currently all use a data format called MEBES that was defined by Etec, the manufacturer of these tools. (Not to be confused with MEBES machines, which are a range of mask writing tools, where the format was first used.)

Any other data formats provided by customers need to be converted into the MEBES format in order to manufacture the mask.
If you are not able to provide MEBES data, our preferred format is Calma GDSII Stream, sometimes known by any of these three names. We use software called CATS to convert this to MEBES. Other acceptable formats are: Electromask, David Mann, Caltech Intermediate Format (CIF), OASIS, Gerber, and Varian.

See separate FAQ on the use of AutoCAD DXF for masks

We cannot accept formats such as IGES, although other companies can provide software or conversion services to convert these formats into Calma GDSII Stream. If you have further questions about data formats, please contact our Data Processing department (customer-support-uk@compugraphics.com).

How is defectivity on the photomask specified?

Defects are any instance where the completed mask does not match the data that was used to write the mask. The most common cause of defects are any impurities in the materials used to make the mask blank, or the effects of the chemical processing of the mask, as it is developed and etched.

There are several different types of defects, however for all but the most advanced masks, customers usually have a single specification that is applied to all types of defects. There are three common methods of specifying a defect specification:

1) No defects on the mask greater than a specified size.

e.g. 0 defects greater than 1 μm

This type of defect specification is usually used for masks that are reticles or contain a small number of die, where no defects can occur on the mask. This is the most demanding specification and is the most expensive specification to apply.

2) An acceptable density of allowable defects at a certain size.

e.g. 1 defect per square inch at 2.0 μm

This type of specification is usually used for masks where the image is an array of identical chips, where defects affecting a small number of the chips are not significant for the wafer yield. This type of defect specification can also be combined with the previous one,
e.g. 1 defect per square inch at 2.0 μm and 0 defects > 10 μm

3) No automatic or microscope inspection.

e.g. Strip and Ship.

Where defectivity is non-critical, then the mask can be given a visual inspection under a bright monochromatic light. The mask will be shipped to the customer if there are no defects large enough to be spotted by the human eye.

In this case there are no automatic inspection or microscopes used to check the defectivity and, as a result, such masks are cheaper.

The Strip and Ship option provides a cost effective method of prototyping where a high yield is of less importance.

How long will it take to get my mask?

Delivery Time

Please note: In the following description the term “order” means that we have received the data, manufacturing instructions and purchase order or other payment method.

If plots are required for approval prior to making the mask, allow 24 hours from receipt of the order.

For reticle or 1X Masters, shipment can be expected within 4 days of receiving the order.

For contact print (copy) masks, dispatch within 10 days of receiving the order.

If you require, we can offer faster delivery times for the first few layers of a mask set. Customers can also choose to pay a premium to ensure that any urgent masks are made to faster timescales.

Can you make greyscale photomasks?

We manufacture binary masks ie. Chrome or not chrome. We can however make approximations to greyscale by having different densities of chrome in different areas.

Please get in touch if you would like to discuss your requirements.

Difference between Quartz and Soda Lime Glass?

Mask Materials

The most common types of glass for manufacturing masks are Quartz and Soda Lime.

Quartz is more expensive, but has the advantage of a much lower coefficient of Thermal Expansion (which means it expands less if the mask gets warm during use) and is also transparent at deeper Ultraviolet (DUV) wavelengths, where Soda Lime glass is opaque.

Quartz needs to be used where the wavelength being used to expose the mask is less than or equal to 365nm (i-line).

How do I place a mask order?

You can place an order by contacting us or sending in a quote request via our Request for Quote (RFQ) form.

Order Process:

  • Step One: Provide information regarding your required photomask type and size, substrate (typically Quartz or Soda Lime), minimum feature size and CD tolerance if you know it.
  • Step Two: Include your delivery address and a purchase order (if you have one).
  • Step Three: We will review your request and contact you if we have questions or find any errors.
  • Step four: We will then generate a quote and send this to you via email

Please note: We prefer to receive data in GDSII format however we do accept other data formats (see Data Formats for more information).

What sizes of mask can you make?

For historical reasons, masks are sized in inches, and are available in the following common sizes and thickness in Quartz and Soda Lime.

  • 3″ x 3″ x 60′
  • 4″ x 4″ x 60′
  • 4″ x 4″ x 90′
  • 5″ x 5″ x 90′
  • 6″ x 6″ x 90′ (Contact prints only)
  • 6″ x 6″ x 120′
  • 6″ x 6″ x 250′ (Quartz only)
  • 7″ x 7″ x 120′
  • 7″ x 7″ x 150 with corners rounded to 7.25″ diameter (Quartz only)
  • 8″ x 8″ x 120′
  • 9″ x 9″ x 120′
  • 10″ x 10″ x 120′
  • 12″ x 12″ x 120′
  • 14″ x 14″ x 120’/190′
  • 16″ x 16″ x 120′

If you need larger area masks (e.g. up to 32″ x 32″) please contact our sales staff.

We recommend that your data leaves at least 10mm at the edge of the mask to allow us to add titles and avoid any of the machine plate holding mechanisms.

What is the smallest feature size possible?

The minimum feature, which can be resolved on a mask, depends on the technology of the writing tool and the mode in which the tool is used, as well as the resist and process chemistry used.

Note that MEBES 5500 photomasks are only available in 6″x6″x250 materials with a dry etch process. (We don’t even have the MEBES 5500 any longer in US or UK)  This makes these photomasks are the more expensive option. Smaller contacts (features) can be manufactured on the 5500 system using Phase Shift Masks, also a more expensive option than using normal Chrome masks.

What is OPC?

OPC is the abbreviation for Optical Proximity Correction.

This is the generic name for a number of different image enhancement techniques where additional features are included on the mask to improve the quality of the image when the mask is used.

The use of OPC becomes important when smaller feature sizes are present, especially as these sizes approach the wavelengths of the light being used in stepper and scanner systems. Some typical examples of OPC techniques are:

  • Adding small rectangles, called serifs, to the corners of lines and contacts to reduce the effect of corner rounding. (Simple OPC)
  • Cutting out of small rectangles, serifs, from the inside edges of angled lines to reduce the over exposure of these areas. (Simple OPC)
  • Using a set of rules relating to feature types and sizes to make changes to features to compensate for the effects of the stepper or scanner. (Moderate OPC)
  • Using computer modelling to make major changes to features to compensate for the effects of the stepper or scanner. (Aggressive OPC)
  • Adding features such as assist bars which will generate interference effects to enhance the nearby images, with the size of the assist bars being small so that they are not being printed. (Aggressive OPC)

In all cases, the resulting mask data when using OPC will be greater than that of a mask without OPC, and will often require to be written using a smaller resolution (pixel size). As a result, OPC masks will usually take longer to expose and require more sensitive inspection, so will cost more than a mask of a similar specification with no OPC features.

Learn more here

What is a pellicle and do I need one?

A pellicle is a transparent membrane supported by a frame.

This protects the surface of the photomask, as well as ensuring that any particles that settle on the pellicle membrane are out of the focal plane of the mask surface, thus reducing the impact of these particles.

There are hundreds of different types of pellicles, with different frame shapes, membranes, and other options such as vented frames, or tacky internal surfaces.

An important aspect of the pellicle is the type of membrane as these may be designed for specific exposure wavelengths or a range of wavelengths. The equipment manufacturer will have specifications for valid pellicles for any given system.

In general, photomasks used with projection alignment systems, Ultratech 1X steppers, and more modern steppers (6″x6″x250′ reticles) have pellicles on the coated side of the photomask, whereas  the older steppers (5″x5″x90′ or 6″x6″x120′ reticles) tend to normally have pellicles on both sides.

The use of pellicles helps to improve the yield in a production environment but, does add to the cost of the photomask. Some users prefer not to use them in a prototype environment.

How is the Reading Sense of a photomask specified?

The Reading Sense of a mask defines the transformation that has been applied between the original design data and the pattern that appears on the photomask.

If you imagine a letter F defined as polygons in the design data, the most common requirement is to want a photomask with the letter F mirrored about it’s vertical axis, when the photomask is viewed with the Chrome surface upwards. This is called an Incorrect or Wrong Reading photomask with the Chrome Upwards, and is equivalent to a Correct Reading photomask with the Chrome downwards.

If the photomask were to be used to make a contact print, then the master would need to be made Correct Reading Chrome Upwards in order for the Contact Printed copy photomask to be Incorrect Reading Chrome Upwards.

Mask makers always prefer to define the reading sense with the Chrome surface upwards, as this is the way that all operations are performed on the photomask during manufacture.

What is registration on a photomask?

Registration is the placement accuracy of the patterned features on the mask from one side of the mask to the opposite side. Registration is guaranteed from our writing tool by SPC. The specification varies by material and writing tool. Based on SPC information, masks without a registration check will conform to the following registration specification.

  • Alta QZ +/- 0.10 um
  • Core QZ +/- 0.15 um
  • Soda Lime masks +/- 0.40 um

Although SL masks can be exposed with a tight registration, any temperature variation when the mask is used will have a more significant effect. Fab temperature alignment should be discussed with customer support as this varies according to mask size.

What is a reticle?

The term Photomask (sometimes abbreviated to Mask) is used to mean any type of glass plate with a pattern etched into an opaque surface.

A Reticle is a special type of photomask where the data for only part of the final exposed area is present.

A Reticle is loaded into a Stepper or Scanner system where multiple exposures are made to cover the full patterned area. Most modern reticles have the features scaled up by 5X or 4X of the final image size and the features will be reduced in scale as the Reticle is exposed. Other scale factors such as 2X, 2.5X and 10X are also possible.

Ultratech Reticles are a special case where the data is at a scale of 1X. When ordering a Reticle, special features for alignment called fiducials also need to be added to the mask. The design of these fiducial features will depend on the manufacturer of the stepper or scanner used. Most steppers and scanners also allow the user to place a barcode on the reticle which can be read on the system as the reticle is used.

Compugraphics makes a wide range of reticles and can advise customers on the additional patterns that need to be added.

Do you make photomasks for excimer laser machining?

Stepper Wavelengths

In addition to making photomasks for use with the wavelengths of visible light, we can also make photomasks suitable for using with i-line (365nm), 248nm and 193nm Deep UV wavelengths.

When ordering photomasks for smaller wavelengths, Quartz glass should always be used.

Also, if a pellicle is required, it is good practice to identify the wavelengths, for which the pellicle should be optimized.

What is the “Strip & Ship” option?

This option means that the photomask will not receive an automatic or microscope inspection step after manufacture.

It means that the photomask may contain a small number of defects.

This type of photomask is cheaper, so is attractive for research or prototyping work where the photomask has multiple regions of similar structures, where such defects could be worked around or would be non-critical to the final product.

How do I specify the tone of the photomask?

There are several different methods which people use to specify the tone (or polarity) of the photomask.

The two main strategies are:

  • Trying to describe how the mask should be processed
  • Trying to describe how the mask should look after processing

We have found that the first of these is the less ambiguous.

If you think about a shape drawn on your CAD system, do you want this to be Dark (Chrome) or Clear (Glass) on the photomask?
e.g. The digitized data should be Dark on the photomask.

Attempts to use descriptions such as Positive / Negative, or Field Tone can be ambiguous in many cases, and are best avoided.

What do any electronically generated plots represent?

It is possible to generate files containing plot information in a variety of ways. The normal convention adopted at Compugraphics is to have any colored areas representing Clear on the mask, and the background area (either black or white)  representing Dark (Chrome) areas on the photomask.

Also any plots will usually show the master mask with the chrome surface upwards.

If contact print copies were being made, this would also represent the copy photomask with the chrome surface downwards.

Any messages sent with plot files should identify the conventions used, if there is any ambiguity – please ask for clarification.

Write space between my data & edge of the photomask?

It is important not to go too close to the edge of the mask, since the resist on masks blanks is slightly thicker in the corners and on the edge of the mask blank.   We recommend that no features are placed within 10 mm of the edge of the mask. 

We would normally add titles out with the exposure (or write) area. The area beyond the exposure area will always be “chrome”.

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