Photomasks are high precision plates containing microscopic images of electronic circuits. Photomasks are made from very flat pieces of quartz or glass with a layer of chrome on one side. Etched in the chrome is a portion of an electronic circuit design. This circuit design on the mask is also called geometry.

Photomasks are used in wafer fabrication, mostly to make ICs (integrated circuits). ICs are used in many products like computers, calculators, cars, cameras, and stereos. Photomasks are also used to make flat panel displays, thin film heads, PC boards, etc.

The steps in making (fabricating) devices like ICs include deposition, photolithography, and etching. During deposition, a layer of either electrically insulating or electrically conductive material (i.e. metal, polysilicon, oxide) is deposited on the surface of a silicon wafer. This material is then coated with a photosensitive resist.

A photomask is then used much the same way a photographic negative is used to make a photo. Photolithography involves projecting the image on the photomask onto the wafer. (If the image on the photomask is projected several times side by side onto the wafer, this is known as stepping and the photomask is called a reticle.)

The wafer is developed and then etched to remove material from the areas exposed with the photomask image.

A new layer of material and resist is then deposited on the wafer and the image on the next photomask is projected onto it. Again the wafer is developed and etched. This process is repeated until the circuit is complete.

Example of the Laying of Photomask Images onto a Wafer

The complete circuit is called a device. The device and each layer of the circuit are named - the photomask used to make a particular layer will have both the device name and the layer name. (The photomask will sometimes also be referred to as a layer.)



SILICON WAFER
with completed circuitry

The customer designs the circuit and digitizes the information. The customer then sends us the digitized data containing the design for each layer. The data can be sent on a floppy disk, magnetic tape, cassette or via modem.

Photronics takes the customer's data and formats it for the write (lithography) tools or systems. This includes fracturing the data, sizing the data if needed, rotating the data if needed, adding fiducials and internal reference marks, and making a jobdeck (instructions for the placement of all the different patterns on the mask).

Fracturing the data means translating the customer data into a language the write tool can understand. The write system uses rectangles and trapezoids - so the customer data is divided up (fractured) into these shapes.

The jobdeck with the fractured data is put on a magnetic tape and sent to the write area or pulled directly to the machines using network software.

There are four types of material used to make photomasks; quartz (the most commonly used and most expensive), LE, soda lime, and white crown. The mask sizes can range from 3 inches square to 7 inches square and 7.25 inches round. The thickness of the masks ranges from 60 mils to 250 mils. Currently the most common sizes of masks used are 5 inches square 90 mils thick and 6 inches square 250 mils thick.

The quartz or glass (or substrate) has a layer of chrome on one side. The chrome is covered with an AR (anti-reflective) coating and a photosensitive resist. The substrate with chrome, AR, and resist is known as a blank.

Lithography is the process of writing the circuit design (geometry) onto the mask. The lithography or write equipment (E-beam or Core) writes the geometry onto the plate by exposing the resist with an electron beam or laser. This exposure changes the molecular composition of the resist. During the developing process any resist that has been exposed will be removed.

The mask is now etched. Etching removes the chrome and AR wherever the resist has been removed.

Strip, the final step in making a photomask, removes all the resist from the mask.

Note: Areas where the chrome has been removed are referred to as clear or glass. Areas where the chrome and AR remain are referred to as chrome, dark or opaque.

1X Masters are used on projection aligners made mostly by Perkin Elmer. The 1X Master mask has many repetitions of the primary pattern (the principle circuit design). Each complete pattern is also called a die. Unlike reticles which are stepped, the patterns on the 1X Master photomask are projected onto the wafer only once - all the repetitions of the device are on the mask itself.

Examples of 1X Masters:

Note the large number of dice on these masks. The five unique dies in both the diagrams are the test patterns or test dies.

Array refers to the arrangement of the dies in rows and columns on the mask.

The patterns on a 5X reticle are stepped onto the wafer - projected many times placing the patterns next to one another. The patterns on the 5X reticle are reduced

5 times when projected onto the wafer. This means the dies on the photomask are

5 times (5X) larger than they are on the final product. There are other kinds of reduction reticles (2X, 4X, and 10X), but the 5X is the most commonly used. Reduction reticles are used on a variety of steppers, the most common being ASM, Canon, Nikon, and GCA.

Examples of 5X Reticles:

Note how much larger the dies are on a 5X compared to a 1X Master.

A UT1X reticle is designed for use on an Ultratech stepper. Patterns on the UT1X reticle are stepped onto the wafer. These patterns are projected onto the wafer at the same size that they are on the mask.

The dies on a UT1X are grouped into separate fields rather than in an array. Each separate field can contain different device patterns or layers. The fields are duplicated and placed in rows (see diagram). Usually on a mask there are two rows containing 3 or 4 fields, but a row can have from 2 to 7 fields.

The identical rows on the UT1X facilitate defect inspection - the rows are compared to one another in a die-to-die inspection. The row with the fewest defects is selected for repair. For older models of Ultratech Steppers, the portion of the mask having this "good" (defect free) row, is cut to 3 X 5 inches and has a pellicle and guides attached to it. Newer models of Ultratech Steppers do not require that the mask be cut or that guides be attached to it.

Example of UT1X Reticles:

A die is a single complete device image. A primary die (also called the primary pattern) has the device design that will be used to make the circuit. The primary die or dies will make up most of the array or fields. A test die contains a simplified functional device (of the same process type as the primary die). The test die is used for process control and monitoring during wafer fabrication and sometimes to test new design ideas.

 

Scribe lines (scribes) are the lines forming a border around each die separating the dies from one another. There may be small patterns placed within these scribes, usually alignment marks or test patterns used in wafer fabrication.

The array is the area made up of the rows and columns of dies on 1X masters, 5X reticles and other reduction reticles.

Fields are the blocks of dies on a UT1X reticle. Primary fields contain primary dies. A test field(s) contains test dies and/or patterns and usually some primary dies.

A row consist of a horizontal line of fields on a UT1X reticle.

Fiducials are patterns on reticles used for alignment on wafer steppers. Each brand and model of stepper has specific types of fiducials. The fiducials are located outside of the array or fields.

KLA/KLARIS reference marks are internal marks placed in the corners of the mask. These marks are used as a points of reference when setting up inspections on an inspection system (such as a KLA system) and when repairing defects found in such inspections. The design and placement of these reference marks may vary from one manufacturing site to another.

Auto-Inspection Marks (KLA Marks) are used on UT1X reticles and located outside each field. These marks are crosses defining the area to be inspected on the KLA or other inspection systems.

Closure Checks are sets of patterns used to monitor the performance and accuracy of the lithography equipment. These marks are located outside the array. The closure check consists of several patterns; half of a pattern is written at the beginning of the write process and the second half of a pattern is written at the end of the write process. Comparing the two halves of the patterns can show any shifts or discrepancies that have occurred while the mask was being written. The actual patterns used in a closure check may vary from one manufacturing site to another.

Blanks and Auxiliaries (Aux.'s) are used on 1X masters and UT1X reticles.

A blank is a pattern, containing no circuitry, used to clear out a specified window area. An auxiliary is a pattern, containing no circuitry, which surrounds a smaller pattern in order to clear out a specified window area and isolate the smaller pattern.

In order for the circuits being made from the photomask imagery to function, the photomasks must meet certain quality standards. The masks are tested for critical dimensions, defects, registration, and contamination before being sent to the wafer fab.

Customers have very specific requirements for geometry sizes and specify certain geometry on the mask to be used as a gauge. These geometries are called critical dimensions or CDs. The customer indicates the target size or spec of the CDs in microns (a thousand microns equals a millimeter) and the acceptable variance from that target (the tolerance).

The amount of time a mask is processed (developed and etched) after the mask is written is important because it affects the size of the geometry. The longer a mask is processed the smaller the chrome geometry will be and the larger the clear areas will be. CDs are measured on a microscope during the processing of a mask to determine process times and to make sure the CD size is acceptable. (Sometimes CDs will also be measured again on registration equipment).

Examples of geometry used as CDs:

A defect is any flaw affecting the geometry. This includes chrome where it should not be (chrome spots, chrome extensions, chrome bridging between geometry) or unwanted clear areas (pin holes, clear extensions, clear breaks). A defect can cause the customer's circuit not to function. The customer will indicate the size of defects that will affect their process (defect spec). All defects that size and larger must be repaired, or if they can not be repaired the mask must be rejected and rewritten.

Examples of some types of defects:

There is also a class of defects known as cosmetic defects. These are defects that may not affect the circuitry 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.

A die-to-die inspection system (KLA) is used to inspect for defects. These machines use two objectives and transmitted (bottom) light to compare similar die patterns to one another.

In this example the left 3 columns of dies are being compared to the right 3 columns of dies.

The image seen through the objectives is divided into pixels. The information in each pixel is digitized and compared to the information found in the other objective. If the pixels do not match the machine registers a discrepancy. This process is known as image processing.

imageS IN OBJECTIVES DIVIDED INTO PIXELS

After the inspection is completed, an operator must view each discrepancy and determine which kind of defect was found and give the defect a code number or classification.

A die-to-database (KLARIS or Orbot) inspection is similar to a die-to-die inspection, except instead of comparing a die to another die it is compared to a database. This inspection is used to insure that the geometry on the mask matches the customer's design.

In this example of a KLARIS inspection the left objective is turned off. The image in the right objective is being compared to data.

The image seen through the objective is compared to the digitized image on the database. If the images do not match the system registers a discrepancy. Again when the inspection is complete, an operator must review each discrepancy and classify it.

Registration equipment (LMS 2000 and Nikon 2i) is used to measure the positioning of patterns on a mask in relation to other layers in the device or to a design grid. This can include checking the size, placement, and rotation of dies, fields, and arrays. This equipment can also be used to measure the placement of fiducials and measure CDs.

Examples of types of registration inspections:

Contamination on a mask will have the same ill effect as a chrome defect when projected onto the wafer. That is why photomasks are made and used in cleanroom environments. Before being shipped to the customer the photomask must be carefully cleaned and then inspected for contamination. Pellicles (metal frames with a protective membrane) are often attached to the masks to help keep them clean.

Photronics uses four ways to inspect for contamination:

1)Gross Light Inspection - the mask is inspected under a high intensity light for gross contamination (contamination that can be seen with the naked eye).

2)Microscope Inspection - an operator manually inspects the mask on a microscope using a high power (50 to 200 times) magnification .

3)Q.C. Optics or KLA Starlight Inspection - an automated inspection on a Q.C. Optics or a Starlight. These machines inspect for contamination using deflected light to detect any particles on the surface of the mask or pellicle.

4)Post-pell die-to-die or die-to-database inspections - an automated inspection on a KLA or Orbot defect inspection system. Though the KLA and Orbot systems are used to detect defects they will also detect contamination in the clear areas on the mask. Post-pell (post-pellicle) inspections are performed after the mask has been cleaned and has had a pellicle attached to it.

OTHER QUALITY INSPECTIONS

Some other quality inspections are performed by operators. These inspections include verifying that the mask has the correct titles, fiducials, field tone, data parity, array rotation and has no cosmetic defects. Manual microscope inspections are used to look for defects in any areas that could not be inspected on automated equipment, this includes test patterns, scribes, and fiducials. Sometimes a microscope is also used to take array and fiducial placement measurements.

[ top ]