Photo mask

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A photo mask

Photomasks ( English reticle ) are projection templates, the main application, the photolithographic patterning in the manufacture of microelectronic circuits or micro-systems is. They normally consist of high-purity quartz glass or calcium fluoride (lithography excimer laser of wavelength 248 nm or 193 nm) and are, for example on one side with a thin patterned layer of chromium is provided.

Background and application

Photo masks are used in the photolithographic structuring of the photoresist ( resist ). In simplified terms, this process can be described as follows. The mask is irradiated with light. The transparent and opaque areas of the mask create a shadow on the photoresist layer and the light causes a chemical reaction in the photoresist in the irradiated areas. After further steps (cf. photolithography (semiconductor technology) ), a structured photoresist layer is created, that is, areas on the wafer are where the photoresist layer is still present or has been removed. This layer is used in subsequent manufacturing steps to deposit structured layers from other materials or to produce them by etching.

Photomasks must be completely error-free, because a failure would in the exposure in each chip or the (single chip masks) on each wafer find. This is why the highest demands are placed on the material in terms of transmission , planarity , purity of the material and temperature stability. This requirement in connection with the necessary precision ( structure widths and positional accuracy of a few nanometers ) require extremely complex and expensive production systems ( laser or electron beam recorder ), in which even fluctuations in the earth's magnetic field have to be compensated in order to produce perfect masks. Due to these high requirements, a photo mask costs up to approx. 250,000 EUR, depending on the specification . Photomasks are generally the most expensive "material" used to make integrated circuits. While raw wafers cost in the range of a few hundred or a thousand euros, a complete set of masks (around 20 to 60 pieces are required for the various process steps) can cost several million euros.

According to an in-house survey in 2011, a good 80% of photomasks were produced on 6-inch substrates. In addition, 5-inch substrates (11%) were also widely used. The majority (more than 50%) of photo masks continued to be manufactured for products in the 250 nm technology node and above; These are usually masks for systems that use the i-line of a mercury vapor lamp or KrF excimer laser as a radiation source. Photo masks for modern circuits (below the 130 nm technology node), on the other hand, only accounted for around 20%. In this area, the proportion of masks for the 65 nm technology node and below, i.e. for today's critical levels of top products from Globalfoundries , Intel , Qualcomm , Renesas , Samsung , TSMC , etc., rose by a good 50% to 9.2% . It should be noted here that although logic products still make up the largest proportion of manufactured masks (56%), masks for memory and microprocessor circuits only accounted for 10.6% and 1.5%, respectively.

Photo masks for "classic" photolithography

In order to increase the resolution of the wafer exposure process, several complex variants of the classic chrome masks were developed. They can be classified as follows:

  • Binary interface mask (BIM)
    • Chromium mask (English chrom on glas , COG): As an industry standard mostly used synonymously with binary mask
    • OMOG mask (English opaque MoSi on glass , OMOG, German opaque MoSi on glass ).
  • Phase mask (Engl. Phase-shifting mask , PSM)
    • Chromeless phase-shift mask (CPM)
    • Alternating phase-shift mask (AltPSM, also called Levenson-PSM )
      • Rim phase-shift mask (Rim-PSM)
    • Halftone phase-shift mask (APSM or AttPSM, also called embedded PSM , EPSM, embedded attenuated PSM , EAPSM, or half-tone PSM , HTPSM)
    • Tritone phase mask ( tri-tone attenuated phase-shift mask , tri-tone AttPSM)

Binary or chrome mask

Intensity profile of a chrome mask

A binary mask is a photomask in which the pattern to be imaged is contained in the form of impermeable, i.e. absorbent, and “open” (transparent) areas. In general, this is done using a highly absorbent thin layer on a transparent substrate. The chrome mask is the "classic" photo mask. It consists of a glass substrate to which a structured, opaque chrome layer has been applied. This type of photomask is the most widely used variant because it is the cheapest and the fastest to manufacture. For many less critical process steps in semiconductor development, their resolution is sufficient. Binary masks are also available with other absorber materials, such as the OMOG mask ( opaque MoSi on glass ), which uses a sufficiently thick layer of so-called molybdenum silicon (MoSi) instead of chromium . In order to optimize the optical properties of such masks, for example to suppress the reflection of light reflected back from the wafer, the layers often have a composition of molybdenum and silicon with oxygen, nitrogen and carbon that changes with thickness, which is why the term molybdenum silicon is not entirely correct . Another variant is the "thin OMOG mask" with a layer stack made of a thinner molybdenum silicon (MoSi) and a thin chrome layer on top.

Chrome-free phase mask

In the case of chromeless phase-shift masks (CPM), there is no coating of the substrate at all. The structural contrast is produced exclusively by phase shifting the light via trenches that are appropriately etched into the glass substrate.

Alternating phase mask

Intensity profile of an alternating phase shift mask

Alternating phase masks are a combination of a chrome mask and a chrome-free mask. In addition to the “opaque” (chrome-coated) and “transparent” (chrome-free) states of a pure binary mask, there are transparent areas where the glass substrate has been etched. They are therefore “deeper” than the regular transparent areas, with the depth being set so that there is a 180 ° phase difference to the normal transparent areas. The areas are applied alternately (therefore "alternating") next to each other, so that a pattern results in which the two variants of transparent areas are always separated by an opaque area ("opaque" / "transparent" / "opaque" / "transparent" 180 ° phase shifted ”etc.). In this way, the contrast of the image can be increased.

Due to the complicated calculation of the distribution of etched glass trenches (possibly contradicting requirements when structure edges meet), the data preparation is extremely time-consuming.

A slightly modified variant is the so-called rim phase mask (from rim = hem). In this case, the transparent and opaque areas are separated by an edging, that is to say hem-like, transparent area which causes a phase shift of 180 °.

Halftone phase mask

An attenuated phase-shift mask (AttPSM) is very similar in structure to the COG or binary mask . The decisive difference between these mask types is the layer thickness of the absorber material. In contrast to binary masks, the areas coated with absorber material are not 100% opaque, but rather weakly partially transparent (hence “halftone”) and the absorber causes a phase shift of 180 °. The degree of transmission (in relation to the light incident on the mask) is between 4 and 20%, but mostly 6%. The layer thickness is chosen so that the radiation used for lithographic imaging experiences a phase shift of 180 ° when passing in comparison to the radiation which only penetrates the glass substrate. This increases the contrast of the structure to be imaged and thus the resolution.

As a rule, all typical materials such as chromium oxide (CrO), chromium oxynitride (CrO x N y ), molybdenum silicon oxide (MoSiO x ) or molybdenum silicon oxynitride (MoSiO x N y ) can be used as absorber material . However, the layer thickness and thus the above-mentioned optical properties must be adapted to the illumination wavelength used. For i-line lithography (365 nm) was and are u. a. Chromoxicarbonitride (CrO x C y N z ) is used. Halftone phase masks for KrF or ArF excimer lithography systems ("DUV lithography" with a wavelength of 248 nm or 193 nm) are largely only masks with a structure-giving layer made of silicon nitride (Si 3 N 4 ), which is about 5 % Molybdenum (Mo) is doped, also referred to as MoSi .

Halftone phase masks were introduced in the early 1990s and are still in use today.

Tritone phase mask

Intensity profile of a Tritone phase mask

The tritone phase mask ("three-tone phase mask") can be understood as a combination of binary and half-tone phase mask. A layer stack consisting of a partially transparent (often MoSi) and an overlying opaque absorber layer (mostly chromium) is fundamental for the function. The layers are structured differently, so that opaque (Cr + MoSi), partially transparent (MoSi) and transparent (uncoated) areas are created. In this way, the chromium layer removes light components that are not required for projection or that would interfere with it, thus increasing the contrast in the photoresist.

Photo masks for next-generation lithography

It is foreseeable that the successor technology of 193 nm immersion lithography (state of the art 2012) will use new, significantly different functional mechanisms. Some of these techniques, which are often referred to as next-generation lithography , are still based on reducing the wavelength of electromagnetic radiation, for example EUV and X-ray lithography . However, since they are in a wavelength range in which the properties of the materials differ significantly from those in the optical and near ultraviolet range, the implementation of these techniques requires new mask shapes and functional mechanisms.

EUV mask

EUV masks are designed for an illumination wavelength in the extreme ultraviolet of 13.5 nm. They are used in EUV lithography. Due to the high absorption of usable substrate materials in this spectral range, EUV masks cannot be used in transmission. However, the degree of reflection of common coating materials is also very low. One makes do with multi-layer systems (English multi-layer , ML), which lie directly below the structured area and function as a Bragg interference mirror . The coating consists of 40 to 50 Bragg pairs , which are usually formed from a molybdenum and a silicon layer. The layer thicknesses are designed for an angle of incidence of 6 ° and degrees of reflection around 65% are achieved. For structuring are tantalum -based absorber used ( tantalum dioxide , tantalum nitride ).


The desired structures are usually produced by direct-writing laser and electron beam lithography , with laser lithography making up the largest share at 60.3% due to the high proportion of masks for 250 nm technology nodes and above. Masks with smaller structures are manufactured using electron beam lithography. The proportion for systems with a variably shaped beam ( vector shaped e-beam ) is a good 37.5% (24.0% use energies greater than 50 keV).

The actual production of the masks is similar to photolithography. In the case of conventional chrome masks, a blank (the bare one ) is coated with a thin chrome layer (often by sputter deposition ). The desired structures are created on a coated substrate by removing unneeded chromium.

Depending on which photolithographic process ( contact exposure , projection exposure , etc.) the mask is intended for, the structures are the same size as the structures in the later photoresist layer on the wafer (scale 1: 1, market share approx. 12.6%) or im Scale 4: 1 (approx. 43.6%) or 5: 1 (approx. 41.4%) enlarged than the later target structures on the wafer.

Manufacturers of photo masks include the Japanese companies Dai Nippon Printing (DNP) and Hoya , the American companies Toppan Photomasks (USA) - involved in the Advanced Mask Technology Center (AMTC, Germany) a joint venture with Globalfoundries - and Photronics (USA) as well as Taiwan Mask Corporation (TMC, Taiwan) and Compugraphics (part of the OM Group , Great Britain / USA).

Possible defects

Various defects can occur during manufacture. For example, particles or other shadows can lead to non-closed areas being created as soon as the chrome layer is deposited. Image errors on the mask can also occur during exposure by the electron beam; these can be both additionally opened areas and unopened areas. In addition, electrostatic discharges are a possible source of defects. In this case, an electrical voltage between two non-connected chrome areas is discharged via an arc (in the sub-micrometer range). Similar to arc evaporation, this leads to sputtering effects on the chrome layer and thus to defects. Due to the high costs per mask, smaller defects are laboriously corrected in individual work instead of having to make the mask again. To correct holes, an additional layer of chrome is applied locally and excess material is removed. For this purpose, various processes based on laser vaporization and deposition, focused ion beam sputter etching , ion beam-assisted deposition , micromanipulation using atomic force microscope techniques and electron beam- assisted processes have been developed in recent years .

The most important "defects" when using photo masks are contamination by particles from the air or by abrasion of the handling systems. The latter, however, can be largely controlled by selecting the appropriate material and minimizing the contact area between the systems and the mask. A pellicle is often also used to protect the very expensive mask from airborne contamination . This is a transparent film (made from nitrocellulose , for example ) that is stretched onto a plastic frame. The frame is attached to the side with the textured layer. This is to prevent particles from getting directly onto the structured layer of the mask and thus leading to imaging errors. Contamination of the pellicle, on the other hand, lies clearly outside the focus of the exposure system due to the distance to the structuring layer (usually 3 or 5 mm) and therefore does not disturb the image or only to a much lesser extent. In addition, deposited particles, such as on the unstructured side of the mask, can easily be blown off with nitrogen without damaging the mask. Furthermore, the risk of mechanical damage is reduced and the replacement of the pellicle is much cheaper than cleaning or repairing the mask itself.

Another defect that is becoming increasingly important is the formation of crystals on the mask surface , the so-called haze (English for 'haze', 'cloudiness'). These are, for example, ammonium sulfate crystals that are formed from residues of ammonia water and sulfuric acid (as a source of sulfur ). Both substances are used to clean the masks. In the case of haze, which slowly grows on the photomask during operation, the sulfur mostly comes from the sulfur dioxide present in the ambient air .

Web links

Individual evidence

  1. a b c d e f Y. David Chan: Mask Industry Assessment: 2011 . In: Proceedings of SPIE . tape 8166 , no. 1 , October 6, 2011, p. 81660D-81660D-13 , doi : 10.1117 / 12.897308 .
  2. ^ The OMOG Photomask Family. Toppan Photomasks, accessed October 10, 2012 .
  3. ^ A b Roger H. French, Hoang V. Tran: Immersion Lithography: Photomask and Wafer-Level Materials . In: Annual Review of Materials Research . tape 39 , no. 1 , 2009, p. 93-126 , doi : 10.1146 / annurev-matsci-082908-145350 .
  4. Dietrich Widmann, Hermann Mader, Hans Friedrich: Technology of highly integrated circuits . Springer, 1996, ISBN 978-3-540-59357-7 , pp. 137 .
  5. Harry J. Levinson: Principles of Lithography . 3. Edition. SPIE Press, 2011, ISBN 978-0-8194-8324-9 , pp. 337 .
  6. ^ A b Harry J. Levinson: Principles of Lithography . 3. Edition. SPIE Society of Photo-Optical Instrumentation Engi, 2011, ISBN 978-0-8194-8324-9 , pp. 287-291 .