Extreme ultraviolet lithography

EUV lithography ( EUVL for short ) is a photolithography process that uses electromagnetic radiation with a wavelength of 13.5 nm ( 91.82 eV), so-called extreme ultraviolet radiation (english extreme ultraviolet, EUV ). This should make it possible in the future to continue the structural miniaturization in the semiconductor industry to produce smaller, more efficient, faster and cheaper integrated circuits can.

Background and key issues

The EUV lithography can be viewed as a continuation of the optical lithography with smaller wavelengths. The technological leap from the currently used 193-nm exposure systems to 13.5 nm requires the solution of a whole series of technical problems. In February 2008, IBM and AMD presented the first complete exposure of a wafer with EUVL with a real chip in 45 nm technology. Technical details were not disclosed.

It is assumed that the technology from an industry standard throughput for lithography equipment from at least 100 wafers / hour is economical. If the technical challenges can be solved in time, it is expected that the EUV lithography is available only after 2019 and only for feature sizes smaller than 16 nm (16 nm technology ).

Reducing the wavelength brings a variety of challenges and technological changes with it that are far more complex than that of previous reductions in wavelength, eg 248 nm ( KrF excimer laser ) to 193 nm ( ArF excimer laser ). In addition to problems that occur at each wavelength reduction, such as the provision of high-quality and stable sources of radiation sufficient radiant power or a photoresist system that will meet the high demands on the resolution and etch resistance, especially following challenges come in the EUV lithography add new:

EUV lithography systems

An EUV lithography system basically consists of the following elements:

• Radiation source with Debrisschutz and collector
• Imaging optics and mask
• Wafer with photoresist

Radiation source

EUV radiation is released during the production of plasmas. Such plasmas ( LPP, LPP radiation source engl. laser -produced plasma) produced in gases by strong electrical discharges ( engl. gas discharge produced plasma, GDPP ) or by focusing of laser radiation. Depending on the nature of the medium is part of the emitted radiation spectrum in the desired range of 2% bandwidth of the central wavelength of 13.5 nm as the medium was first used xenon, due to a higher conversion efficiency, tin ( qv), was able to prevail. This technique has been in recent years by the three major companies in this field ( Cymer, Philips Extreme UV and Gigaphoton ) continuously developed.

The dose of light incident on the photoresist, has direct influence on the duration of the process, and thus the wafer throughput. According to literature, a radiation power of about 100 W for the first generation of EUV lithography equipment required in the range of 13.5 nm, a sufficiently high and reasonably economical throughput of approximately 60 wafers per hour for 28-nm and below products to ensure. In 2009, Cymer for a system with a radiant power of 70 W.

Debrisschutz

The plasma generated in the radiation source consists of ions and electrons, which move at high speed. To prevent this damage cause to the optics used, such particles are intercepted ( foil traps, buffer gas ), or cleaning procedures are applied (chemical processes or thermal processes ) for affected optics.

Nevertheless, it comes to a continuous contamination of the mirror surfaces, which make the cleaning of the mirror surfaces at a distance of about 100 hours, necessary. These short compared to the 193 nm lithography times lead to further Aufwänden.

Collector

The plasma in the source emits radiation in all directions. Thus, this radiation is usable for an exposure process, must the largest possible part of it is reflected toward the actual lithography system by collecting optics (collector). For sources after GDPP principle adapted Wolter telescopes are used in which the radiation is reflected at grazing angles of incidence. LPP sources are multilayer mirror made ​​of molybdenum and silicon at near normal angles of incidence used.

As optical lithography system interface to an intermediate focus is defined (german intermediate focus, IF), at least 100 W EUV radiation (spectral bandwidth percent 2 ) must be provided.

Imaging optics and mask

Through a complex optical system of six or more mirrors, the radiation exposure for the actual process is prepared. For EUV radiation so-called multilayer mirror come (English multilayer mirror) is used, consisting of a large number ( eg 50 or 100) of molybdenum / silicon layer pairs exist. The possibility of producing these Mo -Si - levels and their high relative reflectivities are one of the reasons for the choice of the wavelength of 13.5 nm of the mirror to aging by diffusion of atoms from one layer to another can be by a number of intermediate layers of only atomic layers thickness can be prevented. The surface of the multilayer mirror is protected by a protective layer (English capping layer). The requirements for flatness - about 2 nm for a mirror with a 30 cm diameter - the mirror substrates and the quality of the layers are enormous and provide technological challenges dar. The theoretically achievable reflectivity of these mirrors is about 72 percent, in a six - mirror system go so that more than 86 percent of the radiation at these levels lost. Therefore, for a sufficiently strong radiation sources must - sources with 100 W radiated power at 13.5 nm are probably needed in the first generation of EUV systems - are available, on the other hand, only a very limited number of optical elements are used. Due to the high radiation performance and high absorption, the system must also be cooled more and yet kept at a constant temperature than conventional systems.

EUV radiation is absorbed primarily at the inner orbitals of atoms. The relatively high absorption of oxygen, argon or nitrogen atoms, in addition, that the radiation is attenuated significantly in this optical system. Therefore must the whole optical system, are from the source to the wafer, at least in a weak vacuum. This increases the technical complexity compared to the current 193-nm lithography system further.

Because no transparent media are available for EUV radiation is available, the lithography mask is also executed as a multilayer mirror, which transmits an image of the structures to be produced in its surface. For this purpose, an absorbent layer of chromium or tantalum nitride is patterned on the mask surface by dry etching. A particular difficulty lies in the defect-free version of the mask as a protective against particles pellicle probably can not be used. Both structuring error in the absorber layer as well as defects in the underlying multilayers can lead to imaging errors. Critical defect sizes are significantly below 30 nm is by a particle below the multilayers changed the flatness of the layers, a phase defect can occur. However, there is a particle in the upper part of the multi- layers, produced by the absorption of the particle, an amplitude defect. Defects in the multilayers can often be detected only under EUV radiation, whereby the mask inspection is very expensive. The defect-free multilayers of the mask is one of the greatest technological challenges of EUV lithography.

Due to the oblique incidence of the EUV radiation (typically 5 ° to the surface normal ) is caused by irregularities in the mask surface, a lateral displacement of the mask image, which leads to positional errors of the imaging pattern on the wafer surface. The mask must therefore have a planarity of less than 50 nm, whereby the manufacture of the mask substrate is very complicated and expensive.

Wafer and photoresist

From the mask, the radiation on a suitable photoresist ( resist) coated wafer is reflected. The chemical and photochemical properties of the photoresist largely determine the quality of the produced structures. What is desired is a high sensitivity for EUV radiation, high resolution and low edge roughness, the 3σ deviation of the predicted line edge, eg, 1.5 nm for the 45 nm technology node. The main challenge is to achieve these properties simultaneously with a photoresist. The photoresist is developed after exposure in a process chain, in order to finally to obtain the desired structures.

In the (conventional) Photolithography typically long chain organic polymers are used as photoresists. By radiation in the so-called photo-acid generator (English photo -acid generator PAG) is released a proton, reacts with the organic protecting groups in the polymer side chains. In this way the solubility of the exposed polymer is increased, so that the exposed areas ( the developer) can be removed by an organic solvent. In this case it is a so-called positive photoresist, since the exposed structures are removed. Alternatively, a negative resist may be used, where by radiation-induced crosslinking of the polymer chains, the solubility of the exposed areas is reduced. These coatings achieve but typically at a lower resolution than the positive resists ..

Due to the high energy of the EUV photons carry per area at only a few photons for exposure. Therefore, a high sensitivity to the paints leads to an increase of edge roughness due to random shot noise effects. At the required edge roughness in the range of one nanometer addition the average lengths of the polymer chains are already achieved so that the molecular structure of the coating materials is a limiting factor for the edge roughness. For this reason, short-chain polymers are studied as EUV coatings. However, this can lead to an increased outgassing in vacuum. Characterized there is a risk that the mirror optical system is contaminated by a carbon layer, and the transmittance of the optical system is significantly reduced.

The absorption of radiation is generally an important issue in EUV lithography. Responsible for this are not only the relatively low radiation powers today's EUV sources and the low absorption cross section, which generally decreases with wavelength, but also the fact that the absorption in this wavelength range, mainly at the inner orbitals of the atoms. Therefore, the absorption depends largely on the elemental composition, and not the molecular weight of the photoresists. To view oxygen and fluorine one of the highest absorption coefficient for EUV radiation. The development of EUV resists is thus relatively complicated because existing chemically amplified, high-resolution photoresists are not really suitable. The relatively high absorption of oxygen, argon or nitrogen atoms also means that the radiation is already weakened significantly in the optical system and therefore probably vacuum must be used (see also Section Imaging optics and mask). At a high EUV absorption of the photoresists, the paint layers must be thinner than about 100 nm. This requirement for the structuring of the wafer is a major challenge, as well as the thickness of the paint layer is reduced during dry etching of the wafer. The etch resistance of the resist developed therefore also plays an important role. Must therefore be used under certain circumstances multi-layer photoresists for patterning depending on the application.