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Evolution Of Photoresist

Evolution Of Photoresist

Mar 15, 2018

 Semiconductor photoresist, as the market demands for miniaturization and functional diversification of semiconductor products, constantly increases the limit resolution by shortening the exposure wavelength, so as to achieve the higher density of integrated circuits. With the improvement of IC integration, the process level of the world integrated circuit has been entered into nanoscale stage from micrometer, submicron, deep submicron level.


  In order to meet the requirements of integrated circuit linewidth narrowing, the wavelength of UV photoresist by wide spectrum to the G line (436nm), I (365nm), KrF (248nm), ArF (193nm), F2 (157nm), the transfer of extreme ultraviolet light in the direction of EUV, and the resolution enhancement technology and constantly improve the photoresist the level of resolution.


  Currently, the main photoresist used in semiconductor market includes four kinds of photoresist, such as G line, I line, KrF and ArF. G and I line photoresist is the most widely used photoresist in the market.

 

  

There is a basic relationship between the parts of the exposure system:

  R is the minimum feature size, that is, the minimum distance that can be resolved. K 1 is a constant, and it is also called the Rayleigh constant. Lambda is the wavelength of the exposure light source, and NA is the numerical aperture of the lens. Therefore, we can see that the way to further reduce the minimum characteristic size is to reduce the wavelength of the light source and to increase the value of the NA.

  

  Development of a multiplication decreases with the wavelength of exposure lithography method machine, using wavelength from UV to DUV, light from a high pressure mercury lamp to excimer laser. The most notable ultra violet EUV photoresist introduced by ASML, using the tin vapor of the plasma as the source of the light source, reduces the wavelength to 13.5nm. But the whole photolithography needs to occur in the vacuum environment, and the production speed is low.


  

 The pursuit of higher resolution exposure sources also makes people think of two kinds of non optical light source X - rays and electron beams. Electron beam lithography is now a mature technology that is used to produce high quality mask and magnifying mask.


  This method is different from traditional lithography lithography. It can be directly written by electronic beam and computer control, and it can achieve 0.25? M resolution now. But this way of production is slower and needs to be achieved in a vacuum.


  X ray wavelength of only 4-50 a, is an ideal light source, but the X rays can penetrate most mask and X X-ray photoresist development is difficult due to its not being used.


  But the NA, people also came up with the method of immersion lithography machine, the medium between the lens and the photoresist is replaced by other substances other than the air and greatly increasing the numerical aperture of NA, makes the lithography resolution without changing the exposure source under the condition of a l L. 193nm technology can meet the requirements of the process node of 45nm, but the process node of 28nm can be reached through the immersion technology.


  The combination of immersion and double exposure can reduce the processing node of 193nm lithography to 22nm level, and the limit of process node reaches 10nm, which makes 193nm lithography still widely used in the market.

 

  

 The application of photoresist has to keep pace with the development of the photolithography machine. With the light exposure lithography the continuous upgrading of photoresist from ultraviolet negative photoresist, cyclized rubber negative glue to replace UV positive photoresist, DNQ-Novolac positive, and then to the deep UV photoresist, chemically amplified photoresist (CAR).


  (1 )UV negative photoresist

  In 1954, EastMan-Kodak synthesized the first photosensitive polymer, polyvinyl alcohol cinnamate, and initiated the polyvinyl alcohol cinnamate and its derivatives photoresist system, which is the first photoresist used in the electronics industry. In 1958, the Kodak company also developed a cyclic rubber - diazide photoresist.

  Because this adhesive has good adhesion on silicon wafer, and has the advantages of fast photosensitivity and strong anti wet etching ability, it became the main adhesive in the electronics industry in the early 1980s, accounting for 90% of the total consumption at that time.

  However, due to its development with organic solvents, the film will expand when developing, which limits the resolution of negative glue, so it is mainly used for the fabrication of discrete devices and 5, m, 2~3 m integrated circuits. But with the continuous improvement of the level of integrated circuits, application of negative glue in integrated circuit has been gradually replaced by the positive, but still have many applications in the field of discrete devices.


(2 )UV positive photoresist

Phenolic resin - around 1950 developed a diazonaphthoquinone positive photoresist with alkaline developer, there is no film swelling problem when developing, so the higher resolution, and resistance to dry etching is strong, so it can meet the production of large-scale integrated circuit and large scale integrated circuit. UV positive photoresist exposure machine according to the different, can be divided into broad spectrum UV positive photoresist (2-3 m, 0.8-1.2 m), G (0.5-0.6 m) positive line, I line (0.35-0.5 m) positive, mainly used in integrated circuit manufacturing and LCD manufacturing.

I line technology has replaced the position of G line photoresist in the middle of the 90s, and is the most widely used photoresist technology at present. With the improvement of I line photoetching machine, I line can also make positive linewidth of integrated circuit 0.25um, extend the service life of I line. In a typical device, the 1/3 layer is the real key layer, the 1/3 layer is the key layer, and the other 1/3 is a non critical layer. There is a mixed matching photolithography method that matches the critical state of the photoresist and device technology with the silicon layer. For example, 0.22um DRAM devices, I line stepper can form a key layer device for a total of 20 layer 13 layer pattern, the remaining 7 layer by deep UV step into the front line scanner imaging, and the use of I can reduce the production cost, so the I photoresist will be long a sustained period of time to occupy a certain market share.


(3) deep UV photoresist deep UV photoresist

Unlike UV photoresists, deep UV photoresists are chemically amplified photoresist (CAR). CAR features: added photoacid in the photoresist, under the light radiation, acid decomposition in acid, baking, acid as catalyst, catalytic film-forming resin (plastic), deprotection of groups or catalytic crosslinking agent and crosslinking reaction of binder resin (negative glue);

Moreover, after removing the protective reaction and cross-linking reaction, the acid can be released again, not consumed, and it can continue to play a catalytic role, greatly reducing the energy required for exposure, thereby greatly improving the photosensitivity of photoresist.

The study of 248nm photoresist with KrF excimer laser as the exposure source originated from 1990 and entered the mature stage in the middle and late 1990s. The most widely used photoacidification agent in CAR is the Weng salt or non-ionic photoacidification agent, which produces sulfonic acid, and the main functional polymer is esterified poly (hydroxystyrene).

 248nm photoresist is combined with KrF excimer laser linewidth of 0.25 m, and the development of 256M DRAM and related logic circuit, by increasing the exposure machine NA and improved matching lithography technology, which has been successfully applied to the linewidth of 0.18~0.15 m, 1G DRAM and related devices. With phase shifting mask, off-axis illumination and proximity correction, 248nm photoresist can produce graphics less than 0.1 M and enter 90nm nodes.  

  These results indicate that the 248nm photoresist technology has entered a mature period.

ArF 193nm far ultraviolet chemically amplified photoresist by photoacid and 248nm far ultraviolet photoresist is roughly the same, but in the functional polymer because of 248nm ultraviolet photoresist with film-forming resin containing benzene, have strong absorption at 193nm and cannot be used in the far ultraviolet 193nm photoresist.

  193nm photoresist resin demand is transparent at the wavelength of 193nm, and has good adhesion with the substrate, the glass transition temperature is higher (General requirements 130-170 C), chemically amplified photoresist imaging must also have acid sensitive pendant groups, in order to improve the imaging ability. Commonly used 193nm photoresist materials can be divided into acrylate, fused ring olefin addition, cyclic olefin maleic anhydride copolymer, silicon containing copolymer, multiple copolymerization system, and small molecular materials.

  At present, 193nm is the mainstream solution for the market, and it is also the most advanced solution before the EUV commercialization.


(4) the next generation of EUV photoresist

The ongoing EUV photolithography needs to match its special photoresist, and the technology of the EUV photolithography has also made a very demanding requirement for the EUV photoresist. EUV photoresist need low light transmittance, high transparency, high etch resistance, high resolution (less than 22nm), high sensitivity, low exposure dose (less than 2 10mJ/cm), high environmental stability, low gas and low line edge roughness (less than 1.5nm).

Because this technology uses only 13.4nm light source, it is required that the high absorption elements (such as F) should be minimized in the main material, and the ratio of C/H will also be increased, which will also help reduce the absorption of materials at 13.5nm. A review of the progress of photoresist mentioned in Beijing molecular science laboratory and CAS chemistry indicates that there are mainly 3 kinds of photoresist systems used in EUV lithography, which are reported in the literature.