# RESIN, A DOUBLE RESIN LAYER FOR EXTREME ULTRAVIOLET LIGHT (EUV) PHOTOLITHOGRAPHY, AND AN EXTREME ULTRAVIOLET LIGHT (EUV) PHOTOLITHOGRAPHY PROCESS 

## BACKGROUND OF THE INVENTION 

1. Field of the Invention 

The present invention relates to photolithography tech› niques and aims to provide resins sensitive to extreme ultraviolet light (EUV) wavelengths for making masks resisting plasma etching and enabling structures with dimen› sions of less than 0.1 µm to be obtained. 

2. Description of the Related Art 

The expression "extreme ultraviolet light" generally refers to light at wavelengths less than 100 nm and in particular at wavelengths of around 11 to 13 nm. 

Light in the range of wavelengths from 300 to 100 nm is generally referred to as deep ultraviolet light. The expression "ultraviolet light" refers to light at wavelengths from 700 to 300 nm. 

The wavelengths routinely used in photolithography are in the deep ultraviolet (DUY) range, are generally of the order of 193 to 248 nm, and enable structures with critical dimensions from 0.18 µm to 0.12 µm to be obtained. 

At these wavelengths some resins already exhibit photo› ablation, given that the radiation energy is of the order of 6.4 e V at a wavelength of 193 nm. At a wavelength of 13 nm, which corresponds to an energy of 92.5 e V, photo-ablation or partial sublimation by depolymerization should be observed in many photolithography mask resins. 

These observations have led to the development of vari› ous compositions of the materials of photolithography masks since it is no longer necessary to add photosensitive substances or acid generators to make them sensitive, the materials being quasi-inherently sensitive to wavelengths from 11 to 13 nm. 

Modern photolithography techniques use a 193 nm (DUY) ArF excimer laser and enable the fabrication of structures with critical dimensions of the order of 0.1 µm using phase-shift masks (PSM). 

Problems with the transparency of the masks already begin to appear at this wavelength, and at shorter wave› lengths other limitations are operative, for example exces› sive absorption of conventional chemical amplification resins, absence of transparency of the silica at wavelengths of less than 160 nm, which imply the use of CaF$_{2}$ , and the necessity to operate in a flow of nitrogen or in a vacuum at wavelengths from 172 nm. 

For the above reasons, and to push back the limitations of deep UV lithography without having recourse to PSM techniques, many experiments have been conducted using 55 extreme ultraviolet light wavelengths of around 11 nm to 13 nm, and research has also been conducted into X-ray beam lithography, electron beam lithography and ion beam lithog› raphy. 

One of the main problems arising from the use of lithog› raphy techniques below 193 nm lies mainly in the design of the lithography tool. In particular, the problem of absorption 60 of light in the optics is often encountered, and the only solution is to use reflective optics (mirrors and mask), rather than refractive optics, and what is more in a vacuum. 65 "Photorepeaters" (die by die image repetition equipment) operating by reflection have already been used at a wave- 

length of 13.4 nm, for example. Also, initial experiments indicate that multilayers of silicon and molybdenum offer reflection ratios of the order of 70% at a target EUV wavelength of 13 nm (92.5 e V). These multilayers have low absorption and refractive indices respectively greater than and less than 1 in the range from 70 to 100 e V that corresponds to wavelengths from 18 to 12 nm. These masks are made by depositing palladium on multilayers of silicon and molybdenum. 

As a general rule, the sources used to produce EUV radiation are either a synchrotron or a plasma generated by firing a pulsed laser onto a metallic (Cu, Au and Sn) target or using xenon "cluster" gas jets. 

With regard to photosensitive resins, given the generally high absorption of the polymers used at short wavelengths, it can be assumed that the sensitivity of photosensitive resins in the EUV will be high, but the depth of penetration very small, which is why the use of top surface imaging (TSI) techniques is generally recommended, such as the silylation technique that consists of diffusing a reactive gas based on disilane, the use of double layers consisting of organosilicon resin and planarizing resin, or the use of very thin sensitive layers or photo-ablation layers. 

As just explained, given the high absorption of the prior art resins at short wavelengths, the sensitivity of polymers to EUV radiation is generally high, but the depth of penetration could prove to be too small to authorize the use of resin layers sufficiently thick to act as an etching mask, regardless of the technique employed. 

In particular, for top surface imaging resins using the silylation technique that involves diffusing a reactive gas based on disilane in the exposed areas, the main limitations concern the non-uniformity of silylation, both locally and globally, due to diffusion of the gas and to swelling of the exposed parts, and an increased sensitivity of the resin to many parameters, such as the working pressure or the wavelength used. 

Top surface imaging resins using the double layer tech› nique with organosilicon resin and planarizing resin appear to be more beneficial than silylation techniques. This is because organosilicon resins have great potential because they can consist of molecules that are either richer in carbon or richer in silica which, after EUV exposure, adopt a positive or negative behavior on development. This positive or negative behavior is obtained by developing the exposed parts which are richer in silica in an acid or basic solution or by developing the unexposed parts in a solvent. Experiments have been conducted at 193 nm on polysiloxanes and polysilazanes to fabricate 0.1 to 0.2 nm structures and have shown a satisfactory sensitivity from 30 to 100 mJ/cm 2 at 193 nm. At 13.4 nm the energy is much higher (92.5 e V) and even leads to excitation of the Si 2p and Si 2s electrons. It has nevertheless become apparent that absorption is still the limiting factor. 

Although promising, this approach remains a top surface imaging technique, given the absorption of light at the surface of the layers, and gives rise to associated problems with lateral diffusion and resistance of the patterns. 

Another approach previously mentioned consists in inves- tigating very thin single layers of resin given the high absorption of EUV in resins such as polysiloxane resins, such as those already mentioned above, or methacrylic esters. 

Recent experiments have used resins based on HEMA (hydroxyethyl methacrylate) methacrylic esters without chemical amplification and show acceptable sensitivity at 193 nm to electron beams and to X-ray beams. 

