2  US 6,653,054  B2  1  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.  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.  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.  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.  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.  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.  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.  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 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.  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.  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  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- BACKGROUND OF THE INVENTION  RESIN, A DOUBLE RESIN LAYER FOR  EXTREME ULTRAVIOLET LIGHT (EUV)  PHOTOLITHOGRAPHY, AND AN EXTREME  ULTRAVIOLET LIGHT (EUV)  PHOTOLITHOGRAPHY PROCESS 