US 6,653,054 B2  8  7  The combination shown in FIG.  4  is then obtained, which  has a support layer  1  on one surface of which rests a layer  2  of a substrate to be etched. On the layer  2  rest stacks  8  made up of residual islets  7  of the layer  3  resistant to plasma  etching and residual islets  5  of the layer  4.  The combination obtained in this way is then etched to  eliminate the exposed parts of the layer  2  not protected by  the stacks  8.  The combination shown in FIG.  5  is then obtained, which  includes residual islets  9  of the layer  2  that correspond to the  areas not protected by the stacks  8  and represent the pattern  of the mask  5  from FIG.  1.  The presence of the residual islets  5  is dependent on the resistance of the material used to make  the first polymer resin layer  3.  In the negative second development mode the combina› tion shown in FIG.  3  is subjected to dry development, for  example using an oxygen-based plasma.  The combination shown in FIG.  6  is then obtained, which  includes areas  6  that have been subjected to the deep  ultraviolet light and are resistant to plasma etching resting  on a layer  2  of a substrate to be etched in turn resting on a  surface of a support layer  1.  The areas  6  are present because  they are resistant to the oxygen-based plasma when the layer  3  has been exposed. In this embodiment, a material is  preferably chosen for the layers  3  and  4  that has no intrinsic  resistance to an oxygen-based plasma.  The combination produced in this way is then etched to  eliminate the exposed parts of the layer  2  not protected by  the areas  6  of the layer  3,  in the same way as for positive  development.  The combination shown in FIG. 7 is then obtained, which  has stacks  10  consisting of the areas  6  of the layer  3  and the  residual islets  11  of the layer  2  protected by the areas  6  of  the layer  3.  The negative of the pattern of the mask  5  from  FIG.  1  is then obtained.  I claimed:  1.  A semiconductor substrate etching masking layer com› prising an upper layer and a lower layer, wherein the upper  layer comprises a material sensitive to wavelengths of light  less than 100 nm and at least 10 nm, and wherein the upper  layer material is resistant to wavelengths of light from 300  to 100 nm and/or wavelengths of light from 700 nm to 300  nm, and wherein the lower layer comprises a material  resistant to wavelengths of light less than 100 nm and greater  than 10 nm, and wherein the lower layer material is sensitive  to wavelengths of light from 300 to 100 nm and/or wave› lengths of light from 700 nm to 300 nm, and wherein the  lower layer material is resistant to plasma etching when the  lower layer has developed.  2.  The masking layer claimed in claim  1  wherein said  upper layer comprises a material selected from the group  consisting of resins based on nitrocellulose, resins based on  polyphthalaldehyde and mixtures thereof.  3.  The masking layer claimed in claim  1  wherein said  lower layer comprises a material that can be converted into  pseudo-silica.  4.  The masking layer claimed in claim  3  wherein said  lower layer is convertible by treatment with an oxygen› based plasma.  5.  The masking layer claimed in claim  1  wherein said  lower layer comprises an organosilicon material.  6.  The masking layer claimed in claim  5  wherein said  organosilicon material is selected from the group consisting  of polysiloxanes, polysilazanes and mixtures thereof.  7.  The masking layer claimed in claim  6  wherein said  organosilicon material is selected from the group consisting  of polyphenylmethylsilsesquioxane and perhydrosilazane.  8.  The masking layer claimed in claim  1  wherein said  lower layer is deposited from a polymethylsilane plasma.  9. A  process for fabricating a double masking layer for  semiconductor substrate etching comprising coupling a  photo-ablation layer sensitive to extreme ultraviolet light  wavelengths less than 100 nm and at least 10 nm and  resistant to deep ultraviolet light wavelengths from 300 to  100 nm and/or to ultraviolet light wavelengths from 700 to  300 nm to a second polymer resin layer resistant to extreme  ultraviolet light wavelengths less than 100 nm and at least 10  nm, resistant to plasma etching when said second polymer  resin layer has been developed and sensitive to deep ultra› violet light wavelengths from 300 to 100 nm and/or to  ultraviolet light wavelengths from 700 to 300 nm.  10.  The process claimed in claim  9  wherein said photo› ablation layer comprises a material selected from the group  consisting of resins based on nitrocellulose, resins based on  polyphthalaldehyde and mixtures thereof.  11.  The process claimed in claim  9  wherein said second  polymer resin layer comprises a material that can be con› verted into pseudo-silica.  12.  The process claimed in claim  11  wherein said second  polymer resin layer is convertible by treatment with an  oxygen-based plasma.  13.  The process claimed in claim  9  wherein said second  polymer resin layer comprises an organosilicon material.  14.  The process claimed in claim  13  wherein said orga› nosilicon material is selected from the group consisting of  polysiloxanes, polysilazanes and mixtures thereof.  15.  The process claimed in claim  14  wherein said orga› nosilicon material is selected from the group consisting of  polyphenylmethylsilsesquoxane and perhydrosilazane.  16.  The process claimed in claim  9  wherein said second  polymer resin layer is deposited from a polymethylsilane  plasma.  *  *  *  *  * 