Method to produce masking

Abstract: This is a method for masking a structure 12 for patterning micron and submicron features, the method comprises: forming at least one monolayer 32 of adsorbed molecules on the structure; prenucleating portions 46,48 of the adsorbed layer by exposing the portions corresponding to a desired pattern 36 of an energy source 42; and selectively forming build-up layers 66,68 over the prenucleated portions to form a mask over the structure to be patterned. Other methods are also disclosed.

Description: FIELD OF THE INVENTION This invention generally relates to semiconductor devices and in particular for masking methods to form submicron features. BACKGROUND OF THE INVENTION The semiconductor industry is constantly striving to achieve higher density electronic devices. As the industry has moved into micron and submicron sized features to achieve higher densities, the need for improved masking methods to create such minute features has increased. One way to achieve high resolution of a submicron sized feature is to increase the numerical aperture of the optical imaging system used to produce a patterned image of the feature. Unfortunately, increasing the numerical aperture of the imaging system to achieve higher resolution results in a drastically reduced depth of field. Other problems are associated with conventional masking techniques, such as a lack of uniformity of exposure of resist through a thick layer of resist and scattered light within the layer of resist due to reflective metallized surfaces under the resist. These problems tend to compound the loss of resolution problem by creating ill-defined patterns at the onset. From WO 83/04269 a method of growing patterned films on a substrate is known, wherein the desired region of growth is prenucleated using photodissociation of a thin surface layer of absorbed molecules. Then a spatially uniform, highfluence atom source is used for film growth which occurs selectively in the prenucleated region. However, this document teaches maskless film growth. FR-A-2 319 926 discloses a method of forming an etch mask. According to one embodiment thereof a 100 nm layer of PbI 2 is formed on a layer of SiO 2 . Then portions of the PbI 2 layer are removed by irradiation using an argon laser. The resulting structure is covered by a 100 nm layer of Au. The PbI 2 layer is then dissolved, thereby removing the Au on top of it and leaving a pattern of Au as an etch mask on the SiO 2 layer. There is a need for a method for masking a structure for forming high resolution submicron sized features on a semiconductor device. SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a method for masking a structure for patterning micron and submicron features, as defined in claim 1. Preferably the adsorbed layer is removed from the structure, except at prenucleating sites, before the build-up layers are formed. Also, the prenucleated portions and the build-up layers, if not destructive to the performance of the final structure, may be left as a portion of said final structure; the build-up layers may be used to pattern an organic etch resist; the structure may be a single layer substrate or at least one layer over a substrate layer; the energy source may be a radiant energy source or a particle energy source; the prenucleated portions and/or the build-up layers may be formed in a vacuum chamber, may be formed from an energy dissociable gas, and may be metal. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a cross-sectional view of the formation of the adsorbed layer; FIG. 2 is a cross-sectional view of the prenucleation formation step; FIG. 3 is a cross-sectional view of the unpatterned energy exposure; FIG. 4 is a cross-sectional view of using an etchant to transfer the pattern; Corresponding numerals and symbols in the different figures refer to corresponding parts unless otherwise indicated. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, a structure 12 is placed in a pressure chamber 22 which is made air tight by appropriate seals, not shown, and is provided with an inlet 24 and an outlet 26. Inlet 24 and outlet 26 each have a flow valve 28 and 30, respectively, to control access to the chamber 22. The structure 12 may be a single layer of material, or multiple layers of different materials, depending on the application. The point of importance is that there is a need to pattern a portion of the structure 12. A photosensitive gas is pumped into the chamber 22 via inlet 24. A photosensitive gas such as, for example, trimethyl aluminum (Al(CH 3 ) 3 ), will photodissociate (i.e. break up into its more simple constituents upon exposure to a specified wavelength of light) and deposit one or more of the constituents (e.g. aluminum) on the structure 12 surface. The photosensitive gas is pressurized, for example, to 20 torr, until a monolayer 32 of adsorbed gas molecules forms on the structure 12. After the required time, the unadsorbed photosensitive gas is evacuated through outlet 26. The structure 12 is then placed in an imaging system 34 such as, for example, a projection printer, as shown in FIG. 2. The system 34 comprises a chamber 35 for receiving the structure 12 and a patterning mask 36. The mask 36 is removably fixed to chamber 35 above the structure 12 and is provided with cut-outs, for example, cut-outs 38 and 40, corresponding to the desired pattern to be transferred to the structure 12. An energy source 42 is installed into the chamber 35 to provide the energy needed to photodissociate the adsorbed monolayer 32. The energy source 42 is selected to be compatible with the adsorbed layer 32. The energy rays, as indicated by arrows 44, have an energy sufficient to dissociate the gas in the adsorbed layer 32. For example only, a wavelength of approximately 193 nanometers should be sufficient to photodissociate most gases that may be selected, in an optical imaging system. The adsorbed monolayer 32 is exposed to the energy source 42 through cut-outs 38 and 40 in the mask 36. As previously discussed, the energy source 42 causes the adsorbed monolayer 32 to photodissociate and create prenucleation sites 46 and 48 on structure 12. Prenucleation sites 46 and 48 are comprised of one of the simpler constituents of the photosensitive gas of adsorbed layer 32. For example, if the photosensitive gas comprises trimethyl aluminum, the photodissociation will cause the methyl (CH 3 ) to break from the surface of the structure 12 leaving prenucleation sites comprised of aluminum (Al). The adsorbed layer 32 surrounding the prenucleated sites 46 and 48 remain unaffected by the energy source 42 since mask 36 provides exposure only to the prenucleated sites. It may be desirable at this point to remove the remaining non-nucleated regions of the adsorbed layer 32 by any appropriate method, such as heating, dissolving or etching, which will provide a clean surface for subsequent steps. The prenucleation sites are then used to initiate further deposition which joins sites in the region to form a mask of the desired pattern. This can be done by selective deposition (e.g. tungsten from WF 6 by chemical vapor deposition (CVD) assisted by directed energy) nucleated by the prenucleation sites. By this method, build-up layers 66 and 68 may be selectively deposited on the prenucleated sites 46 and 48, this may or may not be done in a vacuum, depending on the process and materials used. Referring to FIG. 3, the structure 12 with the prenucleated sites 46 and 48 ready for build-up may alternately be placed in an unpatterned energy exposure apparatus 50 (as an alternative to the build-up by the selective deposition of tungsten, for example). The apparatus 50 is appropriately sealed, not shown, to provide a vacuum chamber 52 with an inlet 54 and an outlet 56. Inlet 54 and outlet 56 are each provided with flow valves 58 and 60, respectively, to control access to chamber 52. An energy source 62 is installed in the top of the chamber 52 to provide the energy needed to produce photodissociation during the energy exposure. The energy source 62 must be compatible with the photosensitive gas selected for energy exposure. A photosensitive gas is allowed to fill chamber 52 through inlet 54 as indicated by arrow 64. The energy source 62 is activated to produce energy rays as indicated by arrows 65 which pass through the photosensitive gas filling the chamber 52. The energy rays photodissociate the gas into simpler constituents thereof creating condensable and non-condensable atoms. The condensable atoms will selectively nucleate on the prenucleated sites 46 and 48. Therefore, the prenucleated sites 46 and 48 will build up layers of the condensable atoms to form build-up layers 66 and 68. After a predesignated amount of time, the remaining gas is evacuated through outlet 56 as indicated by arrow 70. Thus, if the photosensitive gas were trimethyl aluminum , the energy rays 66 might dissociate the gas into partially dissociated product. One or two of the three methyl molecules would remain as a non-condensable gas, whereas the aluminum atoms and remaining methyl molecules would be condensable. The condensable atoms would collect upon the prenucleated sites 46 and 48 and further dissociate to form the build-up layers 66 and 68. FIG. 4 illustrate the results of the final stages of the preferred embodiment of the present invention. The desired pattern under the build-up layers 66 and 68 may now be transferred to the structure 12 by any appropriate etch process, such as plasma dry etching with a halogen based etchant, to etch the structure 12 that is not under the build-up layers 66 and 68. The resulting products are layered columns comprising a portion of the structure 12, prenucleated sites 46 and 48, respectively, and build-up layers 66 and 68, respectively. Once the plasma etch is completed, the prenucleated sites 46 and 48, and the build-up layers may be removed, for example, by an etching process, or they may be left as they are, depending on the material used and the effect on subsequent processing steps and the completed device. The final result is the desired submicron pattern formed on the structure 12. The method described above may be used in many applications and with many processes and materials. The energy used to produce a patterned image on the structure 12 may encompass a broad spectrum of sources. These include those commonly used, such as the g-line or the i-line from a Hg arc discharge lamp, or, for example, deep ultra-violet light, x-rays, e-beams, or ion sources may be used. Therefore, limitations due to specific energy sources is overcome. The terms photodissociable and photosensitive are used for convenience in the above description, however, the term photo may be replaced with radiant energy or electromagnetic energy, or particle energy, for example, throughout the discussion (e.g. photodissociable may be replaced with radiant energy dissociable). The deposition processes that may be prenucleated to produce a masking layer are also broad in scope. For example, any fluid used for chemical vapor deposition (CVD), photo-activated or plasma-enhanced CVD, or electron cyclotron resonance deposition may be used (e.g. generally metal-organic or metal halides may be used). The material that is deposited may be metal, insulator, or semiconductor. The choice of material is not limited to the low temperature domain of organic material (which is commonly used in conventional masking techniques), but is driven by the application. As a result, the applications for the masking scheme described above are very diverse. For example, it may be used to mask high resolution ion implants, oxidation of silicon, or etching of underlying films. The masking may be used to pattern another masking material and thus be used to indirectly pattern permanent features on the final structure (e.g. to mask an organic etch resist). In Table 1 below, a nonexhaustive list of possible materials which may be used in the aforementioned method is provided. The table contains potential combinations of masking materials (e.g. the Al prenucleated regions and/or build-up layers), the carrier species (e.g. Al(CH 3 ) 3 as a photodissociable gas or WF 6 as an energy dissociable gas), and the etched material (e.g. the top layer of the structure 12, in which the submicron features are to be formed). The table is meant for clarification and example purposes only and should not be considered in a limiting sense. A preferred embodiment has been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different from those described, yet within the scope of the claims. Words of inclusion are to be interpreted as nonexhaustive in considering the scope of the invention. MASKING MATERIAL CARRIER SPECIES ETCHED MATERIAL Al Al(CH 3 ) 3 SiO 2 Al Al(CH 3 ) 3 Si 3 N 4 Al Al(CH 3 ) 3 Si Al Al(CH 3 ) 3 W Pb Pb(CH 3 ) 4 SiO 2 Pb Pb(CH 3 ) 4 Si 3 N 4 Pb Pb(CH 3 ) 4 Si Pb Pb(CH 3 ) 4 W Mo Mo(CO) 6 Al Mo Mo(CO) 6 Si Mo Mo(CO) 6 TiN Mo Mo(CO) 6 Ta 2 O 5 W WF 6 Si W WF 6 SiO 2 W WF 6 Al

Claims: A method for masking a structure (12) for patterning micron and submicron features, said method comprising: a. forming at least one monolayer (32) of adsorbed molecules on said structure (12); b. prenucleating portions (46, 48) of said adsorbed layer (32) by exposing said portions corresponding to a desired pattern of an energy source (42); c. selectively forming build-up layers (66, 68) over said prenucleated portions (46, 48) to form a mask over said structure (12); said mask being formed over said structure (12) for etching, implanting into or oxidizing a surface of said structure (12) in areas not covered by said mask to form patterned features. The method of claim 1, wherein said adsorbed layer (32) is removed from said structure (12), except at prenucleating sites (46, 48), before said build-up layers (66, 68) are formed. The method of claim 1 or claim 2, wherein said prenucleated portions (46, 48) and said build-up layers (66, 68) are not destructive to the performance of the final structure and are left as a portion of said final structure. The method of any preceding claim, wherein said prenucleated portions (46, 48) and said build-up layers (66, 68) are used to pattern an organic etch resist. The method of any preceding claim, wherein said structure (12) is a single layer substrate. The method of any of claims 1 to 4, wherein said structure (12) is at least one layer over a substrate layer. The method of any preceding claim, wherein said energy source (42) is a radiant energy source. The method of any of claims 1 to 6, wherein said energy source (42) is a particle energy source. The method of any preceding claim, wherein said prenucleated portions (46, 48) and/or said build-up layers (66, 68) are formed in a vacuum chamber. The method of any preceding claim, wherein said prenucleated portions (46, 48) and/or said build-up layers (66, 68) are formed from an energy dissociable gas. The method of any preceding claim, wherein said prenucleated portions (46, 48) and/or said build-up layers (66, 68) are metal. A structure formed by the method of any preceding claim.

Inventors: DOUGLAS MONTE A, STOLTZ RICHARD A, DOUGLAS, MONTE A., STOLTZ, RICHARD A.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1992
Main classification: G03F1
IPC codes: C23C16, G03F1, C23F4, G03F7, G03C5, H01L21