Mask having a tapered profile used during the formation of a semiconductor device

[0002] This is a division of U.S. application Ser. No. 08/650,723 filed May 20, 1996 and issued May 12, 1998 as U.S. Pat. No. 5,750,441.

[0003] This invention relates to the field of semiconductor manufacture, and more particularly to a method and apparatus for forming an opening such as a contact.

[0004] During the manufacture of a semiconductor device a large number of transistors and other structures are formed over a semiconductor substrate assembly such as a semiconductor wafer. As manufacturing techniques improve and transistor density increases as feature size decreases, one manufacturing step which can create difficulties is photolithography.

[0005] A typical photolithography step using positive resist includes the formation of an opening such as a contact to a wafer substrate. To form a contact, a positive photoresist layer is formed over the substrate assembly and the photoresist is exposed in the area where the contact opening is to be formed. The exposed photoresist is removed to expose various underlying layers, then the underlying layers are removed to expose the wafer substrate to which contact is to be made. The opening is typically formed between two structures such as between two adjacent transistor gates. As device density increases the distance between the gates decreases to allow for more transistors per unit area. This distance can decrease only to the limit allowed by photolithography technology, for example allowing for misalignment of the mask or reticle. Photolithography technology is further limited with regard to the minimum size of an opening it can create in the photoresist. Conventional single-layer lithography is capable of resolving line widths less than 0.5 microns on planar, nonreflective substrates. However, when a conventional single layer lithographic technique is used over reflective topography, thickness deviations in the resist lead to poor line width control, and reflections from topographic sidewalls can cause notching. To defeat these problems, dry develop techniques using multiple layers and/or top surface imaging methods have been developed.

[0006] Multilayer lithography techniques include a tri-layer patterning scheme. In this process a thick planarizing layer of novolac resin or some other carbon-based polymer is spun onto the substrate assembly and then baked. Next, an intermediate layer which is resistant to attack in an oxygen plasma (spin-on glass, polysiloxanes, aluminum, silicon nitride, and silicon dioxide, for example) is deposited onto the planarizing layer. A thin layer of resist is applied on top of the intermediate layer to function as an initial imaging layer. The imaging layer is used as a mask to transfer the pattern onto the intermediate layer. Once the pattern is transferred onto the intermediate layer, the intermediate layer is used as the mask for transferring the pattern onto the planarizing layer by way of an oxygen-based plasma. A goal of this process is to form vertical sidewalls such that the opening in the resist is the same at the top as at the bottom.

[0007] A process which extends the useful life of current photolithography equipment and allows for forming smaller, more accurate contacts to an underlying layer would be desirable.

[0008] The present invention provides a new method and apparatus for increasing the accuracy of a contact. One embodiment of the invention comprises the use of a first sloped masking layer which can be a layer of photoresist. The masking layer tapers from top to bottom, being wider at the top, and allows for increased misalignment of a second mask which is used to form the taper in the first mask.

[0009] Using a preferred embodiment, the sloped masking layer is formed by providing a first masking layer such as photoresist over a substrate assembly, then providing a second masking layer over the first layer which can withstand an etch of the first layer. A third patterned masking layer is provided over the second masking layer, and the second layer is patterned. The first layer is then patterned using the second masking layer as a pattern.

[0010] Various objects and advantages will become apparent to those skilled in the art from the following detailed description read in conjunction with the appended claims and the drawings attached hereto.

[0011] FIG. 1 depicts an exemplary in-process wafer assembly having various layers and a patterned material formed thereon, illustrated in vertical section.

[0012] FIG. 2 depicts in verture section the structure of FIG. 1 after an etch of one mask layer.

[0013] FIG. 3 show a cross section of the FIG. 2 structure after an etch of another mask layer to form a tapered hole therein.

[0014] FIG. 4 depicts in vertical section the structure of FIG. 3 after exposing an underlying layer such as a semiconductor substrate.

[0015] It should be emphasized that the drawings herein are not to scale but are merely schematic representations and are not intended to portray the specific parameters or the structural details of the invention, which can be determined by one of skill in the art by examination of the information herein.

[0016] One embodiment of an inventive method for improving the accuracy of a contact to an underlying layer comprises the steps as shown in FIGS. 1-4. As shown in FIG. 1, a substrate assembly 10 is formed. In the example of FIG. 1 the substrate assembly comprises field 12 and gate 14 oxide regions formed over a semiconductor substrate 16 such as a silicon wafer, and transistor gates 18 formed over the substrate. A silicide layer 20 and a tetraethyl orthosilicate layer 22 are also shown, along with a blanket insulation layer 24 such as oxide or nitride formed over the transistor gates. Various other substrate assemblies are likely.

[0017] Subsequently, a blanket first mask layer 26 such as a planar first photoresist layer is formed over the substrate assembly and a blanket second mask layer 28 is formed over the first photoresist layer. The second mask layer is a layer which can withstand an etch of the first mask (i.e. an etch stop layer) such as a low temperature chemical vapor deposited (CVD) oxide or nitride layer although other materials are possible. The second mask is generally formed at a lower temperature than the bake temperature of the first mask layer to ensure the planar first layer is not disturbed.

[0018] Next, a third mask layer 30 such as a planar second photoresist layer is formed over the second mask layer 28. The third mask layer 30 is patterned as shown in FIG. 1, for example using standard photolithography, thereby exposing the second mask layer 28 as shown. If photoresist is used for both the first 26 and third 30 mask layers, the pattern etch of the third mask layer stops on the second mask layer 28. In any case, as shown in FIG. 2, the second mask layer 28 is etched using the third mask layer as a pattern.

[0019] Subsequently, the first photoresist layer 26 is patterned as shown in FIG. 3 using the patterned second mask layer 28 as a pattern. In this embodiment, an anisotropic prograde etch of the first photoresist layer is used and a tapered hole 32 is formed in the first photoresist layer 26. The taper can be smooth as shown, or the slope may be different at various points along the sidewall of the hole. The first photoresist layer can be etched with a dry etch having an etchant such as N₂/O₂, with an optimized nitrogen to oxygen ratio to produce a sloped prograde profile, i.e. a nonvertical slope. For example, using a low pressure inductively coupled plasma system and a 75% /25% ratio of N₂to O₂, a slight slope of 88° was achieved. Increasing the amount of N₂increases the slope . Other possible etchants include O₂/HBr, O₂/SO₂, and O₂/Ar, although other etchants may function sufficiently. The tapered hole has a bottom 34 proximal the underlying layer 16 and a top 36 distal the underlying layer 16. As shown in FIG. 3 the top of the hole is significantly wider than the bottom, for example at least 1.5 times wider than the bottom. The insulation layer 24 is then removed to expose the underlying layer 16 as shown in FIG. 4, for example at active areas of the substrate. As the three mask layers are sacrificial layers, they are removed and wafer processing continues.

[0020] The ratio of the width of the top of the hole to the bottom of the hole (top:bottom) depends on various parameters. For example, the more sloped the etch the greater the ratio. Further, a thicker first photoresist layer will provide an increased top:bottom ratio. A first photoresist thickness of between about 5,000 angstroms (5 K↑) and about 20 K↑ would be sufficient, although other thicknesses could conceivably be used depending on the process.

[0021] The inventive method allows for improved accuracy of contact formation, and also extends the useful life of photolithography equipment. For example, with a top:bottom ratio of 5:1, a 0.5 μm diameter contact to the top resist layer forms a contact with a 0.1 μm diameter at the bottom.

[0022] Any number of top:bottom ratios are possible. A ratio of between about 1.5:1 and about 5:1, for example about 3:1 would be sufficient. With current and near-future processes, a top hole having a width of between about 0.25 microns and about 1.0 microns, and the bottom having a width of between about 0.05 microns and about 0.3 microns may be useful.

[0023] Pressure, temperature, and power are important in determining etch rate, selectivity of resist to oxide, and etch profiles. As pressure increases the etch rate of polymers in oxygen increase due to more reactant being available at higher pressures. To achieve anisotropic etching it is necessary to etch at as low of a pressure as possible. At lower pressure the ions bombarding the substrate suffer fewer collisions in the sheath. This produces a situation where most of the ions arrive at the surface with the full energy associated with the voltage drop from the plasma to the surface. In addition, fewer collisions in the sheath will cause more ions to hit the surface at an angle near 90°. Further, the etch rate of resist increases with temperature. Stripping of resist is commonly carried out at as high of a temperature as possible without decomposing the resist. The dependence of the etch rate on temperature suggests a thermally activated process where, at higher temperatures, the spontaneous chemical etch rate of polymers increases. The higher spontaneous etch rate would coincide with a faster lateral etch rate (undercutting of the masking layer) which would result in critical dimension loss. Finally, the etch rate of polymers increases with power. Higher powers lead to more dissociation and ionization in the plasma. Higher dissociation levels have more atomic oxygen (oxygen radicals) which is the primary species that produces spontaneous chemical etching. Higher ionization levels also lead to an increase in etch rate due to a higher ion flux onto the surface. However, higher powers at the electrode with the wafer lead to lower resist:oxide selectivity, because the higher ion flux and higher ion bombardment energy have a larger effect on the sputtering rate of oxide than on the etch rate of polymer.

[0024] The following table summarizes the ideal settings for pressure, power, and temperature as they relate to etch rates, anisotropic profiles, and resist:oxide selectivity:

[0025] Optimum conditions for anisotropic profiles are low pressure, high power, and low temperature. Unfortunately, low pressure and low temperature produce low etch rates while low pressure and high power decrease resist:oxide selectivity. In fact, no parameter has an ideal setting which satisfies all three criteria of high etch rate, anisotropic profiles, and good resist:oxide selectivity. This indicates that a different strategy which employs a sidewall protection mechanism, may be required. A sidewall protection mechanism could allow higher temperature processing without sacrificing anisotropic profiles.

[0026] Etching resist with O₂and SO₂has been studied (see M. Pons et al., Jpn. J. Appl. Phys. part 1, 33 (2), 991, 1994, and O. Joubert et al. SPIE, vol 1803, 130 (1992) each incorporated herein by reference). It was found that the lateral etch rate of the resist decreased with increasing SO₂, with the lateral etch rate reaching a minimum at 80% SO₂. The lateral etch rate in a 20% O₂/80% SO₂plasma is 15 nanometers/minute (nm/min.), as compared to 110 nm/min. in a pure O₂plasma under the same conditions. The vertical etch rate of the resist also decreases with SO₂addition. Under identical plasma operating conditions, the vertical etch rate in a 20% O₂/80% SO₂plasma is 210 nm/min., as compared to 380 nm/min. in a pure O₂plasma. Fortunately, the decrease in lateral etch rate is more significant than the decrease in vertical etch rate. This produces more anisotropic profiles with better critical dimension control when SO₂is added.

[0027] Further, when ion energy is varied from 20 eV to 120 eV, no lateral etching is observed at ion energies greater than 90 eV. At all energies investigated, the lateral etch rate in O₂/SO₂was at least five times smaller than the lateral etch rate in pure O₂. In this case it appears that there are two factors which control the lateral etch rate, namely passivation of the sidewalls by sulfur and ion energy.

[0028] An in-process semiconductor wafer using an embodiment of the inventive process would comprise a first mask layer, for example the photoresist layer 26 shown in FIG. 4 and a second mask layer, such as the CVD oxide or nitride 28. The first and second mask layers comprise apertures therein which define a single etch opening to a layer underlying the first mask layer, such as layer 16.

[0029] While this invention has been described with reference to illustrative embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as additional embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. For example, any number of top:bottom ratios may prove useful, as well as any sized openings in any of the mask layers. Further, various processes can be used to isotropically and anisotropically etch the different layers. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.