TECHNICAL FIELD

[0001 ] This disclosure relates generally to methods for processing semiconductors and more particularly to methods for processing semiconductors using a combination of electron beam and optical lithography.

[0002] As is known in the art, fabrication of semiconductor devices utilizing a

combimtion of el ectron beam (i.e., e-beam) and optical lithography systems requires alignment features that are optimized for both systems and are placed on the substrate wit minimum, placement errors between the two sets of features. More particularly, optical lithography uses alignment marks etched into a semiconductor substrate while electron beam lithography requires metal alignment marks. Further, there is less backscatter of electrons from the semiconductor than from metal. Further, while electron beam lithography is slo wer than optical lithography, electron beam lithography is able to produce smaller features than optical lithography. Another requirement for these features is that they be capable of standing up to subsequent wafer processes. The best method for insuring minimum placement errors is to pattern both sets of al gnment features in one operation,

[0003] Typically the alignment process includes using an optical alignment tool to etch an alignment mark into the surface of the substrate or a zero layer (where Zero layer is the layer patterned on the substrate to serve as the pattern to which every subsequent layer is aligned) during the first patterning step. More particularly, in the formation of FETs in monolithic microwave integrated circuit (MMIC) mesas hi a semiconductor substrate, one technique used for forming lithographic mask alignment marks is to use a zero layer process where the alignment mark would he formed in the surface of the substrate (i.e., the zero layer) prior to forming the MMIC mesa and another technique would be to combine the zero layer alignment marks with, mesa layer to form the alignment marks simultaneously with the formation of the MMIC mesas while still another technique is the use of a zero layer to etch alignment marks into he substrate prior to the ohmic metal layer in a mesa-less process.

[0004] Daring subsequent processing, as for example forming gates in FETs, electron beam tools capable of creating smaller !inewidth features, such as Schottky gate contacts, than optical tools are required. These electron beam tools use electromagnetic alignment techniques and hence require metal alignment marks.

[0005] More particularly, after the optical lithography processing metal alignment features are created on the previously processed water to provide dequate contrast of secondary or hackscattered electrons for the electron beam alignment tool,

[0006] Thus, prior methods of mixing and matching optical, and e-beam lithograp hy systems developed for compound semiconductors have been based on alignment marks for the electron lithography system which, arc generated with She ohmic metal layer.

This layer is itself aligned to an. earlier layer having registration er ors with respect to the initial optical alignment features imparted by the optical lithography step. Typically gold is a preferred metal layer tor the electron beam alignment features because It's high atomic weight..

[0007] The inventors have recognized that for some semiconductor devices, early use of metal layers can pose a contamination risk during subsequent high temperature operations. This is particularly true in fabricating Ga FETS. More particularly, while with GaAs device fabrication the obmic metal provides an. adequate alignment signal due to differences in the metals composition and alloy conditions; the Inventors have recognized that the alloy conditions and composition of the ohmic metal for GaN is different than the ohmic metal on GaAs and that with GaN the alloyed ohmic metal edges are very rough thereby making Its use as an. alignment mark for subsequent electron beam alignment undesirable nature of the post alloyed ohmic metal.

[0008] A typical process used to form. FETs In a semiconductor MMIC would be to form a mesa by patterning a photoresist layer using optical lithography followed by an etching of the mesa pattern and alignment mark patterns in the surface of the substrate. The first mesa etch then also defines alignment arks for optical stepper. Later masks used to form ohmic contacts arid additional alignment marks are aligned to the etched mesas. Then, metal is deposited for alloying and forming the ohmic source ami drain contacts for the FET. The ohmic contact metal is also used to provides alignment marks for subsequent electron beam lithograph to be used in forming the gate contacts; however, they may not be adequate in providing accurate alignment to the mesas alignment mark. In other words, the gate alignment error would equal the sum of any electron, beam alignment error to the ohmic contact alignment mark plus any errors in the ohmic contact alignment mark to the mesa alignment mark,

[0009] More particularly, ohmic metal has been used for optical alignment as well as electron beam alignment, but the issue remains that you now have multiple layers which are aligned to different features, in addition to that the ohmic metal patterning process must be optimized to define the ohmic contact. The alignment marks defined in an ohmic contact are often not the optimum t ickness or surface roughness lor alignment purposes. As noted above, the inventors have recognized that with a Ga ^' N process the ohmic alignment marks are so rough, that they are not useful for e-beam alignment, and another metal layer has had to be added to define the e~heam alignment marks.

[001 Oj The inventors have discovered a level zero metal layer that can withstand high temperature processing, such as used in GaN ohmic metal formation at 850-950 C for 10-30 seconds, without changing its geometrical shape or physical properdes of the metal This level zero metal layer also severs well as alignment mark for both optical lithography and E-beam li hography..

[0011] hi accordance with one embodiment of the disclosure, a method is provided for providing an alignment mark on a semiconductor structure, comprising: using an optical lithography to form a metal alignment mark on a substrate of the structure; using the formed metal alignment mark to form a first feature of a semiconductor device being formed ou the substrate, such formation comprising using optical lithography, and using the formed metal alignment mark to form a second, different feature for the

semiconductor device being formed, on. the substrate, such formation comprising using electron beam lithography. [0012] in one embodiment, the first feature s a metal feature having a material different from the alignment mark metal,

in one embodiment, the first .feature is an olimic contact

In one embodiment, the second feature is a Schottky contact.

in one embodiment, the metal alignment mark is a refractory metal or a refractory metal compound.

[0013] In one embodiment, the refractory metal or refractory metal compound has an atomic weight greater than 60.

In one embodiment., the metal alignment mark is TaN,

In one embodiment, the semiconductor device is a Ga semiconductor device.

In one embodiment, a semiconductor structure is provided having a metal alignment mark on a zero layer of the structure.

[0 1 ] In one embodiment, a semiconductor structure is provided having a

semiconductor device formed therein, such device having an ohmic contact and a Schottky contact formed in upper layers of the structure, such structure having a metal alignment mark on a lower layer of the structure.

[0015] With such method, sim ultaneous patterning of opti cal alignment features and secondary or backscattered electron alignment features is achieved using a reiractory metal nitride having both sufficient atomic weight and thickness to provide a strong

backscattered or secondary alignment signal and sufficient thickness to create a good optica! alignment signal. Edge quality and is im roved over gold features having larger grain size, and the refractory metal nitride is unaffected by subsequent high temperature operations,

[0016] The use a refractor metal or a refractory metal compound of sufficient atomic weight allows optimization of a dedicated alignment layer for optical alignment signal quality while simultaneously optimising the layer for backscattered and secondary electron alignment signal quality. Specifically the use of a tantalum nitride (TaN) film was shown to exhibit superior alignment signal quality in both optical and e-beam alignment systems and. remain stable when subjected to a high temperature ohmic alloy process step,

[0017] In accordance wi h one embodiment, optical lithography is used to form a metal, for example a refractory metal or reiraclory compound metal, such as for example, TaN, alignment mark on. the zero layer (or when the MM1C mesa, is formed) aad then such alignment mark for optical lithography processing up to the source and drain ohmic contacts of _> for example, a PET such as a GaN FET, and. then electron beam lithography is used to form the Schotiky contact gate using the metal alignment mark,

[0018] The details of one or more embodiments of the disclosure are set forth in the accompanying drawings aad d e description below. Other features, objects, and advantages of the disclosure wi!i be apparent from the descripti on and drawings, and from the claims,

DESCRIPTION OF DRAWINGS

[0019] FIGS, LA through IP, are diagrams showing a process for forming a

semiconductor device, here a Field Effect Transistor (FET), according to the disclosure;

[0020] FIG. 2 shows a metal alignment mark on a semiconductor water having in one region thereof an optical rneia! alignment mark portion and in another region thereof an electron beam metal alignment portion;

[0021] FIG. 3 A shows the alignment signal (amplitude vs. distance) for the indicated portion of the optical alignment mark of FIG. 2 when the mark is comprised of ohmic metal;

[0022] FIG, 3B shows the alignment signal (amplitude vs distance) for the indicated portion of the optical alignment m ark of FIG. 2 when the mark Is comprised of tantalum nitride [0023] FIG. 3C shows three variations of the backscattered electron alignment signals (amplitude vs. distance) according to the PRIOR ART taken at three locations on the wafer of when the electron beam alignment mark is comprised of ohmic metal; each backscattered electron alignment signal being a composite si gnal of the X position and Y position scans over the electron beam alignment mark with variation in the shape and the placement of the X and Y scans in each location and from location to location across the wafer being observed;

[0024] F G. 3D shows one example of a backscattered electron alignment signal according to the disclosure when the electron beam alignment mark is comprised of tantalum nitride; the X and Y scans for these marks overlay closely and can not be discerned from one another, only one example being required since all the marks on the wafer produce a backscattered electron alignment signal which, is substantially the same for all scans.

[0025] Like reference symbols in the various drawings indicate like elements,

DETAILED DESCRIPTION

[0026] Referring now to FIGS, LA through IP, a process for forming a semiconductor device, here a Field Effect. Transistor (FET) 10 (FIG. 1 P) is shown. First, an alignment mark pattern. 12 is formed on an optical lithographic reticle 14 (FIG. .1 A). The pattern 12 has two portions: an electron beam alignment pattern portion 12a and an optical alignment pattern portion 12b. A semiconductor substrate 16, here GaN, having a layer 18 of metal deposited on the upper surface of the substrate 16 (i.e., the zero layer of the process) is coated with a layer of photoresist material 20 (FIG, IB). The metal material in layer 18 is a refractory metal or a refractory metal compound, here for example TaN having a thickness of 500 Angstroms. Other materials may be used for metal layer 18, for example, Ha u , Tantalum, Tungsten, Rhenium, or_compoonds thereof such as when combined with nitrogen, or metals having an atomic weight greater than 60.

[0027] Next, the alignment mark pattern 12 on the reticle 14 is transferred to a corresponding pattern in the photoresist material 20 using conventional optical lithography (FIG , 1 ), Next, exposed regions of the metal layer 18 are etched away as shown in FIG, 1C to form a metal alignment mark .1 , It is noted that the metal alignment mark. 1 has two portions: an electron beam metal alignment portion 19a and an optical metal alignment portion 19b, Next, the patterned photoresist material 20 is removed as shown in FIG. ID leaving the electro beam metal alignment portion 19a and an optical metal alignment portion 19bon the surface of the substrate 10, As will be described, the metal alignment mark 1 will he used as a mask alignment mark for both subsequent optical and electron beam lithography. It is noted thai the electron beam metal alignment portion 19a is used for the electron beam lithography and the optical metal alignment portion 19b is used for the optical lithography

[0028] Next, the surface of fee structure shown in FIG. ID is coated with a photoresist layer 20 (FIG. IE). Next, optical lithography equipment, not shown, uses the metal alignment mark portion 19b of the metal alignment mark 19 to pattern a mesa mask (not shown) into the photoresist layer 20 to thereby expose a portion 22 of the surface of the substrate 16 where a semiconductor mesa 24 is to be formed, as shown in FIG. IF. Next, the exposed portion 22 is etched as shown in FIG, 1G, Next the photoresist layer 20 is lifted off removing portions of the metal layer 20 thereon and leaving the metal alignment mark 1 , as shown in FIG. Hi.

[0029] Next, the surface of the structure shown, in FIG. Ill is coated with a phoioresist layer 28 (FIG. 1.1). Next, optical lithography equipment, not shown, uses the metal alignment mark portion 19b of the metal alignment mark. 1 (FIG. 1H) to pattern an ohmic contact mask (not shown) info the photoresist layer 28 to thereby expose portion 30a, 30b of the surface of the mesa 24 of substrate 16 where source and drain metal ohmic contacts are to be formed, as shown in FIG. 1 J. Next, a metal layer 32 is deposed over the patterned photoresist layer 28 and onto the exposed portions 30a, 30b of the surface of the mesa 24 of substrate 16 where source and drain metal ohmic contacts are to be formed, as shown in FIG. 1 . Here, with GaN, the metal layer 32 includes gold, The structure is then processed to alloy the metal 32 in regions 30a, 30b to form source and drain metal ohmic contacts 34S, 34D, as shown in FJG, IK. Here, the GaN ohmic metal formation is at 850-950 C for 10-30 seconds. Next, the patterned photoresist layer 32 Is lifted off along with the portions of the photoresist layer 28 thereon and leav ng the mask alignment mark 19, as shown in FIG, 1 L.

[00301 Next, the surface of the structure shown, in FIG. 1 L is coated with a photoresist layer 38 (FIG, 1M). Next, electron beam lithography equipment, not shown, uses the metal alignment mark portion. 1 a of the metal alignment mark 1.9 (FIG, 1 L) to pattern a gate Schottky contact mask (not shown) into the photoresist layer 23 to thereby expose the portion 40 of the surface of the mesa 24 of substrate 16 where gate Schottky contact is to be formed, as shown in FIG. IN, Next a metal layer 42 is deposed over the patterned photoresist layer 38 aad onto the exposed portion 40 of the surface of the mesa 24 of substrate gate Schottky contact G is to be formed, as shown in FIG. 10, Here the metal layer 42, with GaN, the metal includes gold. The structure is the processed to alloy the metal 42 in regions 40 to form gate Schottky contact G, as shown in FIG. IP. Next, the patterned photoresist layer 42 Is lifted, off removing the portions of the metal layer 42 t ereon leaving the alignment mask 1 for any further alignment required processing (as for example forming other active, passive and transmission line MMIC circuitry, not shown), .as shown In FIG. 1L,

[0031] FIG. 2 shows an optical metal alignment mark portion 19b of the electron beam metal alignment mark portion 1 a. FIG, 3 A shows the gray value as a function of distance across the optical alignment mark of FIG. 2 when the mark is comprised of ohmic metal F G, 3B is gray value as a function of distance across the optical alignment mark of FIG 2 when the mark is comprised of tantaiam nitride. It is noted that the gray value pattern with the zero layer tantalum nitride optical alignment mark is more well defined than the gray value pattern with the ohmic metal optical alignment mark, FIG. 3C shows variations of electron beam alignment signals across a wafer when the electron beam, alignment mark of FIG 2 is comprised of ohmic metal. Fig 3D shows the electron beam alignment signal for the electron beam alignment mark is comprised of tantalum nitride. It is noted that the alignment signal with the zero layer tantalum nitride electron beam, alignment mark is consistent across the wafer, and in the X and Y directions unlike the highly variable ohmic metal electron beam alignment mark. [0032] Ιί should sow be appreciated a method tor forming an dignment mark on a semiconductor structure according to the disclosure includes: using an optica! lithography to form a metal alignment mark OR a substrate of the structure; using the formed metal alignment mark to form a first feature of a semiconductor device being formed on the substrate, such formation comprising using optica! lithography; ami using the formed metal alignment mark to form a second, different feature for the

semiconductor device being formed on the substrate, such formation comprising using electron beam lithography. The method can also includes one or more of the following features: wherein the first feature is a metal feature having a material different from the alignment mark metal; wherein the first feature is an ohmic contact; wherein the second feature is a Schottky contact; wherein the metal alignment mark is a refractory metal or a refractory metal compound; wherein the refractory metal or refractory metal compound has an atomic weight greater than 60; wherein the metal alignment mark is TaN; or wherein the semiconductor device is a Ga semiconductor device,

[0033] It should now also be appreciated a semiconductor structure according to the disclosure includes a m etal, alignment mark on a zero layer of the structure. The semiconductor structure can also include one or more of the following features: wherein the metal alignment mark is a ref actory metal or a refractory metal compound; wherein the refractory metal or refractory metal compound has an atomic weight greater than 60; wherein the metal alignment mark is TaN; wherein the semiconductor structure is GaN; wherein the metal alignment mark is TaN,

[0034] A semiconductor structure can also include a semiconductor device formed therein, such device having an ohmic contact and a. Schottky contact formed in upper layers of the structure, such st ucture having a metal alignment mark of a material different from the ohmic contact material. The semiconductor structure can also include one or more of the following features: wherein the metal alignment mark is a refractory metal or a refractory metal compound; wherein the refractory metal or refractory metal compound has an atomic weight greater than 60; wherein the metal alignment mark is TaN; wherein the semiconductor structure GaN, [0035] A number of embodiments of the disclosure have bean described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, the optical beam mark and the electron beam, mark may have the same patters. Thus, the metal alignment mark 19 ma have a single portion rather than an electron beam metal alignment portion 1 a and an optical metal alignment portion 19b, Accordingly, other embodiments are within Hie scope of the following claims.