Spin-on Conductor Process for Integrated Circuits

The present invention relates to semiconductor processing and, in particular, to processes which include planarization in producing conductive levels.

Typical manufacturing processes form active devices on a silicon substrate and then interconnect devices using several conductive levels. Insulating layers separate conductive levels, and contact holes (vias) allow interconnection from one conductive level to another.

Methods of forming conductive levels often rely on deposition or photolithographic techniques which suffer when the device surface is uneven. For example, a non-planar wafer surface may prevent a single level from being patterned with a wide range of focus, thereby reducing photolithographic process uniformity and repeatability.

One known technique for the planarization of devices that utilize multi-level metal interconnections involves the deposition of a refractory metal such as tungsten, W, followed by an etchback process to form tungsten "plugs" in contact or via holes.

Although this method has been successfully implemented, it has certain drawbacks that are inherent in the use of a refractory metal. First, a thick deposition of tungsten is required--typically 1.0 to 1.2 times the diameter of the via or contact hole--and a long etch is required to remove the majority of the tungsten from the surface of the wafer. Second, when the tungsten is removed from the surface, the barrier material is exposed at the sides of the hole. If the etch rate for tungsten is less than the etch rate for the barrier material, then the barrier material along the side of the via hole will be removed and leave a void. The third disadvantage arises from the fact that tungsten is deposited by grain growth, and the final tungsten film is composed of individual merged grains. Gaseous tungsten may be unable to diffuse to the bottom of a via before it closes over at the top and forms a seam between the merging wall of tungsten grains.

Other techniques, such as aluminum reflow, have been used to achieve the desired planarization, but these methods also have certain drawbacks. First, aluminum reflow cannot be used if the lower interconnect layer uses aluminum, because reflow requires heating the wafer above the aluminum melting point (>500.degree. C.). Second, the aluminum reflow process is difficult to control and requires very uniform heating and cooling across the wafer within a narrow temperature window. Third, the dielectric film onto which the aluminum is deposited has to be dehydrated by a thermal step prior to deposition. Fourth, a barrier layer (TIN, TIW or W) used to prevent spiking at the junction needs to withstand the high temperatures used in the aluminum reflow deposition. Fifth, the final aluminum thickness--1.0 to 1.5 times the hole diameter--is greater than what is used in a standard process for multilevels of interconnect.

Thus, those working in the art continue their search for alternate methods of device planarization that will eliminate some or all of the limitations of the processes discussed above.

An objective of the present invention is to provide a method of substrate planarization during the formation of multiple conductive levels.

An additional objective of the present invention is to provide a method for applying a conductive material to a substrate wherein the conductive material both provides substrate planarization and becomes a portion of any conductive level formed on the substrate.

A further objective the present invention is to enable the manufacture of monolithic semiconductor devices which utilize the disclosed methods of planarization and interconnection.

A further objective is to provide a fill process which does not form a seam in the middle of a via.

A further objective is to provide a fill process which leaves very little material on the surface of a wafer.

A further objective is to provide a fill process with a coat material that does not need to be etched away prior to aluminum deposition.

A further objective is to provide a fill process with a coat material that may be etched, but with an etch time less than that for tungsten.

In the first of the preferred embodiments, a spin-on conducting material is used to provide partial to complete contact or via fill for integrated circuits having multiple conductive levels. A suspension of a conductive organometallic material in a suitable liquid is applied to the wafer, partially or completely filling the contact or via holes. The wafer is then baked in an elevated temperature, N.sub.2 /NH.sub.3 ambient order to remove the remaining solvent and to decrease the organic content of the spun on material. A higher temperature thermal cure is then done in a furnace for 5 minutes to several hours or a Rapid Thermal Anneal (RTA) system for 5 seconds to several minutes. During the thermal cure, the organic content in the material is lowered which reduces the bulk resistance. The metallic film that remains on the surface of the wafer can be plasma etched away leaving the material behind in the holes. The material remaining in the contact or via holes minimizes the thinning problems associated with the deposition of the next conductive layer into the contact or via holes.

In the second preferred embodiment, a spin-on conductive material is used to provide substrate planarization and as a component layer in a multi-layer conductive level. A completely filling the contact or via holes. The deposition (CVD) of a barrier layer may be needed to prevent the interaction of the spin-on material with the silicon substrate. The barrier material can be Ti, TiN, TiW or W. The wafer is then baked to evaporate the liquid present in the suspension. The wafer is then thermally cured using the same method as described in the previous paragraph. The second conductive layer is then deposited onto the planarized surface. The standard photolithography and plasma etch process can be used to define and etch the conductive layers. The wafer is then etched to remove the exposed layers of conductive material, thereby defining the first conductive level leads. A layer of SiO.sub.2 is deposited on the wafer to provide insulation between the first and second conductive levels. The SiO.sub.2 layer is then planarized using standard existing techniques. The dielectric isolation can be achieved by using other materials besides SiO.sub.2, such as polyimide. The wafer is then patterned and etched to define and open the via holes that will permit connection between the first and second conductive levels. The process sequence used to produce the first conductive level is then repeated to produce a second conductive level. The wafer is then patterned and etched to remove the exposed portion of the conductive level, thereby forming the second conductive level.

A wafer may then be finished with a passivation layer and a thermal sinter prior to pre-assembly test.

The planarization method produces a conductor-filled contact or via hole, and the quality of the metallization is improved with respect to step coverage and resistance to electromigration.

FIG. 1 shows a semiconductor device after deposition of a first conductive level, the deposition and planarization of an overlaying insulating layer, and the opening of via holes to the first conductive level;

FIG. 2 shows the semiconductor device of FIG. 1 after the deposition of the barrier material and the application of three layers of the spin-on material and cure;

FIG. 3 shows the semiconductor device of FIG. 2 after completion of the spin-on material etch;

FIG. 4 shows the semiconductor device of FIG. 3 after the deposition, patterning, and etch of a second conductive level;

FIG. 5 shows a semiconductor device ready for deposition of the first conductive level;

FIG. 6 shows the semiconductor device of FIG. 5 after the application of the first conductive level including deposition of a first barrier metal layer, a layer of the spin-on material and cure, deposition of an aluminum layer, and the deposition of a second barrier metal layer;

FIG. 7 shows the semiconductor device of FIG. 6 after the definition of the first conductive level; and

FIG. 8 shows the semiconductor device of FIG. 7 after deposition and planarization of an interlevel oxide and the deposition of a second conductive level having the same composition as the first conductive level with the exception of the second barrier metal layer.

Although the methods and structures of this invention are explained by showing the evolution of an individual MOS device, it should be understood that the use of this particular structure is illustrative only and is in no way intended to limit the application of the disclosed methods. A wide range of devices and structures can be fabricated using these methods.

It is contemplated that the disclosed methods may be applied in the production of MOS (Metal Oxide Semiconductor), BJT (Bipolar Junction Transistor), and BiCMOS (Bipolar Junction Transistor and Complementary Metal Oxide Semiconductor) devices. It is further contemplated that the disclosed methods could be utilized in other thin film applications.

Intermetal Substrate Planarization

In the first preferred embodiment, a spin-on conducting material is used to provide conductor planarization for an integrated circuit which requires the use of multiple conductive levels. FIG. 1 shows a cross-section of a MOS device, including a substrate 40, source 42, drain 48, gate oxide and gate-poly 44, BPSG 46 and field oxide 50, that has been processed to a point prior to the deposition of a second conductive level. A first conductive level, 10, is already in place forming contacts to the source and drain regions, and a first oxide insulating layer, 11, has been deposited over the first conductive level 10. Via holes, 12, have been opened in the insulating layer to permit connection of the first conductive level to a second conductive level.

Spin-on Material Deposition

As shown in FIG. 2, a barrier layer 56 is applied. Similar to the application of a photoresist layer, a Sol-Gel is (54 of FIG. 2) dispensed onto the wafer and spun to distribute the suspension across the surface of the wafer. (Suitable Sol-Gel suspensions will be discussed in more detail below.) Rotation speed is quickly ramped up to 3000-5000 rpm with a ramp rate on the order of 50,000 rpm/sec. Depending on the materials selected, Sol-Gel may dry on the spinning surface within 20-30 seconds in an ambient atmosphere. The trapped carrier solvent may be removed from the film after the spin operation by heating the wafer on a hot plate or in an oven at a temperature between 100.degree. C. to 350.degree. C., preferably 150.degree.-250.degree. C. and optimally about 200.degree. C., for 5 seconds to several hours. The organics are removed from the film by a thermal NH.sub.3 /N.sub.2 cure in a furnace for 5 minutes to several hours or a RTA system for 5 seconds to several minutes at a temperature between 350.degree.-850.degree. C., optimally about 400.degree. C. for a via or about 800.degree. C. for a contact. During the thermal anneal the organic content in the material is lowered which reduces the bulk resistance.

The application/spin sequence may be repeated as required by the particular device topography, but it is contemplated that a series of 2-5 applications will be sufficient to produce the MOS device structure shown in FIG. 2. To illustrate a multi-application process, three distinct application layers, 13, 14, and 15, are indicated in both the field region of the wafer and in the via hole, 12. The topography of the via hole and the liquid state of the suspension combine to produce thicker application layers in via holes and other surface depressions, thereby planarizing the wafer and producing conductive "plugs" in the via holes 12.

Spin-on Layer Etchback and Plug Formation

After achieving the desired degree of contact or via fill, the wafer is etched to remove a portion of the conductive material. The etch removes the conductive material from the field surface of the wafer and the via hole "plugs" at a similar rate. The etch clears all of the conductive material from the field areas while leaving conductive material in the via hole "plugs," because the conductive material is thicker in the via hole. FIG. 3 shows the MOS device of FIG. 2 after the spin-on layer has been etched to produce conductive "plugs" 16 in the via holes and to remove the spin-on conductive material from the remainder of the wafer surface.

Conductive Level Formation and Passivation

After the spin-on conductive material has been removed from all but the via regions of the wafer, a second conductive layer, typically an aluminum alloy, is deposited on the wafer. This second conductive layer is then patterned and etched to produce the second conductive level leads 17 as shown in FIG. 4. Also shown is a passivation layer, 18.

Multi-layer Conductor Formation

In the second preferred embodiment, a spin-on conducting material is used to planarize the device and serve as a component of a multi-layer conductive level for an integrated circuit having multiple conductive levels. FIG. 5 shows a cross-section of MOS device that has been processed to a point prior to the deposition of a first conductive level. Contact holes 19 opened in the insulating layer 21 permit connection of the first conductive level to the MOS source and drain regions 22. The silicon exposed in the contact regions has also been reacted with a refractory metal to form a silicide layer 20 which facilitates binding of the metal to the source and drain regions. A first layer of a barrier metal 24, typically a 500 .ANG. layer of Titanium Nitride (TiN) is then deposited on the wafer, as shown in FIG. 6.

Spin-on Material Deposition

Similar to the application of a photoresist layer, a Sol-Gel solution is dispensed onto the wafer and spun to distribute the suspension across the surface of the wafer. (Suitable Sol-Gel suspensions will be discussed in more detail below.) Rotation speed is quickly ramped up to 3000-5000 rpm with a ramp rate on the order of 50,000 rpm/sec. Depending on the materials selected, Sol-Gel may dry within 20-30 seconds during the spin operation in an ambient atmosphere. The trapped carrier solvents are removed by baking the films on a hot plate or in an oven at 100.degree. C. to 350.degree. C. (optimally about 200.degree. C.) for 5 seconds to several hours. The film is then cured (organic removed) by a thermal process at between 350.degree. C. and 850.degree. C. in a NH.sub.3 /N.sub.2 ambient which was described earlier. As with the first, it is contemplated that more than one application may be required or desirable.

Completion of First Multi-layer Conductor

The Sol-Gel is followed by a layer of a non-refractory metal, 25, typically 5000 .ANG. layer of an alloy of Al-Cu-Si. These thicknesses will be different for different device geometries. A second layer of a barrier metal 26 is then deposited on the non-refractory metal. This four-layer conductor (barrier, spin-on, refractory, barrier) will be used to produce the first conductive level.

The wafer is then coated with photoresist and patterned to define the first conductive level. The wafer is then etched to remove that portion of conducting the not required for device interconnection. The result of this etch is shown in FIG. 7.

Interlevel Oxide Deposition/Via Pattern and Etch

A layer of an insulating material 30, typically SiO.sub.2, is then deposited on the wafer and planarized to provide dielectric isolation of the first and second conductive levels, as seen from FIG. 8. The wafer is then coated with photoresist and patterned to define the via holes 32 that will allow selective connection of the first and second conductive levels. The wafer is then etched to remove the oxide from the via holes. A 500 .ANG. layer of TiN barrier material 34 is then deposited onto the wafer.

Second Spin-on Material Deposition

After the remaining photoresist is removed, a Sol-Gel material 36 is dispensed onto the wafer and spun to distribute the suspension across the surface of the wafer as described above for other applications. Again, it is contemplated that more than one application may be required or desirable.

Completion of Second Multi-layer Conductor

A second layer of a barrier metal 38, typically a 500 .ANG. layer of TiN is then deposited on the wafer. The barrier metal is followed by a layer of a non-refractory metal 40 typically 5000.ANG. layer of an alloy of Al--Cu--Si. If the particular device requires only two conductive levels, the creation of the second conductive level is complete and a second layer of barrier metal is not deposited.

If, however, there are to be more than two conductive levels, the composition of intermediate layers would be similar to that of the first conductive level in that a third layer of barrier metal would separate the Al-Cu-Si alloy from the spin-on conducting material of the subsequent conductive level. Depending on the type of Sol-Gel material used, there could be an interaction with the aluminum which could cause open interconnect lines or high resistant interconnect lines.

This third conductive level would then be patterned and etched to define leads to the second conductive level leads. If the third conductive level is the last conductive level, a passivation layer may be deposited on the wafer to protect the leads.

Sol-Gel Compositions

The term Sol-Gel is used here broadly to describe inorganic compounds prepared by wet chemical methods. These compounds may be inorganic salts or organometallic compounds. This wet chemical approach allows one to prepare a very pure material at a relative low cost for the application of a thin film to a surface. The compounds may be prepared as described below, suspended in a carrier solvent, and then applied to a substrate for planarization as described above.

The preferred precursors in the Sol-Gel process are metal alkoxides. The alkoxides are easily hydrolyzed and polymerized to form a gel. Particles of the polymerized alkoxides are applied to the substrated.

The elements in the first and second column of the periodic table are often added as salts. The acetates of those elements are used instead of the nitrates because of the fact that nitrates tend to crystallize during the drying operation of the gel and they decompose at high temperatures and present a possible explosion hazard.

The hydrolysis reaction with the metal alkoxides is shown below:

In this reaction, the M is a metallic element and the R is an alkyl (C.sub.n H.sub.2n+1) group. After the hydrolysis the material is condensed by the reaction shown below:

The choice of precursors is important in the preparation of multi-component gels, because the reaction rates of alkoxides vary considerably. Larger alkoxy groups hydrolyze more slowly, which allows the reaction rate of more reactive metals to be slowed down by choosing an alkoxy with a larger R group. The time involved in the reaction is still a concern, because substantial ligand exchange may occur which will change the reaction rate of the process over time.

The structure and density of the polymer that is formed by the hydrolysis and condensation reaction are controlled by the pH of the solution. The base-catalyzed reactions produce a more fully cross-linked polymer while the acid-catalyzed reactions tend toward a less dense, more linear cross-linked polymer.

Sol-Gel SnO

A thin layer of tin oxide is transparent in the visible spectrum range and is conductive. The sheet resistance of such an electrode can be less than 10 ohms per square. The following Sol-Gel process can be used to form a spin-on solution of tin oxide which, in turn, can be used as a material to fill a via hole in the interconnect process.

The hydrolysis reaction with the tin alkoxides, bis(tri-n-butyltin) oxide, is shown below:

A similar hydrolysis reaction can be done using the other tin alkoxides materials as shown in the table below.

Bis(tri-n-butyltin) sulfate

Hexametylditin

Tetraallytin

Tetraethyltin Tetramethyldiacetoxystannoxane

Tetramethyltin

tetra-i-propyltin

Tin (II) acetate

Tin (II) iodide

Tin (II) oxalate

Tin (II) sulfate

Tin (II) tartrate

Tri-n-butyltin deueride

Trimethyltin hydroxide

Triphenyltin hydroxide

Bis(tri-n-butyltin) sulfide oxide

Diallyldibutyltin

Di-n-butyltinbis(2-ethylhexanoate)

Di-n-butyltin dilaurate

Di-n-butyltin sulfide

Dimetyltin oxide

Diphenyltin oxide

Hexa-n-butylditin

Hexaphenylditin

Tetraamyltin

Tetraphenyltin

Terta-n-propyltin

Tin (II) 2-ethylhexanoate

Tin (II) laurate

Tin (II) stearate

Tin (IV) sulfide

Tri-n-butyltin acetate

Tri-n-butyltin hydride

Triphenyltin acetate

Tri-n-propyltin acetate

n-Butyltin hydroxide

Di-n-butyldiphenyltin

Di-n-butyltin diacetate

Di-n-butyltin oxide

Dimethyldiphenyltin

Dimethyltin sulfide

Diphenyltin sulfide

After the hydrolysis reaction the material is condensed by the reaction shown below:

This Sol-Gel solution can be diluted using n-butyl alcohol (or other alcohols) to an appropriate solid content to produce the desired spin-on film thickness.

The iodide and sulfide solutions will have to be cured at a relatively high temperature in the range of 30.degree. C. to 500.degree. C. in order to remove these elements from the final film.

Sol-Gel Solution ITO

The following Sol-Gel process can be used to form an indium tin oxide (ITO) solution. This material is also very conductive (less than 10 ohms per square) and transparent in thin layers. The hydrolysis reaction with the Di-n-butyltin diacetate and triethylindium is shown below:

A similar hydrolysis reaction can be done using the other tin alkoxides or other indium alkoxides materials. The tin alkoxides are shown in the previous table while the indium compounds are shown in the table below.

Cyclopentadienytlindium (I)

Indium acetylacetonate

Indium (I) chloride anhydrous

Indium Chloride tetrahydrate (anhydrous)

Indium fluoride trihydrate

Indium nitrate hydrate (octahydrate)

Indiumphosphide

Indium sulfate hydrate

Indium trifluoroacetate

trifluoroacetylacetonate

Triethylindium

Indium acetate

Indium bromide

Indium chloride anhydrous

Indium (III) fluoride

Indium iodide

Indium perchlorate

Indium sulfate anhydrous

Indium sulfide

Indium

Trimethylindium

After the hydrolysis reaction the material is condensed by the reaction shown below:

This Sol-Gel solution can be diluted using n-butyl alcohol (or other alcohols) to an appropriate solid content to produce the desired spin-on film thickness.

It is not necessary to combine the indium alkoxide with the tin alkoxide in the Sol-Gel solution. Separate Sol-Gel solutions of indium and tin can be made, and then combined into one solution for spin-on application.

Other Sol-Gel Solutions

Barium titanate and cadmium stannate are also conductive oxide materials which can be placed in Sol-Gel solutions.

Titanium Solution

The following example is a titanium solution that can be used in a spin-on via fill process. The following process only involves a basic condensation reaction. The following reaction involves the interaction of Tetrakis(dimethylamino)titanium with titanium hydride in toluene. The byproduct of this reaction is methylamine, which is a flammable gas. This is a heated reaction that must be done in a nitrogen purged vessel because of the methylamine byproduct.

The reaction rate and organotitanium particle size are controlled by the amount of titanium hydride added to the solution and the temperature of the reaction. The Tetrakis(diethylamino)titanium can be used in place of the Tetrakis(dimethylamino) titanium. The byproduct of this reaction would then be ethylamine, a flammable gas byproduct.

Tinitride Solution

Titanium iodide could be substituted in place of the titanium hydride in the above reaction to form a spin-on titanium nitride compound. The by-product of this reaction is methyl iodide, a non-flammable liquid. This is a heated reaction in a nitrogen purged vessel.

The methyl iodide has a boiling point of 42.degree. C. and can be removed from the solution during the reaction as long as the temperature of the reaction exceeds 42.degree. C. The reaction and particle size of the titanium nitride compound is controlled by the amount of titanium iodide added to the solution and by the temperature of the reaction.

Titanium (IV) bromide or titanium (IV) chloride can be used in place of the titanium iodide in the organotitaniumnitride solution. The methyl bromide chloride is a non-flammable liquid with a boiling point of 50.degree. C. which can be removed from the reaction as long as the reaction temperature exceeds 50.degree. C. The methyl chloride is a flammable gas byproduct which can be removed from the Sol-gel during the reaction.

All of these spin-on titanium-solution examples use toluene as a base or carrier solvent which does not participate in the reaction. However, other solvents could be used as long as they do not contain an oxygen atom in their chemical formulation such as hexane or xylene.

For the via spin-on process the lower conductive material that is exposed in a via needs to be titanium or tinitride.

Other Conductive Solutions

Similar processes can be used to form other organometallic solutions for spin-on via fill, conductor or electrode applications. The following list contains some of the metals that could be used to form organometallic solutions: aluminum, palladium, osmium, nickel, niobium, copper, platinum, rubidium, tantalum, neodymium, molybdenum, lanthanum, iridium and gold.