GB2170649A

GB2170649A - Sputtered silicon as an anti-reflective coating for metal layer lithography

Info

Publication number: GB2170649A
Application number: GB08522901A
Authority: GB
Inventors: Tom Chi
Original assignee: Intel Corp
Current assignee: Intel Corp
Priority date: 1985-01-18
Filing date: 1985-09-17
Publication date: 1986-08-06
Also published as: CN85107650A; JPS61171131A; GB8522901D0

Abstract

A sputtered layer of silicon 16 as an anti-reflective coating (A.R.C.) is formed on a metal layer 12 on a semiconductor body 11 prior to patterning. The silicon, by absorbing light used to expose a photoresist layer 13, prevents the light from reflecting off the metal layer and undercutting the photoresist layer. The use of sputtered amorphous silicon eliminates the critical processing steps of A.R.C. bake and spray developing required by spin-on A.R.C.s. The process is performed at low temperature and metal and sputtered silicon are etched simultaneously. The metal layer may be aluminium or a refractory metal silicide. <IMAGE>

Description

SPECIFICATION Amorphous silicon as an anti-reflective coating for metal layer lithography BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of antireflective coatings for use in metal layer photolithography in conjunction with semiconductors.

Prior Art In the manufacture of semiconductor devices. it is desired to form conductive paths on the surface of the device. The prior art achieved this by forming a metal layer on the surface of the semiconductor body and patterning it using standard photomasking techniques. One problem with this prior art technique was the high reflectivity of the metal layer. In the patterning process, a layer of photoresist is coated on a metal layer and is then exposed to light to define the pattern of the conductors. The disadvantage of this method is the exposure of more photoresist than is desired. The incident light reflects off the metal layer at angles such that areas of photoresist adjacent to the desired area are exposed. In subsequent resist developing steps, an undercutting of the photoresist takes place, resulting in a wider opening than was desired.

The prior art attempted to solve this problem by the use of anti-reflective coatings (ARC). Materials used for ARCs include polyimide, Si3N4 and polysilicon. These substances all prevent undercutting of the photoresist layer by absorbing the light incident on the device, thus preventing it from reaching the metal layer and being reflected. However, these materials have created other disadvantages in the photolithography process. For example, SisN4 and polysilicon require high temperatures (350-400 C) for deposit, and promote the generation of hillocks in the metal layer. Additionally, the coating formed from these materials does not have a uniform thickness and generates an excess of particles.

Various spin-on polyimide ARCs have been tried, due to their simple application and low temperature processing. However, spin-on ARCs have a critical bake step which must be accurate to plus or minus 2 C, and a critical developing step requiring tighter control of spray developer. Also, the spin-ARC does not work well with all surface topographies.

It is an object of the present invention to provide an effective solution to the abovestated problems through the use of sputtered amorphous silicon as an ARC.

In Alameddine et al. U.S. Patent No.

4,239,810 the sputtering of amorphous silicon on a metal substrate is taught in conjunction with the manufacture of solar cells. However, Alameddine teaches the deposition of an N doped amorphous silicon layer followed by the application of an aluminum and a silicon layer.

There, the amorphous silicon layer is not used as an anti-reflective coating and is not being used to aid the photolithography process.

Higashi, et al. U.S. Patent No. 4,297,392 teaches the use ofa thin film ofamorphous silicon in the manufacture of a thin film photoconductor. Although it is noted that amorphous silicon will absorb light, its use as an anti-reflective coating in the photolithography process is not disclosed.

SUMMARY OF THE INVENTION By the use of thin sputtered silicon as an anti-reflective coating, notching and necking caused by resist undercutting problems in metal layer photolithography are avoided. The formation of thin layers of sputtered silicon (e.g., approximately 50 to 500 Angstroms) prior to the coating of the photoresist layer results in a perfect resist pattern and profile.

Beside being a low temperature process, the process is not sensitive to surface conditions or surface topography. As a further advantage, the sputtered silicon layer and the metal layer can be etched in one step. In the preferred embodiment of the present invention, amorphous silicon is used as the antireflective coating.

BRIEF DESCRIPTION OF THE DRA WINGS Figure 1 is a cross-sectional elevation view of a prior art semiconductor body with a layer of metal deposited and photoresist coated thereon.

Figure 2 is a perspective view of the body of Figure 1 after openings have been formed in the photoresist layer.

Figure 3 is a cross-sectional elevation of a semi-conductor body with a layer of metal and a layer of sputtered silicon deposited thereon.

Figure 4 illustrates the body of Figure 1 with a layer of photoresist coated thereon.

Figure 5 illustrates the body of Figure 4 after openings have been formed in the photoresist layer.

Figure 6 is a perspective view of the body of Figure 5 after the photoresist layer and the sputtered silicon layer have been removed.

DETAILED DESCRIPTION OF THE INVENTION An improvement in the process of metal layer lithography is described for the patterning of conductors on semiconductors which inhibits undercutting of the photoresist layer while maintaining broad process windows. In the following description, numerous specific details are set forth such as layer thickness, etc., in order to provide a thorough understanding of the present invention. It will be obvious to one skilled in the art however, that the invention may be practiced without the specific details. In other instances, well known processing steps have not been described in detail in order not to obscure the present invention in unnecessary detail.

Prior Art Photolithography Figures 1 and 2 show the process of photolithography as practiced in the prior art. In Figure 1, a metal layer 12 is formed on a substrate 11. Next a layer of photoresist 13 is formed on the metal layer. Light is then directed to certain areas of the photoresist layer 13 in order to form a pattern such that certain portions of the metal layer 12 can be selectively etched away leaving the conductor pattern that is desired for that device. However, as shown in Figure 1, the light entering the photoresist 13 strikes the metal layer 12 and is reflected off at angles into te adjoining photoresist 13 and exposes those portions.

After developing, the exposed portions of the photoresist layer 13 are removed, as in Figure 2. It can be seen that the edges of opening 15, instead of being normal to the metal layer 12, are undercut. This is due to the reflection of the incident light off the metal layer 12. When etching of the metal layer 12 is performed, the resulting strip of metal will be notched in the same manner as the photoresist layer 13. Because current density is related to area of the conducting surface, the resulting conductive strip may be subjected to current densities an order of magnitude greater than desired. This could result in poor performance or even failure of the device.

Working of the Present Invention As in the prior art, and as shown in Figure 3, a metal layer 12 is formed on a substrate 11. At this point, the present invention is applied. A thin coating of sputtered amorphous silicon 16 is formed on the metal layer 12. In the preferred embodiment of the present invention, this amorphous silicon layer 16 is approximately 50 to 500 Angstroms thick. This step is performed at room termperature. The temperature is low enough that the formation of hillocks in the metal layer is not promoted.

Hillocks, small bumps in the metal layer, can create stresses in the metal which canlead to cracking or void formation, both adversely affecting performance. Hillock formation is a problem with prior art nitride and polysilicon ARCs.

Next, the photoresist layer 13 of Figure 4 is formed on the silicon layer 16. No additional processing of the sputtered Si layer is required prior to the formation of the photoresist layer 13. Had a spin-#on ARC been employed, a critical bake step performed at 145 i 20C would have been necessary. The present invention has the advantage of fewer processing steps in general and no critical process steps involved in its application. The resist layer 13 is then baked and exposed in the same manner as if there were no ARC layer. As shown in Figure 4, light is directed to those portions of the photoresist layer 13 as will define the metal pattern. The layer of sputtered silicon 16, absorbs the incident light, preventing reflections back into the photoresist layer 13. Therefore, the profile of the pattern is exactly registered with the incident light.

The resist layer 13 is then developed and undergoes a hard bake. Resulting openings 17 and 18 of Figure 5 are defined in the resist layer 13. The developing step is performed exactly as if no sputtered Si layer had been applied, unlike prior art spinon ARCs, which necessitate a critical spray developing step. At this point the silicon 16 and the metal 12 layers are plasma etched simultaneously down to the surface of the substrate 11. The ability to etch both the sputtered Si layer and the metal layer simultaneously is a great advantage. When the metal layer is Aluminum, an oxide may form on its surface when exposed.

This aluminum oxide layer makes the etching process more difficult to control. Since there is no need to remove the Si layer prior to the metal etch, the metal surface remains covered, inhibiting oxide formation. Thus the metal etch can be accomplished with greater control.

Finally, the remaining resist layer 13 is removed and the remaining silicon layer 16 is stripped from the surface of the metal layer 12 of Figure 6, leaving the desired conductor pattern. The application of the present invention results in perfect patterning of the metal layer 12, with no notching of the patterned strips.

Both DC and RF sputter can be used in applying the silicon layer. Because the sputtering process is not sensitive to the surface topography, no metal line notching, bridging or necking is observed. Although the invention has been described in terms of the patterning of a metal layer, it is also effective in the patterning of refractory metal silicides.

Additional advantages of the present invention are improved metal electromigration and a broader processing window for reworks. During reworks, the sputtered Si is not effected by the photoresist stripper to the degree that prior art ARCs are. This makes stripping of the resist layer during reworks less of a critical step.

Thus, an improvement in metal layer photoli- thography has been described which prevents undercutting of the photoresist layer. By providing a sputtered layer of amorphous silicon, undercutting is eliminated and critical processing steps are avoided.

Claims

1. An improvement in the process of forming a patterned conductive layer on semiconductor bodies by photolithography comprising the steps of: a. forming a layer of sputtered silicon on said conductive layer; b. forming a layer of photoresist on said layer of sputtered silicon, said layer of photoresist formed subsequent to the formation of said layer of sputtered silicon; c. forming a pattern in said conductive layer by said photolithography; whereby light incident to said body during said photolithography is inhibited from reflecting off of said conductive layer and exposing undesired portions of said photoresist layer.

2. The improvement as described in claim 1 wherein said silicon layer is approximately 50 to 500 Angstroms in thickness.

3. The improvement as described of claim 1 wherein said silicon layer is amorphous.

4. The improvement as defined in claim 1 wherein DC sputtering is utilized to form said silicon layer.

5. The improvement as defined in claim 1 wherein RF sputtering is used to form said silicon layer.

6. The improvement as defined in claim 1 wherein said conductive layer is metal.

7. The improvement as defined in claim 1 wherein said conductive layer is a refractory metal silicide.

8. An improvement in the process of forming a patterned conductive layer on semiconductor bodies by photolithography substantially as hereinbefore described with reference to the accompanying drawings.