Dual Glass Contact Process

1. Field of the Invention

This invention relates to the field of forming contacts and contact openings for semiconductor devices.

2. Prior Art

In the processing of semiconductor devices, it is desired to form conductive pads to make contact to the devices. Particularly with transistors, the gate, source and drain must all have contacts. Typically, transistors are three-dimensional with the gate area higher than the surrounding source/drain area. In order to reduce coupling capacitance and prevent shorting of metallurgy to source, drain, and gate regions, the entire device is covered with an insulative dielectric layer. Openings are then formed in this insulative layer over the source of drain and gate and a metal layer is formed therein for contacts.

In the prior art process, the gate, source and drain areas of a field effect transistor are reoxidized. Then a layer of phosphorus doped glass is deposited. The doped glass is subjected to a thermal cycle so that it reflows to smooth corners for good metal coverage. Contacts are then patterned using standard photolithographic techniques. The phosphorus doped glass is wet etched and the thermally grown oxide is anisotropically dry etched. In this prior art process the contacts must be placed sufficiently away from the gate so as to prevent exposing the thermal oxide on a gate to the isotropic wet etch. If the thermal oxide is exposed by the wet etch, it is not sufficient to insulate the gate from the metal.

Therefore, it is an object of the present invention to provide a means of forming a contact opening which results in a gradual slope between elevations.

It is a further object of the present invention to provide a means of forming contacts near gates which still result in contact isolation and without a large increase in the required oxide to silicon etch selectivity.

A dual glass layer is used to provide insulation between a gate of a transistor and a source or drain contact. A layer of borosilicate glass is deposited on the device. Next a layer of borophosphosilicate glass is deposited. A reflow step is performed at approximately 920.degree. C. to smooth the profile of the glass layers. A photoresist layer is applied and openings defined over the contact area. A wet etch is performed which isotropically etches the borophosphosilicate glass and undercuts the photoresist. An anisotropic plasma etch then is used to form an opening through the borosilicate glass to the underlying silicon. The photoresist is stripped and a second reflow step is performed to provide a gradual profile for the contact opening. The result is a contact opening which permits good metal coverage and aggressive design tolerances.

FIG. 1 illustrates a cross-sectional view of a transistor illustrating the gate and drain and having a first dielectric layer formed thereon.

FIG. 2 illustrates the body of FIG. 1 after a second dielectric layer has been formed thereon.

FIG. 3 illustrates the body of FIG. 2 after a reflow step has been performed.

FIG. 4 illustrates the body of FIG. 3 after a photoresist layer has been formed and openings have been defined.

FIG. 5 illustrates the body of FIG. 4 after the second dielectric layer has been etched.

FIG. 6 illustrates the body of FIG. 5 after the first dielectric layer has been etched.

FIG. 7 illustrates the body of FIG. 6 after the remaining photoresist has been stripped and a second reflow step has been taken place.

FIG. 8 illustrates a step in the formation of a contact near a field oxide region.

FIG. 9 illustrates the body of FIG. 8 after the contact opening has been formed.

FIG. 10 illustrates a contact opening formed by prior art methods.

An improved process for forming contacts to semiconductor devices is described. A dual glass layer provides a gradual profile for improved metal coverage while still providing adequate insulating capability. In the following description, numerous specific details are set forth such as etch rates, process temperatures, etc., in order to provide a more thorough understanding of the present invention. It will be obvious however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known steps have not been described in detail in order not to unnecessarily obscure the present invention.

In order to achieve high density in device fabrication, it is necessary to be able to place the contacts close to the edge of the conductors. For example, placing the drain contact of a transistor very near the gate area of the transistor would aid in high density fabrication. In order to achieve this, an insulative layer is formed over the gate and drain area. Openings are formed over the drain area in the insulative layer where a metal layer is deposited to provide a contact. An example of a contact opening formed by prior art methods is illustrated in FIG. 10. A gate 24 is disposed on a silicon substrate 23. A layer of thermally grown oxide 25 covers the gate and silicon substrate. A doped glass layer 26 is deposited on the surface of the device. The doped glass is reflowed to smooth the corners. Contacts are then patterned using standard photolithographic techniques. The doped glass is wet etched and the thermally grown oxide is dry etched. A problem arises if the contact is placed close to the gate because the wet etch exposes the thermally grown oxide. The thermally grown oxide alone is not sufficient to insulate the gate from the metal that is deposited in the contacts. This results in a limitation of the minimum contact to gate space.

In order to overcome the disadvantages of the prior art methods, the present invention utilizes a dual glass process as shown in FIGS. 1 through 7. The device shown includes a polysilicon gate 14, a diffused drain area 11, and a silicon substrate 10. In the preferred embodiment of the present invention, a thin thermal oxide layer 12 is first grown over the gate and drain to provide added isolation between the drain contact and the gate. Next a dielectric layer 13 is formed on the device. This layer must be permeable to hydrogen so that alloying of damage caused by other processing steps can be accomplished. In addition, the layer 13 should have a low dielectric constant near that of the thermal oxide layer. In addition, this first layer 13 should have a high reflow temperature. In the preferred embodiment of the present invention borosilicate glass (BSG) is used. In the present application in the BSG is 4 weight percent boron and is deposited by chemical vapor deposition. This first layer of BSG is approximately 0.4 microns thick.

Next, as shown in FIG. 2, a second dielectric layer 15 is deposited over the first. This layer should have dielectric properties similar to the first layer and also be permeable to hydrogen. However, it is necessary that this dielectric layer 15 have a reflow temperature lower than that of the first dielectric layer 13. In the preferred embodiment of the present invention the second layer 15 is borophosphosilicate glass (BPSG), deposited by chemical vapor deposition. In the present application, the BPSG is 2 weight percent boron and 7 weight percent phosphorus. BPSG has a reflow temperature of approximately 920.degree. C.

Next, a first reflow step is done by heating the device for approximately 10 minutes at 920.degree. C. In the preferred embodiment the heating is done in steam. As a result, the profile of the second dielectric layer 15 is as shown in FIG. 3. A gradual slope between the gate area and the drain area of layer 15 is achieved.

A photoresist layer 16 of FIG. 4 is deposited over the BPSG layer 15. Using standard photolithography techniques, a contact opening 17 is defined in the photoresist layer.

At this point the BPSG layer 15 is etched away at the contact opening as shown in FIG. 5. This etch step should be isotropic so that undercutting of the photoresist layer such as at regions 18 of FIG. 5 results. In the preferred embodiment of the present invention a wet etch using 6 to 1 BHF for 5 minutes is employed. It is necessary that the etching rate of the BPSG layer 15 be faster than the BSG layer 13. This first etching step should not substantially effect the lower BSG layer. In the present invention, BSG has an etch rate of approximately 6 angstroms per second in 6 to 1 BHF while the BPSG has an etch rate of approximately 30 angstroms per second in 6 to 1 BHF (23.degree. C.). Thus the etch rate of the BPSG layer 15 is approximately 5 times the etch rate of the BSG layer 13.

The BSG layer 13 and the thermal oxide layer 12 are then anisotropically etched in a dry (plasma) etch. In this manner, the contact opening 19 of FIG. 6 is in substantial alignment with the photoresist opening 17. It is important that this etch be anisotropic so that aggressive design parameters can be utilized. The thickness of the BSG layer at area 21 (near the corner of the gate) is not substantially effected by this second etch step so that an adequate insulating layer is maintained.

Next, the photoresist layer is stripped away and a second reflow step takes place to provide a better profile for metal coverage. This second reflow step is performed in nitrogen in the preferred embodiment at approximately 920.degree. C. for approximately 10 minutes. This results in the profile as shown in FIG. 7. Because the reflow temperature of the BSG layer 13 is higher than that of BPSG layer 15, there is no substantial thinning of layer 13 at critical area 21. However, at area 20 of layer 15 the second reflow step results in a gradual profile.

Finally, the metal layer is deposited in the contact opening. Good metal coverage is achieved because of the gentle two step contact profile, and adequate insulation between the gate and the metal layer is maintained.

In an alternate embodiment of the present invention, a thermostabilization step is employed following the deposition of the BSG layer 13. In this embodiment, the devices are heated to 920.degree. C. for approximately 5 minutes in order to densify the BSG film and prevent the possible moisture absorption problems. When the thermostabilization step is not employed, the BSG and BPSG layers can be applied in a single sequential step.

The method of forming contacts of the present invention is also advantageously employed when forming contact openings at a junction which is next to field oxide region. An example is shown in FIGS. 8 and 9. In FIG. 8 a BSG layer 13 and BPSG layer 15 have been formed on a silicon substrate which includes the field oxide region 22. The field oxide region includes a "birds beak" region 29, a tapered extension of the oxide along the substrate. A photoresist layer 16 is formed thereon and an opening is defined. After isotropically etching the first BPSG layer 15 so that undercutting of the photoresist occurs, as shown in FIG. 9, the lower BSG layer 13 is anisotropically plasma etched. In the past, etching all the field oxide could result in an electrical short of the underlying substrate to the diffusion. In the prior art this was avoided by disallowing placement of the contact over the field oxide. (The contact opening would extend only to the dotted line of FIG. 9.) As a result of the improved oxide thickness uniformity achieved with the dual glass process less field oxide is removed in the present process, permitting the contact to overlap the field oxide. A reflow step is then employed to ease the profile of the glass layers. Finally, a metal layer is formed in the opening to provide contact to the diffusion 28.

Thus, an improved method for forming contact openings in semi-conductor devices has been described. By using a dual glass layer for insulation, a gently sloping profile is achieved without sacrificing insulating capabilities.