MICROELECTRONIC STRUCTURE

Abstract: A microelectronic structure comprising grooves (30) and contact openings (25) within a first layer (5) or a substrate (5). A metal nitride layer (45) covering at least partially the said first layer (5) or the substrate (5) comprising a material selected from the group consisting of TiN and WN. On top of that metal nitride layer (45) a tantalum layer (50) is deposited. Finally, a metal (75) is deposited to completely fill the grooves (30) and the contact openings (25).

Description: Microelectronic Structure Field of the invention The present invention relates to a semiconductor device and in particular to a metal diffusion barrier for electrical interconnections . Background of the invention A semiconductor device commonly comprises a semiconductor substrate with a plurality of active areas on its surface and multiple metalization layers on top of that surface. The metalization layers are mutually separated by dielectric layers made of Si0 2 , Si 3 N 4 , BPSG or other suitable materials and comprise a plurality of conductive tracks on top of the dielectric layers or in grooves which are formed on the surface of the dielectric layers. These conductive tracks define, in conjunction with contact openings through the dielectric layers, the interconnection structure of the semiconductor device. To form such an interconnection structure a dielectric layer is deposited on top of the semiconductor substrate or on top of a metalization layer and subsequently anisotropically etched to form contact openings (vias) which extend through the dielectric layer to the metalization layer or to the substrate. In the next step, the vias are filled with an electrical conductive material such as tungsten or polysilicon. Finally, a metal layer is deposited on top of the dielectric layer and subsequently structured. In another approach to form a interconnection structure grooves are additionally formed in the dielectric layer after the formation of the contact openings. The grooves which are partially in contact with the contact openings define the location of the conductive tracks. Preferably, the grooves and the contact openings are completely filled with a conductive material in one step. The conductive material is subsequently polished back to the top surface of the dielectric layer in order to obtain a plain surface with completely filled grooves. This method is called a dual damascene process . The materials mainly used for the metalization are aluminum, tungsten and polysilicon. However, as the structuring size of the semiconductor device is scaled down to sub-micron dimensions, the electrical resistance of the vias and the conductive tracks increases due to the reduced cross-section of the conductive structures. To overcome this problem, the use of highly conductive materials such as copper (Cu) has been proposed. Unfortunately, copper tends to diffuse into the dielectric layer and the active areas of the semiconductor substrate and therefore has to be completely encapsulated by a metal diffusion barrier. An appropriate material for such a diffusion barrier is for instance tantalum (Ta) (US 5,714,418; US 5,528,599 and US 5,817,572). Due to adhesion problems of tantalum to a variety of dielectric materials such as Si0 2 , an additional tantalum nitride layer (TaN) between the Ta and the dielectric layer has been suggested (EP 0 751 566 A2) . Since TaN can only be deposited by a PVD process it is difficult to obtain a thin and highly conformal layer, which is very important for reliable interconnections of sub-micron size. Summary of the invention The present invention describes a microelectronic structure comprising: a first layer; - a metal nitride layer covering at least partially the said first layer, the said metal nitride layer comprises a material selected from the group consisting of TiN and WN; a tantalum layer on top of the said metal nitride layer; and a metal layer on top of the said tantalum layer. Using titanium nitride (TiN) or tungsten nitride (WN) instead of tantalum nitride as the adhesion layer, it is possible to get a very thin and uniform layer. Moreover, TiN and WN act additionally as a diffusion barrier against the diffusion of Cu. Preferably, TiN or WN is deposited by a highly conformal CVD-process. The invention further provides a microelectronic structure comprisin : a first layer, which covers at least partially a second layer having conductive regions; the said first layer having an upper and a lower surface, the upper surface having grooves formed therein, some of the grooves having openings extending to the said lower surface to expose the said conductive regions of the said second layer; a CVD-deposited metal nitride layer comprises a material selected from the group consisting of TiN and WN, which lines completely the said grooves and the said openings of the said first layer; - a tantalum layer on top of the said metal nitride layer; and a metal layer on top of the said tantalum layer, whereat the said grooves and said openings are substantially filled by the said metal layer. The present invention further provides a method of forming an microelectronic structure having a first layer, the method comprising the steps: conformally depositing a metal nitride layer on the said first layer, the metal nitride layer comprises a material selected from the group consisting of TiN and WN; depositing a tantalum layer on top of the said metal nitride layer; and depositing a metal layer on top of the tantalum layer. Brief description of the invention These and other features, objects and advantages of the present invention will become apparent upon consideration of the following detailed description of the invention when read in conjunction with the drawing in which: Fig. 1 shows a microelectronic structure with a first layer with integrally formed grooves and contact openings. Fig. 2 shows the microelectronic structure after the deposition of an adhesion layer, a tantalum layer and a catalytic layer. Fig. 3 shows the microelectronic structure after the deposition of a metal layer. Fig. 4 shows the microelectronic structure after a polishing step. Fig. 5 shows the resistivity of a tantalum layer deposited using the present invention. Detailed description of the preferred embodiment A method to form a microelectronic structure having copper wiring (conductive tracks) is described. Referring to Fig. 1, an exemplary microelectronic structure is shown having a first layer 5 which covers at least partially a second layer 10. The first layer 5, often known as inter-level dielectrics (ILD) , is made of Si0 2 , BPSG (boron phosphate silicate glass) , TEOS (Tetraethylorthosilan) , Si 3 N 4 or other suitable dielectric materials having a relative low dielectric constant.- It is also preferred to use a combination of the above mentioned materials for layer 5. Further, in another preferred embodiment layer 5 consists of multiple layers which comprise a thin silicon nitride layer underlying a thick siliconoxide layer. If the second layer 10 functioned as a ILD as well, it preferably consists of the same material as the first layer 5. In that case, the second layer 10 has conductive tracks 15 integrally formed on its surface which represent conductive regions 20. In another preferred embodiment, the second layer 10 represents a semiconductor substrate 10 such as monocrystalline silicon having conductive regions 20 in the form of active areas 20. These areas are usually formed by suitable doping of the semiconductor substrate 10. After depositing the first layer 5 on top of the second layer 10 or the semiconductor substrate 10, at least one opening 25 is etched into the first layer 5. This is done using an etching mask, for instance a photolithographically structurable resist, and an etching medium such as CF , CHF 3 and Argon. Subsequently, grooves 30 are formed in the upper surface 35 of the first layer 5 by another etching step using a second etching mask. During that etching care has to be taken not to completely remove the first layer 5 in the regions which are not masked by the etching mask. This can either be done by using an etch stop layer which is located within the first layer 5 or by premature termination of the etching process. Other possibilities to provide a dielectric layer with grooves and contact openings are described in US 5,726,100 and US 5,612,254, which are introduced herewith as reference . The resulting structure is shown in Fig. 1. The opening or the openings 25 extend from the upper surface 35 to the lower surface 40 of the first layer 5, whereas the grooves 30 are only formed on the upper surface 35. Referring now to Fig. 2, which shows the microelectronic structure after the deposition of several layers. In a first step the microelectronic structure is introduced into a suitable chamber to perform a degas-step. This pretreatment is performed at about 350°C and serves as a cleaning step to remove adsorbed contamination. As a further cleaning step an argon sputter process is preferably applied in a separate chamber to remove, for instance, native oxides. Next, the precleaned microelectronic structure is transferred into a CVD-chamber to deposit a metal nitride layer 45, such as a CVD-TiN layer. The TiN-layer is deposited by chemical vapor deposition using TDEAT (Tetrakis-diethylaminotitanium) precursors at a temperature in the range of about 240°C- 400°C, preferably at about 360°C. The resulting TiN-layer has a thickness of about 5-30nm, preferably lOnm. Subsequently, the microelectronic structure with the deposited TiN-layer 45 is transferred to a chamber, capable of performing an ionized physical vapor deposition (I-PVD) . This chamber is kept at a temperature of about 250 °C. The following sputter deposition of a tantalum layer 50 on top of the TiN-layer 45 is preferably performed at a temperature between 200°C and 300°C, preferably at about 250°C. After the deposition the Ta-layer 50 is about 10-50nm, preferably 40nm thick. In a further deposition step, a so called calalytic layer 55 (about 150 nm thick) is deposited on top of the Ta-layer 50 in an additional I-PVD chamber at about 25°C. The catalytic layer 55 substantially consists of copper. Since this layer is deposited at low temperature it has a very fine crystalline structure which is essential for further deposition of copper. It is preferred to perform all the deposition steps in a system comprising at least a CVD-chamber and two I-PVD chambers connected by a suitable device to carry the processed microelectronic structure from one chamber to the other without interrupting the vacuum. If such a system is not available, one has to perform additional degas and preclean steps before the deposition of the tantalum layer and the catalytic layer. By using a TiN-layer 50 deposited by a CVD-process a highly conformal adhesion layer is provided which shows a uniform covering of the upper surface 35 of the first layer 5, the bottom 70 of the openings 25 and the sidewalls 60 and 65 of the grooves 30 and openings 25, respectively. Another advantage of such a deposited TiN-layer is the low contact resistance between the TiN and the Ta, since CVD-TiN enables the formation of the preferred alpha-phase of tantalum, which has been identified as an important prerequisite for the copper deposition (EP 0 751 566 A2) . The double layer system formed by using the present invention comprises the CVD-TiN and the Ta layer which both materials act as metal diffusion barrier. Moreover, CVD-TiN has a very fine crystalline, partially amorph structure which allows a very conformal covering of fine structures (for instance openings down to 0.2 micron and less) and which supports the formation of the alpha-phase of tantalum. If tungstens nitride (WN) is used instead of TiN, similar results are obtained. Referring now to Fig. 3 which shows the microelectronic structure after the deposition of copper as a metal layer 75 by electroplating. Instead of copper aluminum or a copper- alloy can also be deposited. During that deposition the openings 25 and the grooves 30 are substantially filled with copper. Finally, the metal layer 75 including the TiN-layer 45 and the tantalum layer 50 are etched back preferably by chemical mechanical polishing. The resulting structure is shown in Fig. 4. The back etched copper 75, buried in the grooves 30 and the contact openings 25, serves as conductive tracks have a low resistance and a good electrical contact to the conductive regions 20 of the second layer 10. It is important to note that the copper 75 should be completely separated from the first layer 5 by the TiN-layer 45 and Ta-layer 50 to prevent any contact between copper and the dielectric. Measurements of the electrical resistance have shown that the tantalum layer 50 growing on the TiN-layer 45 deposited by chemical vapor deposition exhibits the alpha phase. This result can be verified by measuring the resistivity of the tantalum layer 50. As shown in Fig. 5 the resistivity measured as described in Sze, "Physics of Semiconductor Devices" 1981, John Wiley & Sons, 2 nd edition pp.31-32 does not exceed 8Ω/sq which is characteristic for the tantalum alpha-phase. Fig. 5 further reveals that back-etching of CVD- TiN prior to the tantalum deposition does not lead significantly to a change of the tantalum layer resistivity. The described measurements were carried out using a lOnm thick CVD-TiN layer deposited at about 340°C and 60 torr underlying a 40nm thick tantalum layer. The step coverage of both layers are very high.

Claims: Claims 1. A microelectronic structure comprising: a first layer; - a metal nitride layer covering at least partially the said first layer, the said metal nitride layer comprises a material selected from the group consisting of TiN and WN; a tantalum layer on top of the said metal nitride layer; and - a metal layer on top of the said tantalum layer. 2. The microelectronic structure of claim 1 wherein the said metal nitride layer is a CVD-deposited metal nitride layer. 3. The microelectronic structure of claim 1 wherein the said metal layer is completely separated from the said first layer by the said metal nitride layer and the said tantalum layer. 4. The microelectronic structure of claim 1 wherein the said metal nitride layer is 5-30nm thick. 5. The microelectronic structure of claim 1 wherein the said tantalum layer is about 10-50nm thick. 6. The microelectronic structure of claim 1 wherein the said metal layer comprises a material selected from the group consisting of Cu, Al or a Cu-alloy. 7. The microelectronic structure of claim 1 wherein the said first layer having an upper surface with grooves formed therein, the said grooves are lined by the said metal nitride layer and the said tantalum layer and are substantially filled by the said metal layer. 8. The microelectronic structure of claim 1 wherein a second layer having conductive regions is covered at least partially by the said first layer, the first layer having an upper and a lower surface and openings extending from the said upper to the said lower surface to expose the said conductive regions of the said second layer, the openings are substantially filled by the said metal layer. 9. A microelectronic structure comprising: a first layer, which covers at least partially a second layer having conductive regions; the said first layer having an upper and a lower surface, the upper surface having grooves formed therein, some of the grooves having openings extending to the said lower surface to expose the said conductive regions of the said second layer; a CVD-deposited metal nitride layer comprises a material selected from the group consisting of TiN and WN, which lines completely the said grooves and the said openings of the said first layer; a tantalum layer on top of the said metal nitride layer; and - a metal layer on top of the said tantalum layer, whereat the said grooves and said openings are substantially filled by the said metal layer. 10. The microelectronic structure of claim 9 wherein the said metal nitride layer is about 5-30nm thick. 11. The microelectronic structure of claim 9 wherein the said tantalum layer is about 10-50nm thick. 12. The microelectronic structure of claim 9 wherein the said metal layer comprises a material selected from the group consisting of Cu, Al or a Cu-alloy. 13. A method of forming an microelectronic structure having a first layer, the method comprising the steps of : conformally depositing a metal nitride layer on the said first layer, the metal nitride layer comprises a material selected from the group consisting of TiN and WN; depositing a tantalum layer on top of the said metal nitride layer; and depositing a metal layer on top of the tantalum layer. 14. The method of claim 13 wherein the said metal nitride layer is about 5-30nm thick. 15. The method of claim 13 wherein the said metal nitride layer is deposited at about 240-400°C. 16. The method of claim 13 wherein the said tantalum layer is deposited at about 200-300°C. 17. The method of claim 13 wherein the said tantalum layer is about 10-50nm thick.

Inventors: HOINKIS MARK, RUF ALEXANDER, SCHMIDBAUER SVEN, SCHNABEL RAINER FLORIAN, WEBER STEFAN, HOINKIS, MARK, RUF, ALEXANDER, SCHMIDBAUER, SVEN, SCHNABEL, RAINER, FLORIAN, WEBER, STEFAN
Applicants: INFINEON TECHNOLOGIES AG
Application year: 2000
Main classification: H01L23
IPC codes: C23C16, H01L21, H01L23