Method for fabricating a semiconductor device in which an insulating layer thereof has a uniform thickness

Abstract: A method for fabricating a semiconductor device comprises the steps of forming an insulating layer (23) by the CVD method on a main surface of a semiconductor substrate (21), fluidizing the insulating layer by heat treatment, unifying a thickness of the insulating layer, opening contact holes in desired points of the insulating layer (23), and forming conductor contacts for interconnection on the contact holes. The insulating layer has a uniform thickness in any area on the semiconductor device, so that over-etching of the insulating layer (23) in opening contact holes can be prevented, and the step coverage is well improved.

Description: FIELD OF THE INVENTION The invention relates to a method for fabricating a semiconductor device in which an insulating layer thereof has a uniform depth. BACKGROUND OF THE INVENTION Planar technology to fabricate semiconductor devices is now important for progress in electronics. Such a type of a conventional method for fabricating a semiconductor device comprises the steps of forming an insulating layer on a main surface of a semiconductor substrate, fluidizing the insulating layer by heating to smooth the surface thereof, opening contact holes in desired points of the insulating layer, and forming conductor contacts on the contact holes for interconnection. In the conventional method for fabricating a semiconductor device, however, there is a disadvantage in that a thickness of the insulating layer is not uniform, so that over-etching of the insulating layer can occur in a patterning step of the contact holes, which causes damage to the surface of the semiconductor substrate. Moreover, some of the contact holes are quite deep and have steep side walls in the ununified insulating layer, so that the step coverage is poor and cracks of the conductor contacts can develop. Various methods have been proposed for reducing the variation in thickness of the insulating film, sometimes involving the use of a spin-on layer which is etched-back. See for example Proceedings Fourth International-IEEE VLSI Multilevel Interconnection Conference, 1987 pp364-370, EP-A-0249173, US 4708770, US 4753866 and GB A-2191338. A semiconductor device in which the contact holes have reflowed and rounded shoulders is shown in EP-A-0.060.613. SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide a method of fabricating a semiconductor device in which the depth of an insulating layer of the semiconductor device is uniform. That consequently prevents over-etching of the insulating layer in opening contact holes and improves the step coverage of the conductor contacts formed on the contact holes. In one aspect the invention provides a method of fabricating a semiconductor device comprising the steps of: forming a first reflowable insulating layer by the CVD method on a surface of a semiconductor substrate; reflowing said insulating layer by heating; providing a spin coat layer such that the total depth of the first insulating layer and the spin coat layer is uniform; etching back both layers until said spin coat layer is removed; and depositing a second reflowable insulating layer on the first insulating layer by the CVD method so that the total depth of the first insulating layer and the second insulating layer is uniform; reflowing said first and second insulating layers by heating; opening contact holes in desired points of said insulating layers; performing further heating to round-off the shoulder edges of the contact holes; and forming conductor contacts for interconnection in said contact holes. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained in more detail in accordance with following drawings wherein, Figs. 1 to 3 are cross-sectional views illustrating a semiconductor device during various stages of production in accordance with the conventional method. Figs. 4 to 11 are cross-sectional views illustrating a semiconductor device during various stages of production in accordance with the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Before describing a method for fabricating a semiconductor device according to the invention, a conventional method for fabricating a semiconductor device briefly described before will be explained in conjunction with Figs. 1 to 3. Fig. 1 shows a cross-sectional view of a semiconductor device under fabrication. The semiconductor device in this stage shown in Fig. 1 comprises a semiconductor substrate 11, two diffusion layers 12, 12 which are formed in the vicinity of the surface of the semiconductor substrate 11, a gate insulating layer 14 which is formed on the surface of the semiconductor substrate 11 between the two diffusion layers 12, 12, a gate electrode 15 which is formed on the gate insulating layer 14, and an insulating layer 13 which is covering the surface of the semiconductor device. The insulating layer 13 is formed, for example, by depositing borophosphosilicate glass (BPSG) using the CVD technique. In this stage, the insulating layer 13 has a rough surface and the step coverage is poor, so that it is necessary to smooth the surface thereof. Fig. 2 shows a cross-sectional view of the semiconductor device of Fig. 1 after the insulating layer 13 is fluidized by reflow treatment with heating to smooth the surface thereof. The surface thereof is now smoothed but rising above the gate electrode 15, so that the insulating layer 13 on the gate electrode 15 is thicker than that in other regions. Fig. 3 shows a cross-sectional view of the semiconductor device of Fig. 2 after contact holes 16 are formed in the insulating layer 13 and conductor contacts 17 are formed on the contact holes 16 for interconnection. The contact holes 16 and conductor contacts 17 are patterned by the photolithography technique. The insulating layer 13 on gate electrode 15 is thicker than that in other regions, especially in the diffusion layer 12, as mentioned above. Therefore the condition of the etching time for opening contact holes should be adjusted to a condition in which a contact hole is formed in the thickest (deepest) region of the insulating layer 13, namely that on the gate electrode 15. As a result, the insulating layer 13 in the thin region such as that on the diffusion layer 12 can be over-etched, causing damage to the surface of the device, namely the diffusion layer 12. On the other hand, the contact hole 16 on the gate electrode 15 is deeper than these in other regions and the side wall thereof is steep, causing cracks 17a of the conductor contacts 17 as shown in Fig. 3. Next, preferred embodiments of the invention will be described in detail in conjunction with Figs. 4 to 11. A semiconductor device is provided, which is fabricated from a semiconductor substrate 21 having two diffusion layers 22, 22, and a gate comprising a gate insulating layer 24 and a gate electrode 25 is also formed on the semiconductor substrate 21. Then a first insulating layer 23, which is made of borophosphosilicate glass (BPSG) for example, is formed by the chemical vapor deposition (CVD) method on the surface of the semiconductor device, as shown in Fig. 4. Then, a reflow treatment (fluidizing treatment by heating) of the first insulating layer 23 is done to smooth the surface thereof as shown in Fig. 5. The surface thereof is now smoothed but rising above the gate electrode 25, so that the first insulating layer 23 on the gate electrode 25 is thicker than that in other regions, which is the same as shown in Fig. 2. A spin coat layer 26 is then formed on the surface of the first insulating layer 23 by spin coating. The thickness of the spin coat layer 26 is determined to get a result that the thickness or depth A of the layer on the gate electrode 25 and the thickness or depth B of the layer on other areas of the semiconductor device are equal, as shown in Fig. 6. The thickness of the spin coat layer 26 is controlled by adjusting the viscosity thereof or by multicoating thin layers. The spin coat layer 26 can be a silica film for example. The silica film is formed by spin coating a silica material and burning that. Then, the surface of the semiconductor device is etched back by the reactive ion etching (RIE) method until the spin coat layer 26 is removed completely, and the remaining first insulating layer 23 has a uniform thickness as shown in Fig. 7. As shown in Fig. 8, a second insulating layer 27, which is made of borophosphosilicate glass (BPSG) for example, is formed by the CVD method on the surface of the first insulating layer 23 to thicken the insulating layer, so as to strengthen insulation thereof. Then a reflow treatment is done like the case of the formation of the first insulating layer 23. Now, the double insulating layer made of the first and second insulating layers 23 and 27 has a uniform thickness in any region on the semiconductor device. Then, as shown in Fig. 9, contact holes 28 are formed through a photolithography step. The depths of the contact holes 28 are equal in any area on the semiconductor device. Then, a reflow treatment is done to smooth the edges of the contact holes 28, so as to improve the step coverage, as shown in Fig. 10. Finally, conductor contacts 29 for interconnection are formed on the contact holes 28 through a photolithography step, as shown in Fig. 11. The step coverage is well improved, so that there is no crack of the conductor contacts 29, especially that of the gate region. Various modifications may be made to the described embodiments. For example, the first insulating layer 23 or the second insulating layer 27 can be made of phosphosilicate glass (PSG) instead of BPSG, or other insulator materials can be usable as far as the reflow performance thereof is excellent. Furthermore, it is not necessary that the first and second insulating layers 23 and 27 are made of the same material, therefore the combination of PSG and BPSG can be adopted for insulating layers. Additionally, the spin coat layer 26 can be an organic film such as a resist film instead of a silica film.

Claims: A method of fabricating a semiconductor device comprising the steps of: forming a first reflowable insulating layer (23) by the CVD method on a surface of a semiconductor substrate; reflowing said insulating layer by heating; providing a spin coat layer (26) such that the total depth of the first insulating layer (23) and the spin coat layer is uniform; etching back both layers until said spin coat layer is removed; and depositing a second reflowable insulating layer (27) on the first insulating layer by the CVD method so that the total depth of the first insulating layer and the second insulating layer is uniform; reflowing said first and second insulating layers by heating; opening contact holes (28) in desired points of said insulating layers; performing further heating to round-off the shoulder edges of the contact holes (28); and forming conductor contacts (29) for interconnection in said contact holes. A method according to Claim 1, wherein the spin coat layer (26) is etched back by reactive ion etching. A method according to Claim 1 or Claim 2, wherein said first and second insulating layers (23, 27) are made of the same material. A method according to Claim 1 or Claim 2, wherein said first and second insulating layers (23, 27) are made of different materials. A method according to any of preceding claim, wherein at least one of said first and second insulating layers (23, 27) is made of phosphosilicate glass (PSG). A method according to any of Claims 1 to 4, wherein at least one of said first and second insulating layers (23, 27) is made of borophosphosilicate glass (BPSG). A method according to any preceding claim, wherein said spin coat layer (26) is made of a silica film. A method according to any of Claims 1 to 6, wherein said spin coat layer (26) is made of an organic material.

Inventors: SATO NATSUKI C O NEC CORPORATI, SATO, NATSUKI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: H01L21
IPC codes: H01L21