Method for forming contact holes of semiconductor memory device

[0002] The present invention relates to a method for fabricating a semiconductor memory device, and more particularly to a method for forming contact holes of a semiconductor memory device.

[0003] FIG. 1 shows a lay-out of a conventional semiconductor memory device. A word line 100 is arranged perpendicularly to an active region 200. A contact hole 300 for a lower capacitor electrode and a contact hole 400 for a bit line are each formed between word lines. During formation of the contact holes 300 and 400, misalignment can cause deviation of the contact holes from active regions 200. Because of such deviation, a silicon substrate 1 can be over-etched more than necessary during the formation of the contact holes 7 and 7a(i.e., during etching of a very thick interlayer insulating layer 5) as shown by dotted circles A and A′ of FIGS. 2 and 3. If the over-etching reaches a well region (e.g., silicon region) of silicon substrate 1, a lower capacitor electrode or a bit line to be formed by following process steps is short-circuited with the well region. In addition, leakage currents can be increased because of the over-etching.

[0004] Accordingly, a need remains for a method for forming contact holes without causing the over-etching problem.

[0005] Accordingly, it is an object of the invention to provide a method for forming contact holes which can prevent a silicon substrate from being over-etched in order that a capacitor electrode or a bit line is not short-circuited with a well region in the silicon substrate.

[0006] According to an aspect of the present invention, a method for forming a contact hole comprises the steps of etching a silicon substrate using a trench forming mask to form a trench therein; filling the trench with a first insulating layer to form an isolation trench; forming a second insulating layer over the silicon substrate including the isolation trench; forming an etch stopper layer on the second insulating layer; forming a third insulating layer on the etch stopper layer; etching the third insulating layer until the etch stopper layer is exposed, to form an opening; and sequentially etching the etch stopper and second insulating layers within the opening to form a contact hole.

[0007] This invention may be understood and its objects will become apparent to those skilled in the art by reference to the accompanying drawings as follows:

[0008] FIG. 1 shows a schematic layout of a typical semiconductor memory device used to explain the present invention;

[0009] FIG. 2 is a cross-sectional view of the semiconductor memory device taken along the line a-a′ of FIG. 1;

[0010] FIG. 3 is a cross-sectional view of the semiconductor memory device taken along the line b-b′ of FIG. 1; and

[0011] FIGS. 4A through 4J are cross-sectional views showing the process steps for fabricating a semiconductor memory device according to the present invention.

[0012] FIGS. 4A through 4J illustrate the process steps for fabricating a semiconductor memory device by cross-sectional views in accordance with the present invention. In these figures, a reference symbol “B” denotes a cross-sectional view of the semiconductor memory device taken along line b-b′ of FIG. 1. A reference symbol “C” denotes a cross-sectional view of the semiconductor memory device taken along line c-c′ of FIG. 1. The memory device that is completed according to the present invention has a layout substantially similar to the layout of the conventional memory device shown in FIG. 1.

[0013] According to the present invention, during the formation of contact holes, the over-etching of a silicon substrate can be prevented using a two-step etching process. The first step is to etch a thick interlayer insulating layer until a thin etch stopper layer, formed beneath the interlayer insulating layer, is exposed. The second step is to over-etch the thin etch stopper layer.

[0014] Referring to FIG. 4A, a pad oxide layer 12 and a silicon nitride layer (e.g., a layer Si ₃N₄) 14 are sequentially formed on a silicon substrate 10. Then a pattern of an oxide layer 16 is formed on the silicon nitride layer 14. Next an etching process is performed, using the oxide pattern 16 as a mask, to form a trench 17. The pad oxide layer 12 is about 200 Å in thickness, and each of the silicon nitride layer 14 and the oxide pattern 16 is about 1000 Å in thickness.

[0015] As shown in FIG. 4B, an oxide layer having a thickness of about 7000 Å fills the trench 17. Then a CMP (chemical mechanical polishing) process is carried out so that an isolation trench 18 can be formed. During the CMP process, the silicon nitride layer 14 is used as an etch stopper layer. After formation of the trench isolation 18, the silicon nitride layer 14 is stripped as shown in FIG. 4C.

[0016] Subsequently, after removal of the pad oxide layer 12, a gate oxide layer 19 is formed on the silicon substrate 10. Then a gate structure is formed as shown in FIG. 4D. The gate structure includes a gate electrode 20 having a thickness of about 1000 Å and an insulating layer (e.g., silicon nitride layer) 22 formed on the gate electrode 20. Next, an n⁻ type impurity implantation is performed, using the gate structure as a mask, to form an n⁻ type doping region 24 in the silicon substrate 10. A silicon nitride layer having a thickness of about 1000 Å is also formed on the semiconductor and is dry-etched to form a spacer 25 on both the sidewalls of the gate structure.

[0017] Referring to FIG. 4E, a silicon nitride layer 26 having a thickness of about 300 Å is deposited over the silicon substrate including the isolation trench 18 and the gate structure. Here, it is important that the silicon nitride layer 26 is used as an etch stopper layer during formation of contact holes to be formed.

[0018] Next, as shown in FIG. 4F, a thick interlayer insulating layer 28 is formed over the silicon nitride layer 26, and is planarized. The interlayer insulating layer 28 formed thus has an upper surface to be planarized.

[0019] In FIG. 4G, conventional photolithographic and etching process steps are performed to form an opening 30 in the interlayer insulating layer 28. During the etching process of the interlayer insulating layer 28, the silicon nitride layer 26 is used as an etch stopper because the interlayer insulating layer 28 has a relatively high etch selectivity to the silicon nitride layer 26.

[0020] With reference to FIG. 4H, the silicon nitride layer 26 in the opening 30 is removed and then the gate oxide layer 19 therein is over-etched until a surface of the silicon substrate 10 is completely exposed. As a result, a contact hole 30ais formed. Here, the over-etching is performed only for the gate oxide layer 19 having a thickness of about 100 Å. Thus, it is very easy to control the over-etching of the gate oxide layer 19 compared to the relatively thick interlayer insulating layer 28.

[0021] Also, even if the etch selectivity of the silicon nitride layer 26 with respect to the gate oxide layer 19 is 1:1, so as to simplify process steps, the over-etching toward the gate oxide layer 19 can be minimized according to the present invention. So, during the over-etching of the gate oxide layer 19, the over-etching of the silicon substrate may also be minimized.

[0022] Referring to FIG. 41, impurities are injected through the contact hole 30ainto the silicon substrate 10 to form a plug region 24aso as to increase doping concentration of the doping region 24. Next, a polysilicon layer fills the contact hole 30aand is then patterned to form the pad electrode 32.

[0023] Finally, as shown in FIG. 4J, an interlayer insulating layer 34 is formed over the silicon substrate including the pad electrode 32 and then patterned to form a contact hole in the interlayer insulating layer 34. A conductive layer is deposited on the interlayer insulating layer 34, filling up the contact hole. As a result, a bit line 36 is formed.

[0024] In this embodiment, it is illustrated that the pad electrode 32 is formed also in the contact hole at the “C” region. However, a lower capacitor electrode (i.e., a storage electrode of a capacitor) may be formed instead of the pad electrode 32 in the contact hole of the “C” region.

[0025] As described above, according to the method of the present invention, a silicon substrate is prevented from being over-etched by using a two-step etching process, one of which is to etch a thick interlayer insulating layer until a thin etch stopper layer formed beneath the interlayer insulating layer is exposed, and the other of which is to over-etch the thin etch stopper layer. Accordingly, a semiconductor memory device formed according to the present invention can prevent a lower capacitor electrode (i.e., a plate electrode) or a bit line from being short-circuited with a well region of the silicon substrate, thereby reducing leakage currents.