Method of fabricating a shallow trench isolation structure

[0002] 1. Field of the Invention

[0003] The present invention relates to a method for forming a semiconductor device. More particularly, the present invention relates to a method for fabricating a shallow trench isolation (STI) structure.

[0004] 2. Description of the Related Art

[0005] Advances in the production of integrated circuits have led to an increase in the level of integration and the miniaturization of semiconductor devices. As the level of integration increases, both the dimensions of each device and size of the isolating structures between devices are reduced. Consequently, device isolation structures are increasingly harder to form. A device isolation structure such as a field oxide layer formed by local oxidation (LOCOS) is no longer suitable for small dimensional devices due to the intensification of bird's beak encroachment. Therefore, the shallow trench isolation (STI) method has been developed for highly integrated circuits, and, in particular, sub-half micron integrated circuits.

[0006] A typical process for shallow trench isolation fabrication generally includes the following steps. First, a shallow trench is formed in a semiconductor substrate by selective etching. Second, an insulating layer is deposited on the entire surface of the semiconductor substrate to fill the trench. The insulating layer is typically formed of silicon dioxide by chemical vapor deposition (CVD), such as atmospheric pressure chemical vapor deposition (APCVD), sub-atmospheric pressure chemical vapor deposition (SACVD) or high density plasma CVD (HDPCVD). Finally, CMP is used to planarize the insulating layer, thus the insulating layer remaining in the trench serves as a STI region.

[0007] Because of the increasing complexity of electronic devices, the dimensions of semiconductor devices are shrinking, while the width of STI regions is decreasing to 0.11 μm or less, and the aspect ratio of STI regions is increasing beyond 3. Even if a HDPCVD with good filling capability is employed, voids or seams still exist in the STI regions and one-step coverage is hard to achieve. When conductive materials are deposited in subsequent processes, these defects cause short circuits between devices, thus reducing the lifetime of the device.

[0008] The object of the present invention is to solve the above-mentioned problems and to provide a method of fabricating a high aspect ratio shallow trench isolation structure.

[0009] The present invention discloses a method of fabricating a shallow trench isolation structure in a semiconductor substrate, comprising the following steps. A trench is formed on the semiconductor substrate. A liner oxide is formed on the bottom and sidewall of the trench. A liner nitride is formed on the liner oxide. The first oxide layer is deposited in the trench by high density plasma chemical vapor deposition. The first oxide layer is spray-etched to a predetermined depth, wherein the recipe of the spray etching solution is HF/H₂SO₄=0.3˜0.4. A second oxide layer is deposited to fill the trench by high density plasma chemical vapor deposition (HDPCVD).

[0010] In a preferred embodiment, the aspect ratio of the trench is greater than 3. The second oxide layer, i.e. an HDP oxide layer, can be annealed to increase the density thereof after the second oxide layer is deposited and the second oxide layer can be subsequently planarized by chemical mechanical polishing (CMP) to form a well-filled shallow trench isolation structure.

[0011] In another preferred embodiment, the trench can be formed by reactive ion etching (RIE). The liner oxide cab be formed by thermal oxidation and the liner nitride can be formed by low pressure CVD (LPCVD).

[0012] The preferred thickness of the first oxide layer in the trench is about 2000-5000 Å. The preferred recipe of the spray etching solution is HF:H₂SO₄=1:2 and the spray etching is performed for 20-30 seconds. The preferred thickness of the second oxide layer in the trench is about 3000-6000 Å. The first and second oxide layer can be silane based HDP oxide.

[0013] The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention. In the drawings,

[0014] FIGS. 1A˜1G illustrate, in cross section, the fabrication process of a shallow trench isolation structure in one embodiment according to the present invention; and

[0015] FIG. 2 is a drawing showing the etching rate of the first oxide layer in different ratios of HF/H₂SO₄spray etching solution.

[0016] FIGS. 1A˜1G illustrate, in cross section, the fabrication process of a shallow trench isolation structure in one embodiment according to the present invention.

[0017] As shown in FIG. 1, a nitride layer (SiN) 102 is formed by low pressure CVD (LPCVD) over the surface of a semiconductor silicon substrate 100. A photo-resist layer (not shown) is patterned by photolithography to form a certain pattern corresponding to the subsequently formed trench on the nitride layer 102. The nitride layer 102 is etched with the patterned photo-resist layer as a mask to form a patterned nitride 102 as a hard mask. A shallow trench isolation structure 104 is formed on the silicon substrate 100 by reactive ion etching (RIE) with the silica nitride 102 as the hard mask. The depth of the shallow trench isolation structure 104 is about 3000˜5000 Å and the width of the opening there of is about 1000 Å. The preferred aspect ratio (depth/width) of the trench 104 is about or greater than 3.

[0018] The silicon substrate 100 is subjected to thermal oxidation to grow a liner oxide layer (SiO₂) 106 on the bottom and sidewall of the trench 104. The thermal oxidation can be wet thermal oxidation performed in a oxygen-hydrogen-containing atmosphere at 800 to 850° C., or dry thermal oxidation performed in a oxygen-containing atmosphere at 900 to 950° C. for 2 hours. The inner silicon of the trench 104 is oxidized by the thermal oxidation to form the liner oxide 106 with a thickness of 100˜200 Å.

[0019] After the liner oxide layer 106 is grown on the inner of the trench 104, a nitride layer (SiN) 108 is formed over the liner oxide layer 106 in the trench 104 and the nitride layer 102 to protect the profile of the trench 104, as FIG. 1C shows.

[0020] AS shown in FIG. 1D, a first oxide layer 110 is deposited as an insulating layer, i.e. HDP oxide, in silane and oxygen by high density plasma chemical vapor deposition (HDPCVD). The preferred deposition thickness is about 2000˜5000 Å, and a more preferred thickness is about 3000 Å.

[0021] When the aspect ratio of the trench 104 is greater than 3, it is hard to achieve thorough coverage of the trench 104 by one step deposition of the first oxide layer 110. There are usually voids or seams formed on the upper portion of the first oxide layer 110. Therefore, spray etching is performed to eliminate the upper portion of the first oxide layer 110 where voids and seams are usually formed and the lower portion of the first oxide layer is retained, as FIG. 1E shows.

[0022] According to the present invention, the spray etching is performed with the mixture of hydrofluoric acid (HF) and sulfuric acid (H₂SO₄) as the etching solution to etch the first oxide layer 110. A preferred recipe of the spray etching solution is HF/H₂SO₄=0.3˜0.4 which has a high etching selectivity between the first oxide layer 110 and the nitride layer 108.

[0023] FIG. 2 shows the etching rate of the first oxide layer in different ratios of HF/H₂SO₄spray etching solution. When the ratio of the HF/H₂SO₄etching solution is between 0.3˜0.4, the etching rate of silicon oxide (SiO₂) 110 is 1279 Å/min and the etching rate of the nitride layer 108 is only 25 Å/min. Therefore, using the HF/H₂SO₄etching solution with a ratio between 0.3˜0.4, the first oxide layer 110 can be etched rapidly without over-etching the profile of the liner nitride 108 in the trench 104.

[0024] Accordingly, the first oxide layer 110 is spray etched with a HF/H₂SO₄etching solution in the ratio of 0.3˜0.4. In a preferred condition, the first oxide layer 110 is spray etched with a etching solution of HF/H₂SO₄=1:2 for 20˜30 seconds.

[0025] As shown in FIG. 1F, second oxide layer 112 is deposited subsequently to fill the trench 104 by HDPCVD. The second oxide layer 112 can be deposited in silane and oxygen by HDPCVD to form a silane based HDP oxide layer. The preferred deposition thickness of the second oxide layer 112 is about 3000˜6000 Å and a more preferred thickness is about 4000 Å.

[0026] In a preferred embodiment, after the trench 104 is filled with the first and second oxide layers, the silicon substrate is subjected to thermal annealing process to increase the density of the oxide layer 112 as a good insulating area.

[0027] Finally, the second oxide layer is planarized by chemical mechanical polishing (CMP) to form a well-deposited shallow trench isolation, as FIG. 1G shows.

[0028] The characteristic of the method according to the present invention is to fill the insulating material in the trench by two-step deposition, which is very suitable for forming a high aspect ratio STI structure in a semiconductor substrate. The upper portion with voids and seams of the first oxide layer 110 is eliminated by spray etching. The spray etching solution is chosen as the mixture of HF/H₂SO₄in a ratio between 0.3˜0.4, which achieves the high etching selectivity between the silicon oxide and nitride. Therefore, according to the present invention, the insulating oxide layer can be etched rapidly without over-etching the liner in the trench 104.

[0029] Accordingly, the advantage of the present invention is to form, by two-step deposition, a well-deposited shallow trench isolation structure without voids or seams.

[0030] The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.