Impurity ion segregation precluding layer, fabrication method thereof, isolation structure for semiconductor device using the impurity ion segregation precluding layer and fabricating method thereof

[0002] 1. Field of the Invention

[0003] The present invention relates to a semiconductor device, and more particularly to a layer for precluding segregation of impurity ions which prevents impurity ion permeation between a device isolation region and a semiconductor substrate, a fabrication method thereof, an isolation structure for a semiconductor device using the segregation precluding layer and a fabrication method thereof.

[0004] 2. Description of the Conventional Art

[0005] A LOCOS (local oxidation of silicon) structure using a LOCOS method has been often used as an isolation structure of a conventional semiconductor device, but there is limit to improvement of integration of the semiconductor device due to generation of a bird's beaks thereof. Thus, there is a tendency to adopt an STI (shallow trench isolation) or PGI (profiled groove isolation) structure in which a trench or a groove is formed in a semiconductor substrate as a device isolation structure and then an insulator is filled therein.

[0006] FIG. 1 is a plan diagram illustrating a cell array unit of a semiconductor device, particularly, a DRAM (dynamic random access memory). A semiconductor substrate 100 is divided into an active region 101 and a non-active region 102 which covers the active region 101, the non-active region 102 being called a device isolation region. The active region is a part where a semiconductor device, that is, a transistor is formed and in which impurity ions are implanted, thus a source 101aand a drain 101bare provided. The device isolation region 102 electrically isolates the semiconductor device and has the STI or PGI structure. A gate electrode 104 is formed on the active region 101. A channel of the transistor is formed in a portion of the semiconductor substrate where the active region 101 and the gate electrode 104 are overlapped.

[0007] FIG. 2A is a cross-sectional vertical view taken along the line IIa—IIa of FIG. 1, the line horizontally crossing the active region 101 in a center point of a channel width of the transistor. As shown therein, the active region 101 of the semiconductor substrate 100 is covered by the device isolation region 102. The device isolation region 102 is etched to a predetermined depth (for example, about 0.5-0.8 mm), thereby forming a trench 102aand an insulator 102bis filled therein. The source 101aand the drain 101b, as shown in FIG. 1, are provided in the semiconductor substrate 100 having a predetermined distance and a gate insulating film 103 and a gate electrode 104 are formed on the semiconductor substrate 100.

[0008] While, FIG. 2B is a cross-sectional vertical view taken along the line IIb—IIb of FIG. 1, the line being perpendicular to the line IIa—IIa of FIG. 1. As shown therein, the trench or groove 102ais formed in the semiconductor substrate 100 and the insulator 102bis filled in the trench or groove 102a. Here, the part 102 in which the insulator is filled corresponds to the device isolation region. The gate insulating film 103 is formed on the semiconductor substrate 100 and a gate electrode 104 is formed thereon, the gate electrode 104 being formed in the active region 101 and extended to the upper surface of the device isolation region 102. 101cis a part where impurity ions in the semiconductor substrate segregate to the device isolation region and thus density thereof is considerably low. Also, 101dis a center part of the channel region of the transistor.

[0009] However, the conventional semiconductor device having the STI or PGI structure, particularly the semiconductor which is an N-channel transistor has problems as follows.

[0010] The N-channel transistor is generally formed in a p-type semiconductor substrate or in a p-type well. Impurity ions, in particular, boron ions in the p-type semiconductor substrate or the p-type wall have a tendency to easily segregate to the device isolation region and accordingly the density of the impurity ions of a portion adjacent to the device isolation region, that is, a portion 101cof the semiconductor substrate adjacent to a sidewall of the trench 102abecomes decreased. Accordingly, an impurity depletion layer is formed in the semiconductor substrate along the sidewall of the trench. As a result, in the center part 101dof the channel region the channel of the transistor is normally formed above a threshold voltage in accordance with a voltage applied to the gate electrode, but in the channel region adjacent to the device isolation region a channel is easily formed below the threshold voltage, so that the threshold voltage decreases. In addition, an electrical characteristic of the semiconductor device is unstable that, for example, a subthreshold current increases and a subthreshold current curve has a hump, which results in the deterioration of the semiconductor device.

[0011] Accordingly, the present invention is directed to an impurity ion segregation precluding layer, a fabrication method thereof, an isolation structure for semiconductor device using the impurity ion segregation precluding layer and a fabricating method thereof which obviate the problems and disadvantages in the conventional art.

[0012] An object of the present invention is to provide an impurity ion segregation precluding layer and a fabrication method thereof that prevent impurity ions in a semiconductor substrate from permeating into a device isolation region thereof.

[0013] Also, an object of the present invention is to provide an isolation structure of a semiconductor device and a fabrication method thereof that stabilize electric characteristics of a semiconductor device using the above impurity ion segregation precluding layer and thereby improve reliability thereof.

[0014] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided an impurity ion segregation precluding layer having a thickness of 1-10 A by placing a semiconductor substrate formed of silicon in a high-temperature furnace and annealing the semiconductor substrate while flowing a nitride gas thereinto at at least 20 l/min.

[0015] Also, to achieve the above objects of the present invention, there is provided an isolation structure of a semiconductor device, which includes a semiconductor substrate, a trench formed in a predetermined portion of the semiconductor substrate, an impurity ion segregation precluding layer formed on a surface of the trench, and an insulator filled in the trench, the impurity ion segregation precluding layer being obtained by placing the semiconductor substrate into a furnace at a high temperature and annealing the semiconductor substrate flowing a nitride gas into the furnace at about 20 l/min.

[0016] Also, to achieve the above objects of the present invention, there is provided a method for fabricating an isolation structure of a semiconductor device, which includes forming a trench in a portion of a semiconductor substrate formed of silicon corresponding to a device isolation region, placing the semiconductor substrate into a furnace at a high temperature and annealing the semiconductor substrate flowing a nitride gas into the furnace at about 20 l/min, and filling an insulator in the trench.

[0017] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.

[0018] In the drawings:

[0019] FIG. 1 is a plan diagram of a conventional DRAM cell array unit;

[0020] FIG. 2A is a cross-sectional vertical view taken along the line IIa—IIa of FIG. 1;

[0021] FIG. 2B is a cross-sectional vertical view taken along the line IIb—IIb of FIG. 1;

[0022] FIG. 3 is a cross-sectional vertical view of an isolation structure of a semiconductor device according to the present invention; and

[0023] FIGS. 4A through 4E are cross-sectional vertical views illustrating a fabrication method of an isolation structure of a semiconductor device according to the present invention.

[0024] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0025] FIG. 3 is a cross-sectional vertical view illustrating an isolation structure of a semiconductor device using an insulating film according to the present invention.

[0026] As shown therein, a trench 301 is formed in a device isolation region in a semiconductor substrate 300 and an impurity ion segregation precluding layer 302 is formed on a surface of the semiconductor substrate 300 along a sidewall and a bottom of the trench 301, the impurity ion segregation precluding layer 302 having a thickness of about 1-9 A. An insulator 303 which is a silicon oxide SiO₂or silicon nitride Si₃N₄is filled in the trench 301. The impurity ion segregation precluding layer 302 at the thickness of 1-9 A formed along the sidewall and bottom of the trench 301 prevents the impurity ions in the semiconductor substrate 300 from permeating through the insulator 303 in the trench 301, thus preventing decrease in the impurity density of the semiconductor substrate. Accordingly, decrease in the threshold voltage in which a channel is formed below the threshold voltage is prevented and thus the reliability of the semiconductor device can improve.

[0027] The impurity ion segregation precluding layer 302 is obtained by putting the semiconductor substrate 300 having trench 301 in a high-temperature furnace (a furnace that is generally used in a semiconductor mass-production line) at about 800□ C. and annealing the semiconductor substrate 300 while flowing nitride gas into the furnace at over 20 l/min. Since the semiconductor substrate 300 is silicon and the annealing process is applied with flowing the nitride gas, the impurity ion segregation precluding layer 302 formed on the bottom and sidewall of the trench 301 of the semiconductor substrate 300 is a silicon nitride which includes silicon and nitride atoms, respectively. In the silicon nitride, it is considered that a combination ratio of silicon to nitride is under 0.75 because of the large inflow of the nitride gas. That is, the silicon nitride has the most stable condition when three (3) silicon atoms are combined with four (4) nitride atoms such as Si₃N₄. However, according to the present invention, it is considered that the combination ratio of the silicon to the nitride is not 3:4, but the ratio of the nitride atom is higher than the silicon. Accordingly, the silicon nitride according to the present invention will be referred to a nitrogen-rich silicon nitride. However, such a nitrogen-rich silicon nitride can be Si₃N_4.1, Si₃N_4.2, Si₃N_4.3, Si₃N_4.4. . . , or Si_2.9N₄, Si_2.8N₄, Si_2.7N₄, Si_2.6N₄, that is, when the silicon atom is 3, the nitride atom is at least 4, or when the nitride atom is 4, the silicon atom is less than 3. Also, the impurity ion segregation precluding layer is considerably thin, of which a thickness is less than 10 A. Since the thickness of the insulating film is less than 10 A, the insulating film is a single nitride atom layer. However, it has been observed through the experiment that the impurity ion segregation effect of this thin layer is excellent.

[0028] The thickness of the impurity ion segregation precluding layer according to the present invention is not a measured value but an estimated one, because “Telcor” of Telcor which the inventor used for measuring the film can only measure a film which has a thickness of at least 10 A. As a result of measuring the thickness of the impurity ion segregation precluding layer, it indicated 10 A, which is the lowest value that the instrument could measure. Thus, it is estimated that the thickness of the film is 10 A or less.

[0029] The reason that it is considered that the film exists, although the thickness of the film was not accurately measured by the instrument is as follows.

[0030] First, as described above, when forming the device isolation region by filling the insulator in the trench and then forming the transistor on the semiconductor substrate, after annealing the semiconductor substrate with the inflow the nitride gas at at least 20 l/min, decrease in the threshold voltage due to the impurity ion segregation of the semiconductor substrate was not shown. Accordingly, it can be assumed that there exists in the device isolation region a film that prevents the impurity permeation between the insulator and the semiconductor substrate.

[0031] Further, the following explains the reason why it is assumed that the impurity permeation preventing film is a silicon nitride.

[0032] After performing the annealing using the nitride gas, the inventor etched the semiconductor substrate with a BOE (buffered oxide etchant=buffered hydrofluorine), formed the device isolation region by filling the insulator in the trench and fabricated a transistor using the semiconductor substrate. Then, such a transistor had the same result as the semiconductor substrate before performing the BOE etching. Accordingly, it is estimated that the impurity permeation preventing film is not the silicon oxide, but there is provided a material of which nitride and silicon are combined, that is, the silicon nitride, since the silicon substrate was annealed using the nitride gas. Further, since the amount of the employed nitride gas is considerably larger than the nitride gas (61 /min at its maximum), which is used for the annealing process in the semiconductor device fabrication process, it is considered to be the nitrogen-rich silicon nitride.

[0033] While, the inventor additionally carried out the following experiment to evaluate the existence of the impurity ion segregation precluding layer and the characteristics thereof.

[0034] Table 1 illustrates results of the experiment that measured a thickness of an oxide formed on the semiconductor substrate when the semiconductor substrate was placed in the furnace at a temperature of at least about 800□ C. and annealed in an oxygen atmosphere while flowing the nitride gas at 121 /min and 201 /min, respectively, thereinto. When the nitride gas was not flowed thereinto, the thickness of the oxide formed on the silicon substrate which was obtained under the same conditions in the oxygen atmosphere was 70 A. However, when the nitride gas was flowed into the furnace at 121 /min and 201 /min, respectively, and annealed in the oxygen atmosphere, the thickness of each oxide formed on the semiconductor substrate was less than 70 A. In other words, when the annealing process was performed in the oxygen atmosphere with the inflow of the nitride gas into the furnace at 121 /min, the oxide formed on the silicon semiconductor substrate had a thickness of 66.8 A on average through experiments of three times, which had nearly the same result as the oxide formed on the semiconductor substrate without the nitride gas inflow into the furnace. Further, when the annealing process was performed in the oxygen atmosphere with the inflow of the nitride gas into the furnace at 201 /min, the oxide formed on the silicon semiconductor substrate had a thickness of 36.6 A on average through experiments of three times, which has about half the thickness of the oxide formed on the semiconductor substrate without the nitride gas inflow into the furnace. Accordingly, it can be seen that when the nitride gas was flowed at least at 201 /min during the annealing process, there is formed a film that prevents oxidization of a surface of the silicon semiconductor substrate.

[0035] Generally, it is known that an oxide is hardly formed on Si₃N₄, which is a stable silicon nitride. Thus, it can be assumed that growth of the oxide was restrained because there was formed the silicon nitride of which the nitride and the silicon were combined. In particular, the oxide grows, but the growth speed of the oxide was controlled while growing. Thus, it is considered that the silicon nitride has an unstable combination of a compound, not a stable one, and thus the inventor referred to the silicon nitride as the nitrogen-rich silicon nitride.

[0036] Further, as a result of the experiments, the oxide, formed on the semiconductor substrate when the nitride gas was flowed into the furnace at 121 /min, has the thickness similar to the oxide which was formed without flowing the nitride gas into the furnace. Thus, it can be assumed that when the nitride gas is flowed at 121 /min, the nitrogen-rich silicon nitride is not sufficiently formed on the semiconductor substrate, but when the annealing process is performed with the inflow of the nitride gas into the furnace at at least 20 l/min, there is formed on the surface of the trench the nitrogen-rich silicon nitride that has a desirable effect of precluding the impurity ion segregation. However, considering the fabrication cost, it is preferred to perform the annealing process with the inflow of the nitride gas into the furnace at 20 to 501 /min.

[0037] Next, a fabrication method of the semiconductor device isolation structure of FIG. 3 according to the present invention will be described with the reference to FIGS. 4A to 4E.

[0038] First, as shown in FIG. 4A, a trench 401 is formed in a part of a semiconductor substrate 400 corresponding to a device isolation region. The trench 401 is formed by the following process. First, a pad oxide 411 is formed over the semiconductor substrate 400. The pad oxide 411 can be formed by oxidizing a silicon substrate by a thermal oxidization method or deposited by a chemical vapor deposition method. A silicon nitride 412 is deposited on the pad oxide 411, and a photoresist film (not shown) is applied on the silicon nitride 412 and a photolithography process is performed, so that the photoresist film remains on the silicon nitride 412, which corresponds to an active region, which becomes a photoresist film pattern (not shown). Using the photoresist film pattern as a mask, the silicon nitride 412 and the pad oxide 411 are etched by a reactive ion etching method and then the semiconductor substrate 400 formed under the etched pad oxide 411 is etched to a predetermined depth, thereby forming the trench 401. Further, to recover a surface of the semiconductor substrate 400 which has been damaged in the etching process to form the trench 401, the semiconductor substrate is annealed at 1050□ C. in an O₂atmosphere, so that there is formed a thermal oxide 402 at a thickness of about 50-200 A on the semiconductor substrate 400 in the trench 401.

[0039] As shown in FIG. 4B, the thermal oxide 402 is removed using an HF solution.

[0040] Next, as shown in FIG. 4C, the semiconductor substrate of FIG. 4bis placed in a furnace at a temperature of at least 800□ C. and annealed while flowing a nitride gas N₂into the furnace at 20-501 /min, thereby forming an impurity ion segregation precluding layer 403 at a thickness of 1-10 A on the semiconductor substrate 400 along an inner wall and a bottom of the trench 401. Here, the impurity ion segregation precluding layer 403 is the nitrogen-rich silicon nitride which has been above described. Also, a nitride ion implantation method can be performed as a method for forming the nitrogen-rich silicon nitride on the semiconductor substrate, however since such an ion implantation method damages the surface of the semiconductor substrate, it is not recommendable. Specifically, when a portion of the semiconductor substrate adjacent to the device isolation region is damaged, a leakage current may be generated, thus it is more desirable to perform the annealing process in the nitride atmosphere, rather than the ion implantation method.

[0041] As shown in FIG. 4D, an insulator 404 is formed on an entire surface of the resultant structure of FIG. 4C including the trench 401 and an annealing process is performed to the resultant structure. Here, the insulator is preferably a silicon oxide or silicon nitride. Then, a chemical mechanical polishing process is performed to the insulator 404 for thereby removing a portion of the insulator formed on the silicon nitride 412, so that a surface level of the resultant semiconductor substrate 400 becomes planarized.

[0042] Next, as shown in FIG. 4E, the silicon nitride 412 and the pad oxide 411 are sequentially removed, thereby completing the fabrication of the isolation structure of the semiconductor device.

[0043] As described above, the thin impurity ion segregation precluding layer, that is, the nitrogen-rich silicon nitride at a thickness of several A is formed on the surface of the part of the semiconductor substrate corresponding to the device isolation region, thereby preventing the inter-permeation of the impurity ions between the device isolation region and the semiconductor substrate, which has an effect of improving the reliability of semiconductor device by stabilizing the electric characteristics of the device.

[0044] It will be apparent to those skilled in the art that various modifications and variations can be made in the impurity ion segregation precluding layer, the fabrication method thereof, the isolation structure for the semiconductor device using the segregation precluding layer and the fabrication method thereof of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.