Method of manufacturing a logic semiconductor device having non-volatile memory

Abstract: The present invention discloses a logic semicon­ ductor device having a non-volatile memory in which a memory cell portion and a logic circuit portion are formed on a single semiconductor substrate, and a floating gate (107a) of the memory cell portion and a gate (112b) of the logic circuit portion are made of different materials, and a method of manufacturing the same.

Description: The present invention relates to a method of manufacturing a semiconductor device in which a non-volatile memory and a logic circuit portion are formed on a single semiconductor substrate, comprising the steps of: forming a first insulating film which is to serve as an element isolating region on a surface of the semiconductor substrate; partially removing said first insulating film to form a memory cell portion forming region and a logic circuit portion forming region of said substrate; forming a second insulating film which is to serve as a first gate insulating film of a cell transistor of said memory cell portion on the entire surface; doping an impurity for controlling a threshold value of said cell transistor in a channel region of said cell transistor of said memory cell portion; forming a first conductive layer on the entire surface; patterning said first conductive layer to form a floating gate of said memory cell of said memory cell portion and to expose said logic circuit portion forming region; forming a third insulating layer on the entire surface; patterning said third insulating layer to expose said logic circuit portion forming region on the substrate; forming a fourth insulating film which is to serve as a sacrificial oxide film of said logic circuit portion on the entire surface; doping an impurity for controlling a threshold value of a transistor of said logic circuit portion in said logic circuit portion through said fourth insulating film; removing said fourth insulating film; forming a fifth insulating film on the entire surface; forming a second conductive layer on the entire surface; doping an impurity in said second conductive layer; and patterning said first and second conductive layers and said third and fifth insulating layers to form a control gate, a second gate insulating film, and a floating gate of said memory cell portion and a gate and a gate insulating film of said logic circuit portion. Such a method is described in DE-A-31 07 543. Further methods are disclosed in US-A-4 663 645 and EP-A-0 187 278. In a conventional logic semiconductor device having a non-volatile memory, the gate of a logic circuit portion, and the floating gate of a memory cell portion are formed in the same step and made of the same material. For example, when the floating gate of the memory cell portion and the gate of the logic circuit portion are formed on a polysilicon layer formed in the same step as these gates, an impurity for controlling threshold values of the memory cell portion and the logic circuit portion must be doped in both the channel regions of the memory cell portion and the logic circuit portion before the polysilicon layer is formed during the manufacture. In the above step, an annealing process for forming a second gate oxide film (inter poly) between the floating and control gates is performed in order to isolate the floating gate of the memory cell portion from the control gate after an impurity doping process and a polysilicon layer formation process. For this reason, the heat of the annealing process affects the impurity for controlling the threshold value doped in the channel region of the logic circuit portion. Thus, when the impurity for controlling the threshold value doped in the channel region of the logic circuit portion is affected by heat generated in the annealing process during formation of the second gate oxide film (inter poly) of the memory cell portion, a channel profile in the logic circuit portion is disturbed. The threshold value of the logic circuit portion is hard to control, and formation of a desired channel profile is difficult. That is, in a conventional logic semiconductor device having a non-volatile memory in which the gate of a logic circuit portion and the floating gate of the memory cell portion are made of the same material, a channel profile in the logic circuit portion is disturbed. The logic circuit portion is not suitable for micropatterning. In addition, in a conventional device, the floating gate of a memory cell portion, and the gate of the logic circuit portion are respectively made of polysilicon layers formed in the same step. For this reason, in a conventional logic semiconductor device having a non-volatile memory, in order to provide a thicknesses difference and characteristics difference between both the gate electrodes in the memory cell portion and the logic circuit portion, a photoresist film must be formed on either of the gate oxide films. Therefore, reliability of the device may be degraded, e.g., a gate breakdown voltage is decreased. It is an object of the present invention to provide a method of manufacturing a highly reliable micropatterned logic semiconductor device having a non-volatile memory wherein the floating gate of a memory cell portion and the gate of a logic circuit portion can be provided with different thickness and characteristics, controllability of the threshold value of a MOSFET in the logic circuit portion can be improved, and a desired channel profile is easily formed. According to the invention, the method initially defined is characterized in that the step of forming said third insulating layer includes the steps of sequentially forming a first oxide film and a first nitride film, oxidizing a surface of said nitride film to form a second oxide film, and forming a second nitride film. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1A to 1F are views showing the steps in manufacturing a logic semiconductor device having a non-volatile memory ; and Figs. 2A to 2F are views showing the steps in manufacturing a logic semiconductor device having a non-volatile memory according to the present invention. A logic semiconductor device having a non-volatile memory and a method of manufacturing the same will be described below with reference to the sectional views sequentially showing the steps shown in Figs. 1A to 1F. As shown in Fig. 1A, a field oxide film 102 having a thickness of 5,000 to 15,000 Å which is to serve as an element isolating region is formed on the surface of a semiconductor substrate 101 by selective oxidation in a steam atmosphere at a temperature of 1,000°C for 1 hr, after forming a well region on the surface of the substrate 101 by oxidation, impurity doping and diffusion. Element region forming portions of a memory cell portion and a logic circuit portion of the substrate 101 are exposed, and a first thermal oxide film 103 having a thickness of 200 to 250 Å which is to serve as a first gate oxide film of the memory cell portion is formed on the entire surface of the resultant structure by thermal oxidation in an oxygen atmosphere at a temperature of 900°C for 0.5 hr. A photoresist film 104 is formed on the entire surface, and a first thermal oxide film 103 on the element region forming portion of the memory cell portion is exposed to form an opening 105. A threshold value control impurity 106 such as boron, phosphorus, or arsenic is ion-implanted in the first thermal oxide film 103 from the opening 105 using the photoresist film 104 as a mask at an acceleration voltage of 35 to 140 keV. In Fig. 1A, reference numeral 106a denotes a region in which the impurity 106 for controlling the threshold value of the memory cell portion is implanted. As shown in Fig. 1B, the photoresist film 104 is removed. After eliminating the first thermal oxide film 103, a thermal oxide film having a thickness of 200 to 250 µm, which serves as a first gate oxide film, is formed. A first polysilicon layer 107 which is to serve as the floating gate of the memory cell portion is formed on the entire surface of the resultant structure by, e.g., CVD. In order to convert the first polysilicon layer 107 into a conductive layer, phosphorus is doped into layer 107, and then thermal diffusion is performed so that the layer 107 has a predetermined resistivity. Alternatively, predetermined impurity ions of phosphorus, or arsenic may be implanted in the first polysilicon layer 107 at an acceleration voltage of 35 to 140 keV. A photoresist film (not shown) is formed on the resultant structure, and the first polysilicon layer 107 is patterned by photoetching to determine a size of the floating gate of the memory cell portion in a direction of channel width and to expose the element region forming region of the logic circuit portion. A second gate oxide film (inter poly) of the memory cell portion and a 200- to 250-Å thick second thermal oxide film 108 which is to serve as a gate oxide film of the logic circuit portion are formed by, e.g., thermally oxidizing the surface of the polysilicon layer 107 and the exposed element region forming region of the logic circuit portion in an oxygen atmosphere at a temperature of 950 to 1,000°C for 1 hr. As shown in Fig. 1C, a photoresist film 109 is formed on the entire surface of the resultant structure, and an opening 110 is formed in a channel forming region of the logic circuit portion by photoetching. An impurity 111 such as boron, phosphorus, or arsenic for controlling the threshold value of the logic circuit portion is ion-implanted in the second thermal oxide film 108 from the opening 110 using the photoresist film 109 as a mask at an acceleration voltage of 35 to 140 keV. In Fig. 1C, reference numeral 111b denotes a region in which the impurity 111 for controlling threshold value of the logic circuit portion is implanted. As shown in Fig. 1D, a second polysilicon layer 112 which is to serve as the control gate of the memory cell portion and the gate of the logic circuit portion is formed on the entire surface of the resultant structure. In order to convert the second polysilicon layer 112 into a conductive layer, phosphorus is doped into layer 112 at an atmosphere of phosphorus. Alternatively, a predetermined impurity 115 such as phosphorus or arsenic may be ion-implanted in the second polysilicon layer 113 at an acceleration voltage of 35 to 40 keV. As shown in Fig. 1E, the second polysilicon layer 112, the second thermal oxide film 108, and the first polysilicon layer 107 are sequentially patterned by a CDE method, an RIE method, wet etching, or the like to determine sizes of the control gate and the floating gate of the memory cell portion in a direction of channel length, sizes of the gate of the logic circuit portion in the directions of channel width and length. At this time, a control gate 112a and a floating gate 107a of the memory cell portion, a gate 112b of the logic circuit portion, a first oxide film 103a and a second gate oxide film 108a of the memory cell portion, and a gate oxide film 108b of the logic circuit portion are formed. An impurity 113 of a conductivity type opposite to that of the semiconductor 101 and the well region is ion-implanted and diffused in the resultant structure in self-alignment using the control gate 112a of the memory cell portion, the gate 112b of the logic circuit portion, and the field oxide film 102 as masks to form source/drain regions 113b. At the same time, source/drain regions (not shown) are formed in the memory cell portion. Alternatively, source/drain regions may be formed at the time when the memory cell portion and the gate of the logic circuit portion are formed. As shown in Fig. 1F, e.g., a CVD SiO₂ film 114 is formed as an insulating interlayer by, e.g., CVD. Note that a phosphosilicate glass film (P₂O₅ film), a borophosphosilicate glass film (BPSG film), or a plasma SiO₂ film can be used instead of the CVD SiO₂ film 114. Thereafter, although not shown, a contact hole is formed in a predetermined position of the semiconductor device through the CVD SiO₂ film 114, and a predetermined wiring layer is formed on the resultant structure. According to the above logic semiconductor device having a non-volatile memory and the method of manufacturing the same, the floating gate 107a of the memory cell portion and the gate 112b of the logic circuit portion respectively consist of polysilicon layers which are respectively formed in different steps. Therefore, the floating gate 107a of the memory cell portion and the gate 112b of the logic circuit portion can be formed so as to have the different thicknesses and characteristics with each other. In addition, after the second thermal oxide film 108 which is to serve as the gate oxide film (inter poly) of the memory cell portion is formed, then the impurity 111 for controlling the threshold value of the logic circuit portion is doped in the resultant structure. For this reason, the impurity 111 for controlling the threshold value of the logic circuit portion is not affected by heat generated in the annealing process for forming the second thermal oxide film 108. When the impurity is not affected by annealing for forming the second thermal oxide film 108, controllability of the threshold value of the logic circuit portion can be improved without disturbance of a channel profile of the logic circuit portion, thereby easily forming a predetermined channel profile. Therefore, a MOSFET suitable for micropatterning can be formed in the logic circuit portion. The first gate oxide film 103a of the memory cell portion can be formed to have a thickness different from that of the gate oxide film 108b of the logic circuit portion. A logic semiconductor device having a non-volatile memory and a method of manufacturing the same according to one embodiment of the present invention will now be described below with reference to sectional views shown in Figs. 2A to 2F showing the steps in manufacturing the device. As shown in Fig. 2A, after well region is formed on a surface of a substrate 201, a 5,000 to 15,000 Å thick field oxide film 202 which is to serve as an element isolating region is formed on the surface of a semiconductor substrate 201 by, e.g., selective oxidation in a steam atmosphere at a temperature of 1,000°C for 5 hrs. Portions of the surface of the substrate 201 on which the element regions of a memory cell portion and a logic circuit portion are formed are exposed, and a 200-to 250-Å thick first thermal oxide film 203 which is to serve as the first gate oxide film of the memory cell portion is formed on the resultant structure by, e.g., thermal oxidation in an oxygen atmosphere at a temperature of 900°C for 2 hrs. A photoresist film 204 is formed on the entire surface of the resultant structure, and an opening 205 is formed in the channel region of the memory cell portion by photoetching. An impurity 206 such as phosphorus, boron, or arsenic for controlling the threshold value of the memory cell portion is ion-implanted in the first thermal oxide film 203 from the opening 205 at an acceleration voltage of 35 to 140 keV. In Fig. 2A, reference numeral 206a denotes a region in which the impurity 206 for controlling the threshold value of the memory cell portion is ion-implanted. Note that the thermal oxide film 203 is removed after the ion-implantation, and then a new thermal oxide film may be formed. As shown in Fig. 2B, the photoresist film 204 is removed. A first polysilicon layer 207 which is to serve as the floating gate of the memory cell is formed on the entire surface of the resultant structure by, e.g., CVD. In order to convert the first polysilicon layer 207 into a conductive layer, phosphorus is doped into the layer 207, and then thermal diffusion is performed. Alternatively, a predetermined impurity such as phosphorus, or arsenic may be ion-implanted in the first polysilicon layer 207 at an acceleration voltage of 35 to 140 keV. A photoresist film (not shown) is formed on the resultant structure, and the first polysilicon layer 207 is patterning by photoetching to determine a size of the floating gate of the memory cell in a direction of channel width and to expose a logic circuit portion forming region. A thermal oxide film 208 having a thickness of 200 Å is formed on the entire surface of the resultant structure by, e.g., thermal oxidation in an oxygen atmosphere at a temperature of 900 to 1,000°C for 1 hr. A first nitride film (Si₃N₄ film) 209 is formed on the second thermal oxide film 208 by a plasma method. The surface of the first nitride film 209 is oxidized in an oxygen atmosphere at a temperature of 850 to 950°C for 1 hr to form a third oxide film (SiON film) 210 having a thickness of 300 Å. A second nitride film (Si₃N₄ film) 211 having a thickness of 50 Å is formed again. A photoresist film (not shown) is formed on the resultant structure, and the second nitride film 211, third oxide film 210, the first nitride film 209, and the second thermal oxide film 208 are removed from a logic circuit portion forming region by photoetching to expose the logic circuit portion forming region of the substrate 201. A fourth thermal oxide film 212 which is to serve as a sacrificial oxide film of the logic circuit portion is formed on the entire surface of the resultant structure in an oxygen atmosphere at a temperature of 900°C for 1 hr. As shown in Fig. 2C, a photoresist film 213 is formed on the entire surface of the resultant structure, and an opening 214 is formed in the channel region of the logic circuit portion by photoetching. An impurity 215 such as boron, phosphorus, or asenic for controlling the threshold value is implanted through the fourth thermal oxide film 212 serving as a sacrificial oxide film from the opening 214 using the photoresist film 213 as a mask at an acceleration voltage of 35 to 140 keV. In Fig. 2C, reference numeral 215b denotes a region in which the impurity 215 for controlling the threshold value of the logic circuit portion is implanted. As shown in Fig. 2D, the fourth thermal oxide film 212 serving as the sacrificial oxide film is entirely removed from the resultant structure to expose the surface of the logic circuit portion forming region. A fifth insulating film 216 which is to serve as the gate oxide film of the logic circuit portion is formed on the entire surface of the resultant structure by, e.g., thermal oxidation in an oxygen atmosphere at a temperature of 900°C. A second polysilicon layer 217 which is to serve as the control gate of the memory cell portion and the gate of the logic circuit portion is formed on the entire surface by, e.g., CVD. In order to convert the second polysilicon layer 217 into a conductive layer, phosphorus is doped into the layer 217, and then thermal diffusion is performed. Alternatively, a predetermined impurity such as phosphorus, or arsenic may be ion-implanted in the second polysilicon layer 217 at an acceleration voltage of 35 to 140 keV. As shown in Fig. 2E, the second polysilicon layer 217, the fifth thermal oxide film 216, the second nitride film 211, the third oxide film 210, the first nitride film 209, the second thermal oxide film 208, first polysilicon layer 207, and first thermal oxide film 203 are sequentially patterned by a CDE method, a RIE method, or wet etching to determine the sizes of the control gate and the floating gate of the memory cell portion in a direction of channel length and the sizes of the gate of the logic circuit portion in the directions, of channel length and width. At this time, a control gate 217a and a floating gate 207a of the memory cell portion, a gate 217b of the logic-circuit portion, and a second gate insulating film (inter poly) consisting of the oxide film 208, the nitride film 209, the oxide film 210, the nitride film 211 and an oxide film 216a, a first gate oxide film 203a, and a gate oxide film 216b of the logic circuit portion are formed. An impurity 218 such as boron, phosphorus, or arsenic having a conductivity type opposite to the semiconductor substrate 201 is ion-implanted in the resultant structure in a self-alignment using the control gate 217a of the memory cell portion, the gate 217b of the logic circuit portion, and the field oxide film 202 as masks at an acceleration voltage of 35 to 140 keV, and then the impurity 201 is diffused to form source/drain regions 218b. At the same time, source/drain regions (not shown) are formed in the memory cell portion. Source/drain regions of the memory cell portions and the logic circuit may be formed in the different step. As shown in Fig. 2F, a CVD oxide film 219 is formed as an insulating interlayer on the entire surface of the resultant structure by CVD method. Thereafter, although not shown, a contact hole is formed in a predetermined position of the semiconductor device through the CVD oxide film 219, and a predetermined wiring layer is formed on the resultant structure. Thus, according to the present invention a logic semiconductor device having a non-volatile memory is manufactured. According to the above logic semiconductor device having a non-volatile memory and the method of manufacturing the same, the floating gate 207a of the memory cell portion and the gate 217b of the logic circuit portion consist of polysilicon layers formed in difference steps. Therefore, the thicknesses, characteristics, and the like of the floating gate 207a of the memory cell portion and the gate 217b of the logic circuit portion can be changed as in the method described with reference to Figures 1A-1F. In addition, the second gate insulating film (inter poly) having a multilayered structure consisting of the nitride film and the oxide film of the memory cell portion, and then the impurity 215 for controlling the threshold value of the logic circuit portion is doped in the resultant structure. For this reason, the impurity 215 for controlling the threshold value of the logic circuit portion is not affected by heat generated in the annealing process for forming the second gate insulating film as in the method described with reference to Figures 1A-1F. Therefore, a MOSFET suitable for micropatterning can be formed in the logic circuit portion. Since the second gate insulating film having a multilayered structure consisting of the nitride and oxide films is formed on the first polysilicon layer 207, the sacrificial oxide film 212 can be formed in the logic circuit portion. By forming the sacrificial oxide film 212, the quality of the gate oxide film 216b in the logic circuit portion is excellent. The gate breakdown voltage of a MOSFET in the logic circuit portion can be increased, thereby improving the reliability of the MOSFET. The first gate oxide film 203a and the second gate insulating film (inter poly) of the memory cell portion and the gate oxide film 216b of the logic circuit portion can be formed to have different thicknesses. Note that the three-layered insulating film consisting of the nitride film 209, the oxide film 210, and nitride film 211 is used as the second gate insulating film (inter poly) in order to obtain excellent characteristics of the memory cell portion such as a high breakdown voltage. Thus, after the second gate insulating film (inter poly) of the memory cell portion is formed (it is normally formed at a temperature of 950 to 1,100°C), an impurity for controlling the threshold value of the logic circuit portion is doped in the second gate insulating film. For this reason, the logic circuit portion is rarely affected by a thermal influence, and a MOSFET suitable for micropatterning can be formed. Therefore, the logic circuit portion can be micropatterned as in a single logic LSI. In particular, the present invention is very effective for a logic semiconductor device having a non-volatile memory in which the area of a memory cell is smaller than that of a logic circuit portion. Needless to say, a well region is formed prior to the formation of each of the field oxide films 102 and 202 shown in Figs. 1A and 2A. In the method of the present invention, since sacrificial oxidization is performed for the logic circuit portion, the thickness of the second insulating film for electrically isolating the control gate of the memory cell portion from the floating gate and the thickness of the gate insulating film of the logic circuit portion can be independently controlled. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A method of manufacturing a semiconductor device in which a non-volatile memory and a logic circuit portion are formed on a single semiconductor substrate, comprising the steps of: forming a first insulating film (202) which is to serve as an element isolating region on a surface of the semiconductor substrate (201); partially removing said first insulating film (202) to form a memory cell portion forming region and a logic circuit portion forming region of said substrate (202); forming a second insulating film which is to serve as a first gate insulating film (203) of a cell transistor of said memory cell portion on the entire surface; doping an impurity for controlling a threshold value of said cell transistor in a channel region of said cell transistor of said memory cell portion; forming a first conductive layer on the entire surface; patterning said first conductive layer to form a floating gate (207) of said memory cell of said memory cell portion and to expose said logic circuit portion forming region; forming a third insulating layer (208-211) on the entire surface; patterning said third insulating layer to expose said logic circuit portion forming region on the substrate (201); forming a fourth insulating film (212) which is to serve as a sacrificial oxide film of said logic circuit portion on the entire surface; doping an impurity for controlling a threshold value of a transistor of said logic circuit portion in said logic circuit portion through said fourth insulating film (212); removing said fourth insulating film (212); forming a fifth insulating film (216) on the entire surface; forming a second conductive layer (217) on the entire surface; doping an impurity in said second conductive layer (217); and patterning said first and second conductive layers (207, 217) and said third and fifth insulating layers (208-211, 216) to form a control gate (217a), a second gate insulating film, and a floating gate (207a) of said memory cell portion and a gate (217b) and a gate insulating film (216b) of said logic circuit portion, characterized in that the step of forming said third insulating layer includes the steps of sequentially forming a first oxide film (208) and a first nitride film (209), oxidizing a surface of said nitride film (209) to form a second oxide film (210), and forming a second nitride film (211).

Inventors: HANADA NAOKI C O INTELLECTUAL, HANADA, NAOKI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H01L27
IPC codes: H01L27, H01L29, H01L21