SHORT CHANNEL FLASH EEPROM DEVICE HAVING A DOUBLE DIFFUSED SOURCE AND METHOD OF MANUFACTURING THE SAME

Abstract: A system and method for providing a very short channel memory cell having a double diffuse implant junction is disclosed. The system and method comprise the sequential steps of providing a junction implant (110), providing a spacer (108), and providing a double diffuse implant (112). Because the double diffuse implant is provided after the spacer, the double diffuse implant does not extend as far under the gate of a memory cell after processing. Thus, the memory cell has a graded junction that does not substantially shorten the effective length of the channel. The memory cell can, therefore, function even as the size of the memory cell is decreased. In addition, the thermal cycling of the double diffuse implant may be decoupled from that of the junction implant. This is achieved without complicating processing. Consequently, overall system performance is enhanced.

Description: SHORT CHANNEL FLASH EEPROM DEVICE HAVING A DOUBLE DIFFUSED SOURCE AND METHOD OF MANUFACTURING THE SAME FIELD OF THE INVENTION The present invention relates to memory elements, and more particularly to a method and system for providing a double diffuse implant junction in a very short channel FLASH EPROM device BACKGROUND OF THE INVENTION A typical memory chip includes a plurality of memory cells A particular cell is comprised of a source junction ("junction"), a drain, and a gate The area between the junction and the drain is the channel Between at least a portion of the gate and the channel is a tunneling oxide One conventional method tor making the junction of a cell uses a single implant, typically arsenic However, this method results in an abrupt junction. Because the junction is abrupt, the electric field at the corner of the gate is extremely strong during erasure This strong electric field causes band-to-band tunneling of electrons from the valence band to the conduction band of the silicon, leaving behind holes causing a variety of problems First, the holes create a large substrate current Second, because of the lateral electric field near the junction, the holes can also move laterally This lateral motion can cause interface damage Finally, the holes may also travel onto the gate Holes traveling to the gate may cause an accidental erasuie of the cell In oidei to remedy the problems caused by band-to-band tunneling of electrons, typical cells implement a double diffuse implant The double diffuse implant provides a graded junction This reduces band-to-band tunneling The implant is typically comprised of phosphorus. In conventional semiconductor processing, the double diffuse implant is carried out prior to the junction implant. The junction implant is typically comprised of arsenic ("As") Because phosphorus diffuses more rapidly than As, after thermal cycling, the As junction will be shallow while the double diffuse implant will be deeper The double diffuse implant will also extend farther under the gate, thereby shortening the channel length In addition, the diffusion rate of the double diffuse implant will constrain the thermal cycling used in processing the junction. Because a high density of storage elements are desired, it is necessary to make storage elements smaller. As the cell becomes smaller, the shortening of the channel length limits the ability of the device to function Consequently, for very short channel length or a high density of devices, there are conflicting constraints. A graded junction is desired in order to reduce band-to-band tunneling However, the effective channel length must be greater than a minimum size in order for the device to function For FLASH EPROM devices, one way of increasing density is through the use of a self aligned source ("SAS") etching technique An example of this type of technique is disclosed in U S Pat. No. 5,120,671 entitled "PROCESS FOR SELF ALIGNING A SOURCE REGION WITH A FIELD OXIDE REGION AND A POLY-SILICON GATE " The SAS technique, however, can damage the tunneling oxide and gouge the area where the junction is formed. A description of the problems of the SAS technique and a method for addressing these problems is disclosed in U S Pat No 5,470,774 .->. entitled "METHOD PROTECTING A STACKED GATE EDGE IN A SEMICONDUCTOR DEVICE FROM SELF ALIGNED SOURCE (SAS) ETCH." Although these patents address SAS techniques, neither discloses how these techniques could be utilized with a very short channel FLASH EPROM device. Accordingly, what is needed is a system and method for a cell having a graded junction which does not substantially shorten the effective channel length. In addition, because each step in semiconductor processing is complex, it is desirable to produce such a device without adding a substantial number of steps to the process. Finally, in the context of FLASH EPROM devices, it would be beneficial if the SAS technique could be employed while processing very short channel FLASH cells having a double diffuse implant junction. The present invention addresses such a need. SUMMARY OF THE INVENTION A system and method for providing a very short channel memory cell having a double diffuse implant junction is disclosed, the system and method comprise the sequential steps of providing a junction implant, providing a spacer, and providing a double diffuse implant. Because the double diffuse implant is provided after the spacer, the double diffuse implant does not extend as far under the gate of a memory cell after processing. Thus, the memory cell has a graded junction that does not substantially shorten the effective length of the channel. The memory cell can, therefore, function even as the size of the memory cell is decreased. In addition, the thermal cycling of the double diffused implant may be decoupled from that of the junction implant. This is achieved without significantly complicating processing. Consequently, the performance of the memory cell is enhanced. BRIEF DESCRIPTION OF THE DRAWINGS Figure la is a diagram of a typical FLASH cell. Figure lb is a diagram of a typical FLASH cell having a graded junction and made using the SAS technique. Figure lc is a simplified flow chart of a typical process used to produce a memory element using the SAS technique. Figure 2a is a diagram of a FLASH cell in which the stacked gate edge is protected from a self aligned source etch. Figure 2b is a simplified flow chart of a process used to produce a FLASH cell in which the stacked gate edge is protected from a self aligned source etch. Figure 3 is a simple flow chart of the process for providing a storage element in accordance with the method and system. Figure 4 is a detailed flow chart of the process for providing the storage element in accordance with the method and system. Figure 5a is a diagram of a FLASH cell in accordance with one embodiment of the method and system. Figure 5b is a diagram of a FLASH cell in accordance with a second embodiment of the method and system. Figure 5c is a diagram of a FLASH cell in accordance with a second embodiment of the method and system after the SAS etch has been completed and the SAS mask has been removed. DESCRIPTION OF THE INVENTION The present invention relates to an improvement in binary storage elements. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. Although the embodiments are described with reference to FLASH EPROM cells and the self aligned source (SAS) technique, the present invention is not limited to these types of cells or to use only with this processing technique. Figure 1 a is a diagram of a typical memory cell 40. Memory cell 40 is comprised of control gate 42, floating gate 44, junction 46, drain 48, and channel 49. Memory cell 40 is shown with SAS mask 45. The junction is abrupt because it has a rapid change in dopant concentration between the n+ doped junction 46 and channel 49. Thus, memory cell 40 will have problems associated with band-to-band tunneling of electrons. Figure 1 b is a diagram of a typical FLASH cell 10 having a double diffuse implant junction and made using a conventional SAS process. FLASH cell 10 is comprised of control gate 12, floating gate 14, tunneling oxide 16, junction implant 18, double diffuse implant ("DDI") 20, drain 22, and channel 24. FLASH cell 10 is shown with SAS mask 1 1 in place. Since DDI 20 has been provided, the junction is graded. However, because of the SAS technique used, damage, shown generally by 25, has been incurred in tunneling oxide 16 and junction implant region 18. Figure lc is a simplified flow chart depicting the major steps used in processing typical FLASH cell 10. First, the DDI is performed at step 30. After the DDI step 30, the junction implant is performed via step 32. After both the DDI and junction implant have been performed, the SAS technique is performed in step 34. Figure 2a displays a FLASH cell 50 made in accordance with the technique disclosed in U.S. Pat. No. 5,470,773. This technique minimizes the damage to FLASH cell 50 during the SAS technique. FLASH cell 50 is comprised of control gate 52, floating gate 54, tunneling oxide 56, drain 58, junction implant 60, DDI 62, spacer 64, and channel 66. FLASH cell 50 is shown with SAS mask 68. Because of DDI 62, the junction is graded. In addition, FLASH cell 50 has not suffered damage induced by the SAS technique used during processing. Figure 2b shows a simplified flow chart of the process used to form FLASH cell 50. First, DDI implant step 70 is performed. The junction implant is then performed through step 72. Next, the spacer is provided at step 74. Finally, the SAS process is employed in step 70. Because the spacer providing step 74 is formed prior to the SAS implant step 76, the process of Figure 2b results in FLASH cell 50 not having damage in tunneling oxide 56 and junction 60. Although the FLASH cells shown in Figures lb and 2a perform their desired functions, those with ordinary skill in the art will realize that these cells will not function at very short channel lengths or high densities of FLASH cells. Referring back to Figure la, although junction 27 is abrupt, channel 29 has not been shortened. Referring now to Figure lb, although DDI 20 results in a graded junction, DDI 20 has also reduced the effective length of channel 24. Similarly, channel 66 shown in Figure 2a has also been shortened due to DDI 62. Thus, although the SAS techniques used to make FLASH cells 10 and 50 allow for higher density, as the FLASH cells become smaller, the cells can no longer function. In addition, because the DDI dopant is implanted prior to the junction implant and typically diffuses more rapidly than the junction implant, the diffusion of the DDI dopant constrains the thermal cycling of the junction. Finally, it would be desirable if these problems in processing and a FLASH cell could be remedied without significantly complicating processing of the FLASH cell. The present method and system for a very short channel memory cell having a graded junction. The method and system will be described with reference to a FLASH cell processed using an SAS technique. In addition, the method and system will be described using an arsenic ("As") junction implant and a phosphorus ("P") DDI. However, one of ordinary skill in the art will understand that the method and system are not limited to this type of cell, these processing techniques, or these dopants. The present method and apparatus provide the benefits of conventional DDI, without the large reduction in effective channel length. Figure 3 is a simplified flow chart of one embodiment of the method. First, the As junction is formed in step 200. After the As junction is formed, a spacer is introduced using step 300. Before or after the spacer formation is completed, the DDI is carried out in step 400. Finally, the SAS process is employed at step 500. In the method, the order of implantation of the junction and DDI dopants are reversed as compared to the conventional methods of Figures lc and 2b. Figure 4 shows a more detailed flow chart of one embodiment of the process in accordance with the method and system. Referring back to Figure 3, junction formation 200 is carried out through steps 202-210 of the flow chart in Figure 4. First, the self aligned etch is carried out at step 202. The pre-implant oxidation step is shown in step 204. Next, the core source/drain ("core S/D") implant is carried out in step 206. In the preferred embodiment, the core S D implant is As. The core S D anneal is performed in step 208. Thus, the As junction implant and thermal cycling are performed in steps 202-208, prior to the DDI. The spacer is provided in steps 302-304 ofFigure 4. First, the spacer is deposited via step 302. In one embodiment of the invention, a spacer etch is performed in step 304. This step is optional. In addition, the spacer etch step 304 could be performed after the DDI implant step 402, discussed below. In an embodiment where the spacer etch step 304 occurs after the DDI implant step 402, the DDI implant step 402 is carried out before spacer formation is completed. The SAS mask is applied in step 502. The DDI implant is then performed in step 402. In the preferred embodiment, the DDI implant is phosphorus. Next, the SAS process is continued in the SAS etch step 504 and the SAS implantation step 506. Note that some conventional steps may have been omitted. Because the DDI implant, step 402 in Figure 4, is performed after the junction implant and anneal, steps 202-210 in Figure 4, the diffusion of the phosphorus is not a consideration in the junction (As) anneal. Since the SAS process is carried out after the spacer has been provided, the stacked gate edge is protected and the SAS process does not induce the damage of conventional SAS processing. Finally, additional steps have not been introduced in processing. Thus, processing is easier. To more particularly illustrate the method and system, refer now to Figures 5a and 5b. Figure 5a shows a FLASH cell 100 made in accordance with one embodiment of the method and system. FLASH cell 100 is comprised of control gate 102, floating gate 104, tunneling oxide 106, spacer 108, junction implant 1 10, DDI 1 12, drain 1 14, and channel 1 16. FLASH cell 100 is shown with SAS mask 1 18. In the preferred embodiment, the dopants used for junction implant 1 10 and DDI 1 12 are As and P, respectively. Because spacer 108 is provided before DDI 1 12, DDI 1 12 is offset from gate 104 by spacer 108 during implantation. The phosphorus used as a dopant for DDI 1 12 does not diffuse as far under gate 104 during subsequent thermal cycling. Thus, even after diffusion, DDI 1 12 is offset from gate 104. Unlike conventional DDI storage elements, the effective length of channel 1 16 has not been substantially reduced. Thus, a very small FLASH cell 100 having a graded junction can be provided. In addition, because the As junction implant and anneal are performed prior to the DDI implant, an optimal As junction can be formed without constraints due to phosphorus diffusion. Another embodiment of a FLASH cell 150 in accordance with the method and system is shown in Figure 5b. FLASH cell 150 is formed when spacer etch step 304 of Figure 4 is omitted. FLASH cell 150 is comprised of control gate 152, floating gate 154, spacer 158, junction implant 160, DDI 162, drain 164, and channel 166. FLASH CELL 150 is shown with SAS mask 1 18. Figure 5c shows one embodiment of FLASH cell 150 after the SAS etch has been completed and SAS mask 1 18 has been removed. In the preferred embodiment, the dopants used for junction implant 160 and DDI 162 are As and P, respectively. Referring now to Figure 5b, because spacer 158 is provided before DDI 162, DDI 162 is offset from gate 154 by spacer 158 during implantation. The phosphorus used as a dopant for DDI 162 does not diffuse as far under gate 104 during subsequent thermal cycling. Thus, even after diffusion, DDI 162 is offset from gate 154. Unlike conventional DDI storage elements, the effective length of channel 166 has not been substantially reduced. Consequently, a very small FLASH cell having a graded junction can be provided. As with FLASH cell 100, the processing of As junction 160 is decoupled from that of DDI 162. Figure 5c depicts one embodiment of FLASH cell 150 after the SAS etch has been completed and the SAS mask 1 18 has been removed. Note that spacer 158 is now comprised of two portions. One portion comprises a spacer on same side of the gate 154 as the As junction 160. The other portion of spacer 158 is on the same side of the gate 154 as the drain 164. A method and system has been disclosed for a very short channel memory cell having a DDI junction. A preferred embodiment of the method and system performs the DDI after the junction implant, junction anneal, and spacer formation have been completed. Thus, the junction implant and thermal cycling are decoupled from that of the DDI. Because the spacer is formed prior to the DDI, the DDI is offset from the gate. Consequently, the effective channel length is not reduced. The method and system thus provide a graded junction in a very small memory cell, such as a FLASH cell. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims. -I I -

Claims: CLAIMS What is claimed is: 1. A method for providing a memory cell having a very short channel comprising the sequential steps of: (a) providing a junction implant; (b) providing a spacer; and (c) providing a double diffuse implant. 2. The method of Claim 1 wherein the junction implant providing step (a) further comprises the steps of: (al) providing a junction implant; and (a2) providing a junction anneal. 3. The method of Claim 1 wherein the spacer providing step (b) further comprises: (b 1 ) depositing a spacer; and (b2) providing a spacer etch. 4. The method of Claim 2 wherein the spacer providing step (b) further comprises: (b 1 ) depositing a spacer; and (b2) providing a spacer etch. 5. The method of Claim I further comprising the step of (d) providing a spacer etch. 6. The method of Claim 2 further comprising the step of (d) providing a spacer etch. 7. The method of Claim 1 wherein the memory cell is a FLASH cell and the method further comprises the step of: (d) providing a self aligned source process. 8. The method of Claim 4 wherein the memory cell is a FLASH cell and the method further comprises the step of: (d) providing a self aligned source process. 9. The method of Claim 6 wherein the memory cell is a FLASH cell and the method further comprises the step of: (e) providing a self aligned source process. 10. The method of Claim 1 wherein the junction providing implant step (a) further comprises the step of (al ) providing an arsenic junction implant. 1 1 . The method of Claim 10 wherein the double diffuse implant step (c) further comprises the step of (c 1 ) providing a phosphorus double diffuse implant. 12. The method of Claim 2 wherein the junction implant providing step (a) further comprises the step of (al ) providing an arsenic junction implant. 13. The method of Claim 12 wherein the double diffuse implant step (c) further comprises the step of (c 1 ) providing a phosphorus double diffuse implant. 14. The method of Claim 3 wherein the junction implant providing step (a) further comprises the step of (al ) providing an arsenic junction implant. 15. The method of Claim 14 wherein the double diffuse implant step (c) further comprises the step of (c 1 ) providing a phosphorus double diffuse implant. 16. The method of Claim 5 wherein the junction implant providing step (a) further comprises the step of (al) providing an arsenic junction implant. 17. The method of Claim 16 wherein the double diffuse implant providing step (c) further comprises the step of (c 1 ) providing a phosphorus double diffuse implant. 18. The method of Claim 4 wherein the junction implant providing step (a) further comprises the step of (a 1 ) providing an arsenic junction implant. 19. The method of Claim 18 wherein the double diffuse implant providing step (c) further comprises the step of (c 1 ) providing a phosphorus double diffuse implant. 20 The method of Claim 6 wherein the junction implant providing step (a) furthei comprises the step of (al ) providing an arsenic ]unctιon implant 21. The method of Claim 20 wherein the double diffuse implant providing step (c) further comprises the step of (c 1 ) providing a phosphorus double diffuse implant 22 A method for providing a FLASH cell comprising the sequential steps of (a) providing a self aligned etch, (b) providing a pre-implant oxidation, (c) providing a core source/drain mask and implant, (d) providing a spacer, (e) providing a self aligned source mask, (f) providing a double diffuse implant, (g) providing a self aligned source etch, and (h) providing a self aligned source implant 23. The method of Claim 22 wherein spacer providing step (d) further comprises the sequential steps of: (dl ) depositing a spacer; and (d2) providing a spacer etch. 24. The method of Claim 22 wherein the core source/drain implant of step (c) is arsenic; and the double diffuse implant of step (f) is phosphorus. 25. The method of Claim 23 wherein the core source/drain implant of step (c) is arsenic; and the double diffuse implant of step (f) is phosphorus. 26. A method for providing a FLASH cell comprising the sequential steps of: (a) providing a self aligned etch; (b) providing a pre-implant oxidation; (c) providing a core source/drain mask and implant; (d) depositing a spacer; (e) providing a self aligned source mask; (f) providing a double diffuse implant; (g) providing a spacer etch; (h) providing a self aligned source etch; and (i) providing a self aligned source implant. 27. The method of Claim 26 wherein the core source/drain implant of step (c) is arsenic; and the double diffuse implant of step (f) is phosphorus. 28. A memory cell comprising a gate; a drain; a channel having an effective length; a junction comprising a junction implant and a double diffuse implant; and a spacer located adjacent to the gate, the spacer having a width; the double diffuse implant further comprising a first edge, the first edge being offset by approximately the width of the spacer; the offset thereby ensuring that the double diffuse implant does not substantially shorten the effective length of the channel. 29. The memory cell of Claim 28 wherein the junction implant further comprises an arsenic implant and wherein the double diffuse implant further comprises a phosphorus implant. 30. A memory cell comprising a drain; a gate including a side; a channel including an effective length; a junction comprising a junction implant and a double diffuse implant; and an oxide layer, a portion of the oxide layer being disposed on the side of the gate, the portion of the oxide layer disposed on the side of the gate having a thickness; the double diffuse implant further comprising a first edge, the first edge being offset by approximately the thickness of the portion of the oxide layer disposed on the side of the gate; the offset thereby ensuring that the double diffuse implant does not substantially shorten the effective length of the channel. 31. The memory cell of Claim 30 wherein the junction implant further comprises an arsenic implant and wherein the double diffuse implant further comprises a phosphorus implant. 32. A FLASH EPROM device comprising a drain; a gate; a channel having an effective length; a junction comprising a junction implant and a double diffuse implant; and a spacer located adjacent to the gate, the spacer having a width; the double diffuse implant further comprising a first edge, the first edge being offset by approximately the width of the spacer; the offset thereby ensuring that the double diffuse implant does not substantially shorten the effective length of the channel. 33. The memory cell of Claim 32 wherein the junction implant further comprises an arsenic implant and wherein the double diffuse implant further comprises a phosphorus implant. 34. A FLASH EPROM device comprising a drain, a gate including a side, a channel including an effective length, a junction comprising a junction implant and a double diffuse implant, and an oxide layer, a portion of the oxide layer being disposed on the side of the gate, the portion of the oxide layer disposed on the side of the gate having a thickness; the double diffuse implant further comprising a first edge, the first edge being offset by approximately the thickness of the portion of the oxide layer disposed on the side of the gate; the offset thereby ensuring that the double diffuse implant does not substantially shorten the effective length of the channel. 35. The memory cell of Claim 34 wherein the junction implant further comprises an arsenic implant and wherein the double diffuse implant further comprises a phosphorus implant.

Inventors: CHEN JIAN, HSU JAMES, TANG YUAN, CHEN, JIAN, HSU, JAMES, TANG, YUAN
Applicants: ADVANCED MICRO DEVICES INC, ADVANCED MICRO DEVICES, INC.
Application year: 1997
Main classification: H01L29
IPC codes: H01L21