Method of fabrication a non-volatile semiconductor memory device with shielded single polysilicon gate memory part

Abstract: Process for manufacturing a semiconductor memory device comprising the formation, in a same semiconductor material chip, of at least a first memory cell (18) comprising a MOS transistor (19) with a first gate electrode (21) and a second gate electrode (23) superimposed and respectively formed by definition in a first (12) and a second layer (17) of conductive material, and of at least a second memory cell (1) shielded by a layer (32) of shielding material for preventing the information stored in the second memory cell (1) from being accessible from the outside, said second memory cell (1) comprising a MOS transistor (2) with a floating gate electrode (4) formed simultaneously with the first gate electrode (21) of the first cell (18) by definition of said first layer of conductive material (12). Said layer of shielding material (32) is formed by definition of said second layer of conductive material (17).

Description: The present invention relates to a process for obtaining a semiconductor memory device comprising mass-storage memory cells and shielded memory cells for storing reserved information. It is known that in some applications it is desired that at least part of the information stored in the cells of a semiconductor memory device are not easily accessible from the outside, so to increase the security of the stored information. This is for example the case of memories embedded in Smart Cards. Therefore, it is necessary to provide suitable shielding layers. In particular, as far as electrically programmable non-volatile semiconductor memory devices such as EPROM, Flash EEPROM or EEPROM devices, shielding of the memory matrix, or of a portion thereof wherein reserved information is to be stored is generally obtained by forming a metal layer over the surface to be shielded, to prevent intrusion from the outside. Such metal layer is an additional metal layer other than those necessary to form interconnections, and significantly increases the costs and the manufacturing time of memory devices. It is also known that electrically programmable non-volatile semiconductor memory cells such as EPROM, Flash EEPROM and EEPROM cells can be formed with a single level of polysilicon (SP) or with a double level of polysilicon (DP). An example of single-polysilicon level electrically programmable non-volatile memory cells are single-polysilicon level FLOTOX EEPROM memory cells comprising a sensing transistor with a floating gate and a selection transistor for selecting the cell to be programmed in a group of cells of a matrix. The floating gate is capacitively coupled to an N+ diffusion forming the control gate of the sensing transistor. The floating gate of the sensing transistor and the gate of the selection transistor are formed in a same, unique layer of polysilicon. The cell also comprises, in a portion of its area, a tunnel oxide between the floating gate and the drain of the sensing transistor, wherein the passage of electrons during write and erase operations of the cell. An example of single-polysilicon level EEPROM cell is provided in the European patent application No. EP-A-0471131 in the name of the same Applicant. An example of double-polysilicon level electrically programmable non-volatile memory cells are double-polysilicon level FLOTOX EEPROM memory cells, well known to the skilled person, comprising a floating gate transistor and a selection transistor for selecting the cell to be programmed inside a group of cells of a matrix. The gate of the transistor comprises a floating gate over which a control gate is insulatively disposed, with the interposition of a layer of oxide. The two gates are respectively formed in two different layers of polysilicon. Single-polysilicon level cells have the advantage that they can be fabricated by means of simpler process steps with respect to double-polysilicon level ones, but for a same technological resolution they occupy an area at least twice that of the double-polysilicon level cells, because the single-polysilicon level cells are to be formed with the control gate and the floating gate one beside the other, instead of superimposed. It is apparent that double-polysilicon level cells are more suitable for applications where high integration degrees are desirable. An object of the present invention is to provide a process for obtaining a semiconductor memory device comprising storage memory cells and shielded memory cells for preventing reading from the outside of information contained therein, said process not affected by the drawbacks of the known processes. According to the present invention, such object is achieved by means of a process for manufacturing a semiconductor memory device as defined in claim 1. Thanks to the present invention it is possible to obtain a semiconductor memory device comprising both storage memory cells and shielded memory cells, without additional process steps for forming the shielding layer. In fact, the shield is advantageously obtained using the second level of conductive material by means of which the control gate of the storage memory cell is formed. The features of the present invention will be made apparent by the following detailed description of one embodiment thereof, described as a non-limiting example in the annexed drawings, wherein: Figure 1 is a top-plan view of a single polysilicon level EEPROM memory cell; Figure 2 is a sectional view of the single polysilicon level EEPROM cell along line II-II in Figure 1; Figure 3 is a sectional view of the single polysilicon level EEPROM cell along line III-III of Figure 1; Figure 4 is a top-plan view of a double polysilicon level EEPROM memory cell; Figure 5 is a sectional view of the double polysilicon level EEPROM cell along line V-V in Figure 4; Figures 6 to 13 are schematic views showing, in vertical section and in time sequence, some steps of a process according to the invention for simultaneously forming double polysilicon level EEPROM cells and single polysilicon level EEPROM cells, the latter shielded by means of a second level of polysilicon. With reference to Figures 1 to 3, there is shown a single polysilicon level (SP) FLOTOX EEPROM memory cell 1 comprising a floating-gate sensing transistor 2 and a selection transistor 3 for selecting the cell to be programmed inside a group of cells of a matrix of memory cells. The floating gate 4 of transistor 2 is isolated from a P type substrate 5 by means of a gate oxide layer 6, while source and drain regions 7,8 of transistor 2 are formed by N type implants in the substrate 5. The selection transistor 3 comprises a source region 9 and a drain region 10, both of type N, separated by a channel region over which there is disposed, with the interposition of a gate oxide layer 60, a gate electrode 11. The floating gate 4 of the sensing transistor 2 and the gate 11 of the selection transistor 3 are formed in a first layer of polysilicon 12. The floating gate 4 of transistor 2 is superimposed over an N+ diffusion 13 formed in substrate 5, so as to be capacitively coupled therewith; over the diffusion 15 there is disposed a thin oxide 14 called tunnel oxide, that forms the region through which electrons pass during write and erase operations of the cell. The floating gate 4 of the sensing transistor 2 is also capacitively coupled with an N+ diffusion 15, forming the control gate of the cell, over which there is disposed an oxide layer 160 thicker than the tunnel oxide 14. Over the whole surface of the cell 1 there is formed a layer of intermediate dielectric 16, and the whole cell 1 is then covered by a second polysilicon layer 17, and if desired by a layer of silicide 29, so as to form a shield 32 preventing the information stored in the cell 1 from being accessible from the outside. In Figure 1 the region over which there is provided the shield 32 is indicated by dash-and-dot line. With reference to Figures 4 and 5, there is shown a double polysilicon level (DP) FLOTOX EEPROM memory cell 18 formed in the same P type substrate 5 and comprising a floating gate transistor 19 and a selection transistor 20 for selecting the cell to be programmed inside a group of cells of the matrix of memory cells. The floating gate 21, formed in the first level of polysilicon 12, is formed on a gate oxide layer 600 having a thinner tunnel oxide portion 22 through which electrons pass during write and erase operations of the cell 18. A control gate 23, formed by means of the second level of polysilicon 17, is insulatively disposed above the floating gate 21 with the interposition of the intermediate dielectric layer 16. Source and drain regions 25, 24 are formed by means of N type dopant implants in the substrate 5. Similarly, an N+ region 260 is formed at the drain 24, extending under the tunnel oxide 22. The selection transistor 20 comprises an N type source region coincident with the drain region 24 of transistor 19, and a drain region 25' formed in the P type substrate 5. The region of the substrate 5 comprised between the drain region 25' and the source region 26 forms a channel region of transistor 19, over which there is insulatively disposed (by the interposition of a gate oxide layer 600') a gate electrode. Conventionally, the gate electrode comprises two polysilicon electrodes 26, 27 superimposed on each other respectively formed in the first and second level of polysilicon 12, 17 with the interposition of the intermediate dielectric layer 16; electrodes 26, 27 are electrically short-circuited in regions of the matrix not shown in the drawings (short-circuited double polysilicon or "DPCC"). With reference to Figures 6 to 13, there are described the steps of a process according to the invention for the simultaneous manufacturing of a single polysilicon level memory cell 1 and a double polysilicon level memory cell 18. The process steps providing for the formation of the N+ diffusion 15 forming the control gate of the SP EEPROM cell 1 and the N+ region 260 of the DP EEPROM cell 18 are not described in detail because conventional. On the substrate 5 there is grown a gate oxide layer 70 forming the gate oxide 6, 60, 600 and 600' of the transistors of cells 1 and 18, and the gate oxide of the transistor of the circuitry (not shown). Then, using a conventional photolithographic technique, the oxide layer 70 is selectively removed and the tunnel oxide 14, 22 for transistors 2 and 19 of the cells 1 and 18 is grown (Figure 6). In the following steps of the process the first level of polysilicon 12 is deposited and doped by ion implantation (Figure 7). As shown in Figure 8, using a photolithographic technique providing for the deposition of a photoresist layer 28 and a selective etch thereof, the floating gate 4 of the sensing transistor 2 and the gate 11 of the selection transistor 3 of cell 1 are defined in the first level of polysilicon 12. The first level of polysilicon is not removed from over the cell 18 (Figure 9). Still referring to Figure 9, in the following step the photoresist layer 28 is used to perform an N- implant for forming connecting regions for the sensing transistor 2, and the source 9 and drain 10 of the selection transistor 3 of cell 1. In particular, this step can be accomplished according to the known Lightly Doped Drain (LDD) technique for obtaining regions with low dopant concentration. The intermediate dielectric layer 16 is then formed over the whole surface of cells 1 and 18 (Figure 10); conventionally, layer 16 will be removed from the regions where the transistors of the circuitry will be formed. The second level of polysilicon 17 is then deposited. Preferably, over the second level of polysilicon a layer of silicide 29 is formed (Figure 11). Still referring to Figure 11, another layer of photoresist 30 is deposited, and layer 30 is selectively removed for forming a mask for the etch of layers 29 and 17. In this way there are defined the shield 32 for the SP EEPROM cells 1 and the control gate 23 of cells 18. Obviously, the layout of shield 32 can be designed according to the necessity, provided that the areas wherein drain and source contacts for cells 1 are to be formed are left uncovered. In a preferred embodiment, the shield 32 can be contacted by lines with low resistivity, and biased to reduce the capacitive couplings in the matrix. As shown in Figure 12, using a mask comprising a double layer of photoresist formed by layer 31 and an overlying layer 31, a selective etch of the intermediate dielectric layer 16, the first level of polysilicon 12 and the gate oxide layer 70 for forming the gate of transistors 19 and 20 of cell 18 (Figure 13). The following steps conventionally provide for forming spacers 32, performing drain 24 and source 25, 25' implants of cell 18, forming contacts, forming metallization lines, and final passivating the whole matrix. In the previous description reference has been made to double level of polysilicon EEPROM cells, used to store a large amount of data, and single polysilicon level EEPROM cells shielded by means of the second level of polysilicon for storing reserved information. However, it is to be pointed out that the present invention allows in general to obtain memory devices integrating in a same semiconductor chip double polysilicon level electrically programmable non-volatile memory cells, either EPROM, Flash-EEPROM or EEPROM, and single polysilicon level electrically programmable non-volatile memory cells, either EPROM, Flash-EEPROM or EEPROM, over which a shield is formed by means of the second level of polysilicon for preventing the information stored therein from being accessible from the outside. The modifications to the process previously described are straightforward for the skilled person: for example, in the case both the double polysilicon level cells and the single polysilicon level cells are EPROM cells, it is not necessary to selectively etch the oxide layer 70 and the growth of the tunnel oxides 14, 22, and it is not necessary to implant dopants to form N+ regions 13, 260 under the tunnel oxides. There is thus obtained a process for forming a semiconductor memory device comprising double polysilicon level storage memory cells and single polysilicon level shielded memory cells, wherein the shield is formed by means of the second level of polysilicon provided for the formation of the double polysilicon level cells.

Claims: Process for manufacturing a semiconductor memory device, comprising: forming in the same semiconductor material chip of at least one first memory cell (18) which comprises a MOS transistor (19) by forming a first gate electrode (21) in a first layer (12) of conductive material and a second gate electrode (23) in a second layer (17) of conductive material which is superimposed on the first layer (12) of conductive material; forming in the chip at least one second memory cell (1) having a control gate (15) and a floating gate electrode (4) that are both shielded by a layer (32) of shielding material preventing information stored in the second memory cell (1) from being accessible from the outside; wherein said floating gate electrode (4) is formed simultaneously with the first gate electrode (21) of the first memory cell (18) by definition of the first layer (12) of conductive material, and in that said layer (32) of shielding material is formed by definition of said second layer (17) of conductive material. Process according to claim 1, characterized in that said first (12) and second layer (17) of conductive material comprise each a layer of polysilicon. Process according to claim 2, characterized in that said first cell (18) is a double polysilicon level EPROM memory cell. Process according to claim 2, characterized in that said first cell (18) is a double polysilicon level Flash EEPROM memory cell. Process according to claim 2, characterized in that said first cell (18) is a double polysilicon level EEPROM memory cell. Process according to any one of claims 3 to 5, characterized in that said second cell (1) is a single polysilicon level EPROM memory cell. Process according to any one of claims 3 to 5, characterized in that said second cell (1) is a single polysilicon level Flash EEPROM memory cell. Process according to any one of claims 3 to 5, characterized in that said second cell (1) is a single polysilicon level EEPROM memory cell. Process according to any one of the preceding claims, characterized in that said layer of shielding material (32) has a variable shape. Process according to any one of the preceding claims, characterized in that said layer of shielding material (32) is contacted and biased by low-resistance lines.

Inventors: BOTTINI ROBERTA, DALLA LIBERA GIOVANNA, PIO FEDERICO, VAJANA BRUNO, BOTTINI, ROBERTA, DALLA LIBERA, GIOVANNA, PIO, FEDERICO, VAJANA, BRUNO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1997
Main classification: H01L21
IPC codes: H01L21, H01L27, H01L29