Use of an etch to reduce the thickness and around the edges of a resist mask during the creation of a memory cell

[0002] This application claims the benefit under 35 U.S.C. § 119(e) of the United States provisional application Ser. No. 60/189,724, filed on Mar. 16, 2000.

[0003] Related subject matter is disclosed in commonly-assigned, co-pending patent applications entitled “PROCESS FOR CREATING A FLASH MEMORY CELL USING A PHOTORESIST FLOW OPERATION” and “USE OF AN ETCH TO ROUND THE EDGES OF A RESIST MASK DURING THE CREATION OF A MEMORY CELL”, both cases filed on even date herewith.

[0004] This invention relates generally to non-volatile memory devices. In particular, the present invention relates to a method and process for manufacturing a non-volatile memory device.

[0005] Non-volatile memory devices are currently in widespread use in electronic components that require the retention of information when electrical power is terminated. Non-volatile memory devices include read-only-memory (ROM), programmable-read-only memory (PROM), erasable-programmable-read-only memory (EPROM), and electrically-erasable-programmable-read-only-memory (EEPROM) devices. EEPROM devices differ from other non-volatile memory devices in that they can be electrically programmed and erased. Flash EEPROM devices are similar to EEPROM devices in that memory cells can be programmed and erased electrically. However, Flash EEPROM devices enable the erasing of all memory cells in the device using a single electrical current pulse.

[0006] Typically, an EEPROM device includes a floating-gate electrode upon which electrical charge is stored. The floating-gate electrode overlies a channel region residing between source and drain regions in a semiconductor substrate. The floating-gate electrode together with the source and drain regions forms an enhancement transistor. By storing electrical charge on the floating-gate electrode, the threshold voltage of the enhancement transistor is brought to a relatively high value. Correspondingly, when charge is removed from the floating-gate electrode, the threshold voltage of the enhancement transistor is brought to a relatively low value. The threshold level of the enhancement transistor determines the current flow through the transistor when the transistor is turned on by the application of appropriate voltages to the gate and drain. When the threshold voltage is high, no current will flow through the transistor, which is defined as a logic 0 state. Correspondingly, when the threshold voltage is low, current will flow through the transistor, which is defined as a logic 1 state. Data resides in a certain logic state on the floating-gate electrode. During a read operation, selected data from a selected floating-gate electrode can be output to an external communication unit using a bit-line.

[0007] In a flash EEPROM device, electrons are transferred to a floating-gate electrode through a dielectric layer overlying the channel region of the enhancement transistor. The electron transfer is initiated by either hot electron injection, or by Fowler-Nordheim tunneling. In either electron transfer mechanism, a voltage potential is applied to the floating-gate by an overlying control-gate electrode. The control-gate electrode is capacitively coupled to the floating-gate electrode, such that a voltage applied on the control-gate electrode is coupled to the floating-gate electrode. The flash EEPROM device is programmed by applying a high positive voltage to the control-gate electrode, and a lower positive voltage to the drain region, which transfers electrons from the channel region to the floating-gate electrode. The flash EEPROM device is erased by grounding the control-gate electrode and applying a high positive voltage through either the source or drain region of the enhancement transistor. Under erase voltage conditions, electrons are removed from the floating-gate electrode and transferred into either the source or drain regions in the semiconductor substrate.

[0008] Product development efforts in EEPROM device technology have focused on increasing the programming speed, lowering programming and reading voltages, increasing data retention time, reducing cell erasure times and reducing cell dimensions. Many of the foregoing research goals can be addressed through development of materials and processes for the fabrication of the floating-gate electrode. Recently, development efforts have focused on dielectric materials for fabrication of the floating-gate electrode. Silicon nitride in combination with silicon dioxide is known to provide satisfactory dielectric separation between the control-gate electrode and the channel region of the enhancement transistor, while possessing electrical characteristics sufficient to store electrical charge.

[0009] One important dielectric material for the fabrication of the floating-gate electrode is an oxide-nitride-oxide (ONO) layer. During programming, electrical charge is transferred from the substrate to the silicon nitride layer in the ONO layer. Voltages are applied to the gate and drain creating vertical and lateral electric fields, which accelerate the electrons along the length of the channel. As the electrons move along the channel, some of them gain sufficient energy to jump over the potential barrier of the bottom silicon dioxide layer and become trapped in the silicon nitride layer. Electrons are trapped near the drain region because the electric fields are the strongest near the drain. Reversing the potentials applied to the source and drain will cause electrons to travel along the channel in the opposite direction and be injected into the silicon nitride layer near the source region. Because silicon nitride is not electrically conductive, the charge introduced into the silicon nitride layer tends to remain localized. Accordingly, depending upon the application of voltage potentials, electrical charge can be stored in regions within a single continuous silicon nitride layer.

[0010] Non-volatile memory designers have taken advantage of the localized nature of electron storage within a silicon nitride layer and have designed memory devices that utilize two regions of stored charge within an ONO layer. This type of non-volatile memory device is known as a two-bit EEPROM. The two-bit EEPROM is capable of storing twice as much information as a conventional EEPROM in a memory array of equal size. A left and right bit is stored in physically different areas of the silicon nitride layer, near left and right regions of each memory cell. Programming methods are then used that enable two-bits to be programmed and read simultaneously. The two-bits of the memory cell can be individually erased by applying suitable erase voltages to the gate and to either the source or drain regions.

[0011] While the recent advances in EEPROM technology have enabled memory designers to double the memory capacity of EEPROM arrays using two-bit data storage, numerous challenges exist in the fabrication of material layers within these devices. In particular, fabricating the p-type and n-type regions within a memory cell presents several challenges. Sometimes, in the fabrication of a memory cell, an ONO layer is formed having a first silicon dioxide layer overlying the semiconductor substrate, a silicon nitride layer overlying the first silicon dioxide layer, and a second silicon dioxide layer overlying the silicon nitride layer. A layer of photoresist is then spun on the ONO layer. The photoresist is patterned into a resist mask and the semiconductor substrate is doped with a p-type dopant such as boron using ion implantation at a large angle of incidence relative to the principal surface of the semiconductor substrate to allow the p-type implant to be located away from a subsequent n-type dopant. The wafer is then rotated 180° and the semiconductor substrate is doped a second time with a p-type dopant using ion implantation at a large angle of incidence relative to the principal surface of the semiconductor substrate. Doping the semiconductor substrate with a p-type dopant creates p-type regions. The semiconductor substrate is then doped with an n-type dopant such as arsenic using ion implantation at an angle substantially normal to the principal surface of the semiconductor substrate. Doping the semiconductor substrate with n-type dopants creates n-type regions. Typically, the ONO layer is etched before the semiconductor substrate is doped with n-type dopants in order to make the implant of n-type dopants a more controlled implant. Once the n-type dopants have been implanted in the semiconductor substrate, the resist mask is stripped and cleaned from the ONO layer and a bit-line oxide region is thermally grown onto the semiconductor substrate.

[0012] There are several problems that occur with the above-described prior art method for fabricating a memory cell. One problem is that the resist mask has to meet two conflicting requirements: the resist mask has to be thin enough to accommodate the large angle of incidence of the p-type implant, and yet the resist mask has to be thick enough to withstand the n-type implant. If the resist mask is too thick, the p-type implant must be performed with a smaller angle of incidence, however if the resist mask is too thin the n-type implant cannot be performed at all because the resist mask would have been too heavily degraded. Accordingly, advances in memory cell fabrication technology are necessary to insure patterning of high density memory cells used in two-bit EEPROM devices.

[0013] The present invention is for a process for fabricating a memory cell in a non-volatile memory device, preferably in a two-bit EEPROM device. Fabrication of a two-bit EEPROM device having a memory cell requires the formation of p-type regions and n-type regions with good critical dimension control. This is because proper functionality of the two-bit EEPROM device during a programming operation requires voltages to be applied to the p-type regions and n-type regions. In particular, the p-type regions must be positioned at the edges of the ONO layer for fabrication of high density devices. However, fabrication of high density devices with p-type regions positioned at the edges of the ONO layer is hard to obtain due to the limitations of the resist mask. By reducing the thickness of the resist mask and rounding the edges of the resist mask, a high-density two-bit EEPROM device with good critical dimensions control can be manufactured.

[0014] In one form, a process for fabricating a memory cell includes providing a semiconductor substrate and forming an ONO layer over the semiconductor substrate. A layer of photoresist is then deposited overlying the ONO layer and patterned into a resist mask. The resist mask is thick enough to withstand an n-type implant. The semiconductor substrate is then doped with an n-type dopant such as arsenic, preferably by using ion implantation. The doping of the semiconductor substrate with an n-type dopant causes n-type regions to form in the semiconductor substrate. Preferably, the n-type implant is a direct implant, which is an implant at an angle substantially normal with respect to the principal surface of the semiconductor surface. After doping the semiconductor substrate with n-type dopants, the resist mask is then etched to reduce the thickness of the resist mask and round the edges of the resist mask. Once the resist mask is etched, the semiconductor substrate is doped with p-type dopants such as boron, preferably by using ion implantation. The p-type implant is an angled implant, which is an implant at an angle substantially acute with respect to the principal surface of the semiconductor substrate. The doping of the semiconductor substrate with p-type dopants causes p-type regions to form in the semiconductor substrate. After doping the semiconductor substrate with p-type dopants, the resist mask is removed and the bit-line oxide region is formed. The rounded edges and reduced thickness of the resist mask allow for a more angled implant. The more angled implant allows for the fabrication of a memory cell with tighter critical dimensions. In one preferred embodiment, the resist mask is etched to reduce the thickness of the resist mask and round the edges of the resist mask, before the doping the semiconductor substrate with n-type dopants and p-type dopants.

[0015] FIG. 1 illustrates, in cross-section, a portion of a memory cell containing p-type regions, in accordance with one preferred embodiment of the invention; and

[0016] FIGS. 2-6 illustrate, in cross-section, process steps for the fabrication of a memory cell, in accordance with one preferred embodiment of the invention.

[0017] It should be appreciated that for simplicity and clarity of illustration, elements shown in the Figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other for clarity. Further, where considered appropriate, reference numerals have been repeated among the Figures to indicate corresponding elements.

[0018] Shown in FIG. 1, in cross-section, is memory cell 20 suitable for use in a two-bit EEPROM device. Please note that while all the FIGS. 1-6, illustrate only one memory cell 20, a two-bit EEPROM device may have many more memory cells identical to memory cell 20. For the sake of clarity, these additional memory cells have been left out of FIGS. 1-6.

[0019] Memory cell 20 includes p-type regions 44 and n-type regions 46 located in semiconductor substrate 22 and separated by channel region 23, as illustrated in FIG. 1. Bit-line oxide region 50 overlies p-type regions 44 and n-type regions 46, respectively. Oxide-nitride-oxide (ONO) layer 24 includes first silicon dioxide layer 26 overlying channel region 23. Silicon nitride layer 28 overlies first silicon dioxide layer 26. Second silicon dioxide layer 30 overlies silicon nitride layer 28. In the operation of memory cell 20, data resides in a certain logic state on memory cell 20. During a read operation, selected data from a selected memory cell 20 can be output to an external communication unit by using n-type regions 46. Those skilled in the art will recognize that in order to create higher density memory cells 20, p-type regions 44 and n-type regions 46 must be formed with great accuracy. In order to form p-type regions 44 with great accuracy, the resist mask has to be thin enough to accommodate the large angle of incidence of the p-type implant, and yet the resist mask has to be thick enough to withstand the n-type implant.

[0020] Referring to FIG. 2, ONO layer 24 is formed overlying the surface of semiconductor substrate 22. Preferably, semiconductor substrate 22 is a single crystal silicon substrate. Semiconductor substrate 22 has principal surface 38 previously processed and cleaned to remove debris and native oxides. Preferably, ONO layer 24 comprises first silicon dioxide layer 26, silicon nitride layer 28, and second silicon dioxide layer 30. First silicon dioxide layer 26 is formed overlying the semiconductor substrate 22. Preferably, first silicon dioxide layer 26 has a thickness of about 50 to about 150 angstroms and, more preferably, a thickness of about 80 angstroms. After forming first silicon dioxide layer 26, silicon nitride layer 28 is deposited overlying first silicon dioxide layer 26, as illustrated in FIG. 2. Preferably, silicon nitride layer 28 is formed by reacting ammonia NH3 with either dichlorosilane (SiCl₂H₂) or silane (SiH₄). Preferably, silicon nitride layer 28 is deposited overlying first silicon dioxide layer 26 having an initial thickness of about 120 to about 150 angstroms and, more preferably, an initial thickness of about 135 angstroms. After depositing silicon nitride layer 28, second silicon dioxide layer 30 is formed overlying silicon nitride layer 28, as illustrated in FIG. 2. As second silicon dioxide layer 30 is formed overlying silicon nitride layer 28, silicon nitride layer 28 decreases in thickness to a final thickness of about 50 to about 80 angstroms and; more preferably, a final thickness of about 60 angstroms. Preferably, second silicon dioxide layer 30 has a thickness of about 50 to about 150 angstroms and, more preferably, a thickness of about 80-95 angstroms.

[0021] Once ONO layer 24 has been formed, photoresist layer 36 is deposited overlying ONO layer 24. Photoresist layer 36 comprises any known photoresist.

[0022] Photoresist layer 36 must be thick enough to withstand the doping of semiconductor substrate 22 with n-type dopants. Preferably, photoresist layer 36 has a thickness of about 4,000 to about 10,000 angstroms and, more preferably, a thickness of about 5,000-6,000 angstroms. After photoresist 36 has been deposited, photoresist layer 36 is patterned into resist mask 34, as illustrated in FIG. 3. Preferably, photoresist layer 36 is exposed to a light source, such as an ultra-violet light source, which allows for the patterning of photoresist layer 36 into resist mask 34. Resist mask 34 has sharp edges 35, as illustrated in FIG. 3.

[0023] Once photoresist layer 36 has been patterned, semiconductor substrate 22 is then doped with an n-type dopant using resist mask 34 as a doping mask. Preferably, n-type dopant comprises an element selected from the group consisting of antimony, arsenic, and phosphorus. Preferably, semiconductor substrate 22 is doped with arsenic. In one preferred embodiment of this invention, the semiconductor substrate 22 is doped with an n-type dopant, such as arsenic, by using ion implantation. Ion implantation allows for the fabrication of memory cell 20 with higher density and smaller feature sizes. Using resist mask 34 as an ion implant mask, n-type implant 60 is then performed, as illustrated in FIG. 3. The n-type implant 60 is preferably arsenic implanted at a dose of about 1×10¹⁴cm⁻²to about 1×10¹⁶cm⁻²at an implant energy of about 20 KeV to about 80 KeV. Preferably, n-type implant 60 is performed through Ad ONO layer 24, but is masked out by resist mask 34, as illustrated in FIG. 3. The n-type implant is preferably a direct implant performed at an angle substantially normal to the principal surface 38 of the semiconductor substrate 22, such as about a 90° angle, as illustrated in FIG. 3. The n-type implant 60 provides a high breakdown voltage for the drain node, which is needed during cell erasure. The n-type implant 60 also forms the buried bit-line, which is a sub-surface conductive channel that carries a signal to each bit in the memory cell 20.

[0024] Once semiconductor substrate 22 has been doped with an n-type dopant, forming n-type regions 46, the thickness 72 of resist mask 34 is reduced and the edges 35 of resist mask 34 are rounded, as illustrated in FIG. 4. In one preferred embodiment, the thickness 72 of resist mask 34 is reduced and the edges 35 of resist mask 34 are rounded before semiconductor substrate 22 has been doped with an n-type dopant. Preferably, the thickness 72 of resist mask 34 is reduced and the edges 35 of resist mask 34 are rounded by performing a resist flow operation onto semiconductor substrate 22, by applying an etch to resist mask 34, or by any other process or method known by one of ordinary skill in the art for rounding edges 35 and reducing the thickness 72 of resist mask 34.

[0025] In one preferred embodiment, an etch is applied to resist mask 34, as illustrated in FIG. 4. Preferably, the etch is a anisotropic etch, however the etch may also be an isotropic etch, a plasma etch, ion milling, a reactive ion etch, a wet etch, or any other type of etch known to one of ordinary skill in the art. The etch is applied to reduce the thickness 72 of resist mask 34 and round the edges 35 of resist mask 34. The thickness 72 of resist mask 34 is defined as the distance from top surface 68 of resist mask 34 to bottom surface 70 of resist mask 34, as illustrated in FIGS. 3 and 4. In one preferred embodiment, the thickness 72 of resist mask 34 is reduced about 5% to about 75% and more preferably about 25%. The percentage of which the thickness 72 of resist mask 34 is reduced, and the amount of etch that is applied, depends on the dose of p-type dopant required to create p-type regions 44.

[0026] In one preferred embodiment, a blanket anisotropic etch is performed to reduce the thickness 72 of resist mask 34 and round the edges of resist mask 34, as illustrated in FIG. 4. In one preferred embodiment, the blanket anisotropic etch is performed using an etch comprising an element selected from the group consisting of nitrogen, hydrogen, chlorine, and helium, since nitrogen, hydrogen, chlorine, and helium will cause the edges of resist mask 34 to become rounded without significant loss in the critical dimensions of memory cell 20. In one preferred embodiment, a plasma etch is performed using an etch comprising a compound selected from the group consisting of helium, oxygen, CHF₃, and argon. In one preferred embodiment, an isotropic etch is performed to reduce the thickness 72 of resist mask 34.

[0027] Applying an etch to resist mask 34 thins down resist mask 34 and causes edges 35 of resist mask 34 to become rounded. The rounded edges 35 and thinness of resist mask 34 allow for a more angled p-type implant 62, as illustrated in FIG. 5. The more angled implant allows for the fabrication of a memory cell 20 with tighter critical dimensions. In one preferred embodiment, an etch is applied to the exposed ONO layer 24 to expose part of semiconductor substrate 22, as illustrated in FIG. 4. Preferably, the etch applied to expose part of semiconductor substrate 22 is applied after the n-type implant 60 is performed.

[0028] Once the thickness 72 of resist mask 34 is reduced and the edges of resist mask 34 are rounded, semiconductor substrate 22 is doped with a p-type dopant, as illustrated in FIG. 5. In one preferred embodiment of this invention, semiconductor substrate 22 is doped with a p-type dopant by using ion implantation. Preferably, p-type dopant comprises boron. Using the same resist mask 34 as an ion implant mask, p-type implant 62 is performed to fix the threshold voltage of the select transistor, as illustrated in FIG. 5. The p-type implant 62 is preferably boron implanted at a dose of about 1×10¹²cm⁻²to about 1×10¹⁴cm⁻²at an implant energy of about 20 KeV to about 80 KeV. In one preferred embodiment, the p-type implant 62 is performed directly onto semiconductor substrate 22, but is masked out by resist mask 34, as illustrated in FIG. 5. Preferably, the p-type implant 62 is an angled implant, that is, an implant at an angle acute with respect to the principal surface of the semiconductor substrate. If the thickness 72 of resist mask 34 is reduced and the edges of resist mask 34 are rounded, p-type implant 62 can be performed at an angle substantially acute with respect to principal surface 38 of the semiconductor substrate 22. Preferably, p-type implant 62 can be performed at an angle of about 0° to about 45° with respect to principal surface 38 of the semiconductor substrate 22, and more preferably at an angle of about 25° with respect to principal surface 38 of the semiconductor substrate is 22.

[0029] Once semiconductor substrate 22 has been doped with n-type and p-type dopants, resist mask 34 is stripped and cleaned from ONO layer 24 and bit-line oxide region 50 is formed overlying the exposed semiconductor substrate 22, as illustrated in FIG. 6. Bit-line oxide region 50 is preferably formed by thermally growing a layer of silicon dioxide into and on top of the exposed semiconductor substrate 22.

[0030] The following example is provided for purposes of illustrating the operating parameters of the etch applied to reduce the thickness 72 and round the edges 35 of resist mask 34, according to one preferred embodiment of the present invention.

[0031] Table 1 and Table 2 list the operating parameter of an etching unit in accordance with one preferred embodiment of this invention. Table 1 and Table 2 illustrate 4 steps, entitled STABLE, MAIN ETCH, STABLE, OVER ETCH, and a DECHUCK process. Operating parameters such as the maximum time for each step (maximum step time), the pressure, and other parameters which may be necessary to operate the etching machine are listed. The chamber temperature and the electrode temperature for the etching unit are two parameters that are constant throughout the entire etching process. In the following example, the chamber temperature and the electrode temperature are both 15° C. While other parameters may be used, these parameters are listed to serve as an example of one possible set out of many possible sets of parameters for performing an etch to ONO layer 24, as described above.

[0032] Thus, there has been disclosed in accordance with the invention, a process for fabricating a memory cell using a resist mask that fully provides the advantages set forth above. Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those. illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications that fall within the scope of the appended claims and equivalents thereof.