Reduction of field edge thinning in peripheral devices

[0001] This application is a division of U.S. patent application Ser. No. 10/180,415, filed Jun. 24, 2002, which is hereby incorporated by reference herein in its entirety.

[0002] This invention relates to reducing gate oxide thinning at the field edge of complimentary metal oxide semiconductor (CMOS) devices and, more particularly, to reducing field edge thinning in peripheral devices.

[0003] Some CMOS devices are fabricated in such a way that a thin gate oxide and a thick gate oxide are grown on the device. In some processes for fabricating these peripheral CMOS devices, the thin gate oxide devices are sometimes stripped with a wet oxide etch prior to gate oxidation. In other processes, the thick gate oxide devices are sometimes stripped with a wet oxide etch prior to gate oxidation. When a device is stripped with a wet oxide etch, the silicon corners at the field edges may be exposed. This results in increased gate oxide thinning at the field edges (e.g., at the exposed corners). This increased thinning may lead to undesirable variability in the electrical parameters of the peripheral devices.

[0004] For example, the increased thinning at the field edges may cause a MOSFET (metal oxide semiconductor field effect transistor) to break down at the corners due to the non-uniformity of the gate oxide. This results in what is sometimes called a threshold kink. That is, a MOSFET may turn on closer to the device's field edges before the rest of the device turns on. When enough voltage is applied to the rest of the gate, the rest of the device may turn on. In essence, this results in two devices in parallel. This variability and other factors may lead to degradation in functionality, yield, and reliability in these devices.

[0005] It would therefore be desirable to be able to provide fabrication processes that reduce field edge thinning in peripheral devices.

[0006] It is an object of this invention to provide fabrication processes that reduce field edge thinning in peripheral devices.

[0007] Some peripheral devices (e.g., CMOS) include a low voltage device and a high voltage device separated by a field oxide or isolation dielectric. Field oxides or isolation dielectrics may also separate these peripheral devices from other devices. The invention is directed to the fabrication of these low and high voltage devices.

[0008] Low voltage and high voltage devices are typically fabricated in silicon. The field oxide may be a dielectric such as silicon dioxide. The fabrication processes of the invention eliminate a wet oxide etch used in other fabrication processes. Eliminating the wet oxide etch reduces the exposure of the silicon corners of the peripheral devices at their field edges. Reducing the exposure of the silicon corners results in more uniform gate oxides for such devices. More uniform gate oxides leads to less variable and more reliable devices.

[0009] The invention uses a dielectric layer as an oxidation and wet oxide etch barrier to prevent exposure of the silicon corners of the thick and thin gate oxide devices. Thinning of the gate oxides at the field edges is therefore reduced. The dielectric layer may be an interpoly dielectric layer, which may be, for example, an oxide-nitride-oxide.

[0010] The dielectric layer is deposited on both the low voltage and high voltage devices of a peripheral device. The low voltage device will ultimately have a thin gate oxide grown over it, while the high voltage device will ultimately have a thick gate oxide grown over it. Generally, the dielectric layer is not removed from a low or high voltage device until that device is ready to be processed. That is, the dielectric layer remains on a device until the process of growing a gate oxide on the device has begun. Masks may be deposited on or placed over the device to selectively etch dielectric layers.

[0011] A portion of the thick gate oxide may be grown on a high voltage device before the thin gate oxide is grown on a low voltage device. That is, the gate oxide for the high voltage device may be the thick gate oxide grown over the high voltage device and an additional layer of oxide grown over the device (e.g., a thin layer of oxide grown over the low and high voltage devices—the thin layer of oxide grown over the low voltage device may be the gate oxide for that device). In other words, the thin gate oxide is grown directly over the thick gate oxide. The thin and thick gate oxides grown over the high voltage device will be the gate oxide for that device.

[0012] When masks are not lined up exactly with the dielectric layers they are intended to cover, a small portion of the dielectric layer may not be removed from the device when oxides are grown. These small portions are sometimes referred to as hedges. Hedges may be removed by exposing a trench in the field oxide near the hedge. Field oxides may be locally exposed by depositing or placing a mask with openings or windows on or over the device. The field oxide is then etched at the hedges which will in turn remove the hedges.

[0022] The invention is directed to the fabrication of peripheral devices. Peripheral devices may be used to implement storage cells in memory devices such as, for example, SRAMs, DRAMs, and memory devices used in flash applications.

[0023] In the fabrication of some peripheral memory devices, a dielectric layer is deposited over the peripheral devices and the memory devices to which the peripheral devices are coupled. The dielectric layer may be an interpoly dielectric layer. Such an interpoly dielectric layer may be, for example, oxide-nitride-oxide or aluminum-oxide. The dielectric is used as an oxidation and wet-oxide etch barrier. In some peripheral device fabrication, the dielectric layer is removed from the entire peripheral device but not the memory device. The dielectric layer is removed from the peripheral device to allow thin and thick gate oxides to be grown over the peripheral device. That is, the low and high voltage devices are exposed before the growth of the thin and thick gate oxides.

[0024] In processes such as these, one device may be stripped more times with a wet oxide etch prior to gate oxidation than the other device (e.g., a low voltage device compared to a high voltage device). This increases exposure of the silicon corners at the field edges resulting in increased gate oxide thinning at the field edges.

[0025] FIGS. 1-6 and 8-20 show peripheral devices having sources and drains perpendicular to face of the page.

[0026] FIGS. 1-6 illustrate a process of fabricating complimentary metal oxide semiconductor (CMOS) field effect transistor (FET) peripheral devices. As illustrated in FIGS. 1-6, a thin gate oxide device (depicted on the left of the FIGS.) may see more wet oxide strips prior to gate oxidation than a thick gate oxide device (depicted on the right of the FIGS.). FIGS. 1-6 illustrate a process that results in the exposure of silicon corners in low voltage devices. Other processes may result in the exposure of silicon corners in high voltage devices.

[0027] Thin gate oxide devices and thick gate oxide devices are sometimes referred to as low voltage and high voltage devices, respectively. The thin and thick gate oxide devices are respectively referred to as such because of the amount of voltage required to turn the devices on. Hereinafter, “low voltage device” and “high voltage device” are used to refer to thin gate oxide and thick gate oxide devices, respectively.

[0028] As mentioned above, a dielectric layer is sometimes deposited over a memory device as well as peripheral devices. FIG. 1 illustrates a peripheral device 10 with a dielectric layer removed from the entire device in order to grow thin and thick gate oxides over the low voltage device 20 and the high voltage device 30, respectively. The dielectric layer is not removed from the rest of the memory device (not shown) in order to protect the rest of the memory device from the processing of the peripheral devices. Devices 20 and 30 are preferably silicon devices. A field oxide 15 separates devices 20 and 30 from each other as well as from other devices. Field oxide 15 may be any suitable oxide such as silicon dioxide.

[0029] As shown in FIG. 2, an oxide layer 40 is grown over peripheral device 10 after the dielectric layer is removed. Oxide layer 40 will be part of the gate oxide for high voltage device 30. The remainder of the gate oxide for high voltage device 30 is grown when the gate oxide for low voltage device 20 is grown. After oxide layer 40 is grown, a mask 50 is laid over device 10 such that low voltage device 20 is not covered, as shown in FIG. 3. Mask 50 may be any suitable mask such as an implant mask. The portion of oxide layer 40 that is not covered by mask 50 is then stripped by a wet oxide etch, as shown in FIG. 4. This wet oxide etch increases the exposure of the silicon corners 60 at the field edges. Mask 50 is then removed, as shown in FIG. 5. Another oxide layer is then grown over devices 20 and 30, as shown in FIG. 6. Oxide layer 70 is grown directly over oxide layer 40. The oxide layers grown over high voltage device 30, (i.e., oxide layers 40 and 70) are the gate oxide for device 30. That is, the thickness of the gate oxide for device 30 is the sum of the thicknesses of oxide layer 40 and oxide layer 70. Oxide layer 70 is the gate oxide for low voltage device 20. In this example, oxide layer 70 is thinner than oxide layer 40. (None of the FIGS. are drawn to scale.)

[0030] FIG. 6 illustrates the increased thinning of oxide layer 70 at the field edges because of the increased exposure of the silicon corners of low voltage device 20. FIG. 7 is a more detailed illustration of FIG. 6. As shown, oxide layer 70 is thinner at silicon corner 60 than at the rest of the device.

[0031] Although the processes of the invention relate primarily to the fabrication of CMOS peripheral devices, these processes may be used to reduce or prohibit gate oxide thinning at field edges in other devices such as, for example, capacitors, other types of field effect transistors, and other related MOS devices.

[0032] The fabrication processes in accordance with the invention reduce the increased gate oxide thinning at the field edge by, among other things, using a dielectric layer as an oxidation and wet oxide etch barrier for the low and high voltage devices. FIGS. 8-17 illustrate an exemplary process of reducing the exposure of silicon corners and the increased gate oxide thinning at field edges. In FIGS. 8-17, a low voltage device 90 is shown on the left, and a high voltage device 100 is shown on the right.

[0033] FIG. 8 illustrates a peripheral device 80 before gate oxides are grown on low voltage device 90 and high voltage device 100. As illustrated in FIG. 9, a dielectric layer 110 is deposited over devices 90 and 100. Dielectric layer 110 may be any suitable dielectric layer that behaves as an oxidation and wet oxide etch barrier. Such a dielectric layer may be an interpoly dielectric layer of, for example, oxide-nitride-oxide or aluminum-oxide. Dielectric layer 110 preferably has a thickness suitable to suppress oxidation and behave as a wet oxide etch barrier. For a dielectric layer 110 of oxide-nitride-oxide, such a thickness may be, for example, about 150 angstroms.

[0034] After dielectric layer 110 is deposited, a mask 120 is deposited on or placed over peripheral device 80 such that the portion of dielectric layer 110 covering high voltage device 100 can be removed, as shown in FIG. 10. As shown in FIG. 11, the exposed portion of dielectric layer 110 is removed. This removal may be accomplished by a dry etch, a wet etch, or a combination of a dry and wet etch. Generally, those portions of a dielectric layer that are covered by a mask (e.g., mask 120) are preferably not removed. That is, when a mask is deposited on or placed over portions of a dielectric layer, those portions are preferably not removed by, for example, a dry or wet etch.

[0035] As illustrated in FIGS. 10 and 11, mask 120 may be an implant mask. Alternatively, mask 120 may be any other suitable type of mask such that portions of dielectric layer 110 may be selectively removed and oxide layers (e.g., oxide layer 130) selectively grown. Mask 120 may be of any suitable material such as a photoresist.

[0036] As illustrated in FIG. 12, mask 120 is removed after the exposed portion of dielectric layer 110 is removed. Oxide layer 130 is then grown over high voltage device 100, as shown in FIG. 13. The portion of dielectric layer 110 covering low voltage device 90 prevents the growth of any oxide over low voltage device 90.

[0037] Oxide layer 130 is preferably a portion of the total gate oxide for high voltage device 100. That is, the fabrication process may subsequently grow more oxide on top of oxide layer 130. Oxide layer 130 in conjunction with additional oxide may serve as the gate oxide for device 100.

[0038] After oxide layer 130 is grown over high voltage device 100, a mask 140 is deposited on or placed over oxide layer 130, as shown in FIG. 14. The portion of dielectric layer 110 protecting low voltage device 90 is then removed, as shown in FIG. 15.

[0039] As illustrated in FIGS. 14 and 15, mask 140 may be an implant mask. Mask 140 alternatively may be any other suitable type of mask that permits portions of dielectric layer 110 to be selectively removed. Mask 140 may be of any suitable material such as a photoresist.

[0040] FIG. 16 shows mask 140 removed. Oxide layer 150 is then grown over low voltage device 90 and high voltage device 100. As illustrated in FIG. 17, oxide layer 150 may be grown directly over oxide layer 130. When oxide layer 150 is grown over oxide layer 130, less oxide will be grown over oxide layer 130 than over silicon. This occurs because the growth of oxide on silicon is greater than the growth of oxide on oxide. For example, if about 30 angstroms are grown on low voltage device 90, only about 20 angstroms of oxide may be grown on high voltage device 100.

[0041] During the fabrication of peripheral devices, masks may not line up exactly with the oxide layers or dielectric layers that they are intended to cover. At times, a mask may unintentionally cover a small portion of a dielectric layer. This is illustrated in FIG. 18. As shown, mask 310 covers a portion of dielectric layer 320. Portion 330 will not be removed when the rest of dielectric layer 320 is removed, and as shown in FIG. 19, portion 330 may remain on the peripheral device and have oxide 332 grown over it.

[0042] Portions such as portion 330 are sometimes referred to as hedges. Hedges are often undesirable because they may lift off in subsequent fabrication processing, unpredictably disrupting the grown oxide layer. Hedges may be controllably removed by exposing trenches in the field oxide under the hedges. To expose a trench in the field oxide, a mask may be deposited on or placed over a device. The mask may have openings that line up with desired trench locations (i.e., where the hedges are located). The field oxide is exposed at those openings, and the mask is then removed.

[0043] As illustrated in FIG. 20, a trench 340 was exposed in field oxide 350 causing portion 330 (i.e., a hedge) to be removed. Hedges may be alternatively removed using any suitable method or technique. (FIGS. 18-20 are not drawn to scale.)

[0044] The fabrication process illustrated in FIGS. 8-17 and 20 is depicted in the flow chart of FIG. 21. At step 2102, a dielectric layer is deposited over the low voltage and high voltage devices. At step 2104, a first mask is then deposited on or placed over the low voltage device. Then, at step 2106, the dielectric layer is removed from the high voltage device. The first mask is then removed from the low voltage device and a first oxide layer is grown on the high voltage device (steps 2108 and 2110).

[0045] Subsequently, a second mask may be deposited on or placed over the high voltage device (step 2112). The dielectric layer is then removed from the low voltage device at step 2114. Then, the second mask is removed from the high voltage device and a second oxide layer is grown on the low voltage device (steps 2116 and 2118). Any hedges that result from this process may be removed by exposing a trench in the field oxide at step 2120.

[0046] FIG. 22 depicts another embodiment of a process that reduces gate oxide thinning at field edges in accordance with the invention. At step 2202, a dielectric layer is deposited over a peripheral device. The peripheral device includes a first device and a second device (e.g., a low voltage device and a high voltage device, MOS devices in parallel, and various other CMOS devices; capacitors, field effect transistors). At step 2204, a first mask is deposited on or placed over the first device of the peripheral device to protect the dielectric from being removed from the first device. At step 2206, the dielectric not protected by the mask is removed. This exposes the second device of the peripheral device. That is, the second device is no longer protected by the dielectric.

[0047] The first mask is then removed from the first device and a first oxide layer is grown on the second device (steps 2208 and 2210). A second mask is then deposited on or placed over the second device (step 2212). The dielectric layer is then removed from the first device at step 2214. Then, the second mask is removed from the second device and a second oxide layer is grown on the first device (steps 2216 and 2218). Any hedges that result from this fabrication process may be removed by exposing a trench in the field oxide at step 2220.

[0048] Thus it is seen that fabrication processes for reducing field edge thinning in peripheral devices are provided. One skilled in the art will appreciate that the invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the invention is limited only by the claims which follow.