A SEMICONDUCTOR DEVICE HAVING A SELF-ALIGNED P-WELL WITHIN A P-BURIED-LAYER

Abstract: A self-aligned, maskless method of forming a p-well is accomplished by implanting a fast p-type diffuser into a p+ buried layer, prior to epitaxial growth. This way the p-well is self-aligned with the p+ buried layer because one mask is used to define both, and the need for a separate p-well mask is eliminated. The fast diffuser, such as aluminum (Al), diffuses toward the silicon surface during the various thermal steps of the process such as epitaxial growth, field oxidation, sinker diffusion, and final drive-in, etc. The dosage of aluminum used and the parameters of the thermal drive determines the p-well doping level. Similarly, an n-well can also be formed in an n-type buried layer in a self-aligned fashion.

Description: A SEMICONDUCTOR DEVICE HAVING A SELF-ALIGNED P-WELL WITHIN A P-BURIED-LAYER Field of the Invention The method of fabrication and resulting structure as described in the present invention may be used to realize high-frequency and low noise fully complementary Bipolar processes (vertical NPN and PNP transistors in particular). The processes can also be used in CMOS technologies employing single or dual wells and single or dual buried layers to suppress device latch-up. Backeround of the Invention The general concept of forming p-wells in a buried layer is well known in the prior art. According to such methods, a p-type buried layer was formed in a p-type substrate (or wafer) by using a mask, and then an epitaxial layer was grown over the p-type buried layer. Subsequently, an impurity, such as aluminum (Al), was implanted from the top of the substrate and driven down towards the buried layers, to form the p-well. This step required an additional mask and was followed by a step of thermal diffusion of the implanted impurity. Summary of the Invention The present invention avoids having to use an additional mask step while providing additional advantages by using a self-aligned, mask-less method for forming a p-well. According to this method, a fast p-type diffuser is implanted into a p+ buried layer, prior to epitaxial growth. This way the p-well is self-aligned with the p+ buried layer because the same mask is used to define both, and the need for a separate p-well mask is eliminated. The fast diffuser, which may be aluminum (Al), will diffuse toward the silicon surface during the various thermal steps of the process such as epitaxial growth, field oxidation, sinker diffusion, and final drive-in, etc. The dosage of aluminum used and the parameters of the thermal drive will determine the p-well doping level. Thus, the doping level of the p-well can be easily controlled. In instances where an n-well is used to separate the p+ buried layer from the p- substrate, compensation for the n-well is non-problematic so long as the p-well implant dose is less than the p+ buried layer implant dose. Brief Description of the Drawings Figure 1 is an illustration of a structure according to the present invention after the buried layer is implanted using a mask. Figure 2 is an illustration of the structure of Figure 1 after the p-well is implanted. Figure 3 is an illustration of the structure of Figure 2 after the removal of the photoresist mask, etching of an oxide layer and growth of an epitaxial layer. Figure 4 is an illustration of the structure of Figure 3 after pad oxide growth, nitride deposition and masking, and field oxidation steps. Figure 5 is an illustration of the cross-sectional doping profile after epitaxial growth. Figure 6 is an illustration of the cross-sectional doping profile at the end of the process. Detailed Description of the Preferred Embodiments The preferred embodiments of the present invention will now be discussed in more detail in connection with Figures 1-6. The method of fabrication as shown in Figures 1-4 will be discussed first. A p-type substrate 10 is shown in Figure 1. An n-well 15 is formed in substrate 10 using conventional techniques. A layer of pad oxide 20 is grown over substrate 10 and a photoresist mask 25 is patterned over the layer of pad oxide 20. Growth of pad oxide layer 20 is optional. Mask 25 is used - 1 - to define the location of buried layer 30, which in this case is p-type. It is to be understood that although the formation of a p-type buried layer is shown and described, an n-type buried layer could instead be formed. Buried layer 30 is formed by implanting a dopant by using the photoresist mask 25. In this case, where a p-type buried layer is desired, boron (B) can implanted preferably to a depth of .1 to .5 microns. This can be accomplished by using a dosage between 5 x 10 and 2 x 10 atoms/cm . A dosage of 2 x 10 atoms/cm is preferred. The energy level should be between 60 and 220 keV with 140 keV being the preferable amount. Once the buried layer 30 has been formed, p-well 35 can be implanted as shown in Figure 2. In this preferred embodiment, aluminum (Al) is implanted into buried layer 30 using the same photoresist mask 25. The Al can be implanted to any depth in the substrate, but a depth between .05 and .2 microns is preferred. The Al should be implanted at a dosage between 1 x 10 13 and 5 x 101 atoms/cm 1 and at an energy level between 30 keV and 160 keV. Preferably, the implanting is performed at a dosage of 2 x 10 atoms/cm and at 50keV. Thus, the aluminum is implanted at a lower energy level than is boron. This is done in order to increase p-well thickness for a fixed sinker drive, which is discussed later in more detail. After p-well 35 has been implanted a rapid thermal anneal (RTA) step is performed for 10 to 120 seconds at 1000-1200 degrees Celsius. Preferably, RTA is performed for 20 seconds at 1100 degrees Celsius. Next, as shown in Figure 3, photoresist 25 is removed, oxide layer 20 is etched away and an n- type epitaxial layer 40 is grown over the substrate 10. Oxide layer 20 may be etched using such chemistries as CF 4 + H 2 , CHF3 + 0 2 , CHF3 - --2 F 6' or CHF3 + CF 4 . Epitaxial layer 40 is preferably grown to a thickness of 0.5-10 microns. During this step some diffusion of boron and aluminum occurs. Then, as shown in Figure 4, a layer of pad oxide 42 is grown over epitaxial layer 40. Subsequently, nitride layer 45 is deposited over pad oxide layer 42 and then masked and etched in order to define active regions of epitaxial layer 40. The regions of epitaxial layer 40 not covered by nitride layer 45 are subjected to localized oxidation of silicon (LOCOS) forming oxide regions such as 44 (of course, other oxidation methods may also be used). Region 44 is preferable grown to a thickness of 1 micron. The LOCOS step is done simultaneously with a sinker drive step in which a collector region (not shown) is formed. Sinker drive is preferably performed at 950 degrees Celsius for approximately 560 minutes. It is during this sinker drive step that most of the p-well diffusion occurs, resulting in the structure shown in Figure 4. As is apparent from the figure, aluminum is a faster diffuser than boron and therefore diffuses a greater distance. The extent to which the diffusion occurs is shown in Figures 5 and 6. Figure 5 shows the amount of diffusion that results after epitaxial layer 40 is grown. Figure 6 shows how much diffusion has taken place after the sinker drive step has been performed. As these experimental results show, p-well 42 has extended to a depth of approximately 4.5 microns in substrate 10, while simultaneously approaching the surface of epitaxial layer 40. If a flat p-well profile is desired, a sufficient thermal drive should follow growth of epitaxial layer 40. Also, in some cases, having the gradient of the p-well profile reach the surface of epitaxial layer 40 is helpful in improving device performance. For example, in bipolar processes, collector resistance R ς will be reduced without sacrificing Early voltage. As noted earlier, an n-well can be formed together with a n-type buried layer by using a fast diffusing impurity (such as phosphorous) for the n-well implant, and a slow diffusing implant (such as arsenic) for the n-type buried layer implant. Although the present invention has been described with reference to the preferred embodiments disclosed, one of ordinary skill in the art would be enabled by this disclosure to make various modifications to the described embodiments and still be within the scope and spirit of the present invention as claimed below. - 3 - SUBSmWE SH

Claims: What is Claimed is: 1. A method for forming a well within a buried layer of a semiconductor substrate comprising the steps of: aligning a photoresist mask upon the substrate; implanting a first dopant into the substrate using the mask to form a buried layer; implanting a second dopant into the substrate using the mask; removing the photoresist mask; growing an epitaxial layer over the substrate; growing a pad oxide layer on top of the epitaxial layer; depositing a nitride layer on top of the pad oxide layer; masking and etching the nitride layer; and performing a sinker drive. 2. The method according to claim 1, wherein the well and buried layer are p-type, and the first dopant is boron, the boron being implanted at a dosage between 5 x 10 and 2 x 10 atoms/cm . 3. The method according to claim 2, wherein the second dopant is aluminum. 4. The method according to claim 3, wherein the steps of growing an epitaxial layer and performing a sinker drive cause the second dopant to diffuse, thereby forming a p-well. 5. The method according to claim 4, wherein the step of performing the sinker drive is combined with a localized oxidation of silicon (LOCOS) step which produces field oxide regions. 6. The method according to claim 1, wherein the well and buried layer are n-type and the first dopant is arsenic. 7. The method according to claim 6, wherein the second dopant is phosphorous. 8. The method according to claim 7, wherein the steps of growing an epitaxial layer and performing a sinker drive cause the second dopant to diffuse, thereby forming an n-well. 9. The method according to claim 8, wherein the step of performing the sinker drive is combined with a localized oxidation of silicon step which produces field oxide regions. 10. A method for forming a well within a buried layer of a semiconductor substrate comprising the steps of: aligning a photoresist mask upon the substrate; implanting a first dopant into the substrate using the mask to form a buried layer; implanting a second dopant into the substrate using the same photoresist mask; growing an epitaxial layer over the substrate; performing a sinker drive such that the second dopant diffuses sufficiently to form the well within the substrate. 1 1. The method according to claim 10, wherein the well and buried layer are p-type and the first dopant is boron. 12. The method according to claim 1 1, wherein the second dopant is aluminum. - 4 - LE 26 13. The method according to claim 12, wherein the step of implanting a second dopant results in the second dopant being implanted into the buried layer. 14. The method according to claim 10, wherein the well and buried layer are n-type and the first dopant is arsenic. 15. The method according to claim 14, wherein the second dopant is phosphorous. 16. The method according to claim 15, wherein the step of implanting a second dopant results in the second dopant being implanted into the buried layer. 17. A semiconductor device produced in accordance with the method of claim 10. - 5 -

Inventors: BASHIR RASHID, CHEN DATONG, DE SANTIS JOSEPH A, BASHIR, RASHID, CHEN, DATONG, DE SANTIS, JOSEPH, A.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1995
Main classification: H01L21
IPC codes: H01L21