INTEGRATED CIRCUIT CONTACT METHOD

Abstract: Shorting together of adjacent, oppositely doped semiconductive regions (23, 21) at the surface of a substrate caused by misalignment of contact windows (W') over portions of both regions is avoided by providing a first set of contact regions (21') of one (p) conductivity type of sufficient width to easily avoid the misalignment caused problem, and a second set of contact regions (21) of the opposite (n) conductivity type and of the minimum possible width (to obtain maximum device density on the substrate) which, owing to their small width, give rise to the misalignment problem. The doping (p type) of the regions (21') of the first set is made higher than the doping (p type) of the regions (23) adjacent to the regions (21) of the first set. Prior to applying the conductive contact materials through the contact windows, impurities are non selectively implanted through the windows at a concentration sufficient to reverse the conductivity type (p) of any portions of the adjacent regions (23) exposed by the misaligned windows (W'), but of insufficient concentration to reverse the conductivity type (p) of the second set of regions (21').

Description: INTEGRATED CIRCUIT CONTACT METHOD Background of the Invention As the size of active impurity regions in semiconductor integrated circuit substrates shrink the size of the electrical contacts to those regions likewise shrink. A limiting factor is the minimum line that can be produced in a given technology. When the impurity region is reduced to that minimum dimension the contact "window" that is made prior to forming the contact cannot, as is customary, be made smaller than the region to be contacted. As a result both features are made with the minimum feature size and the contact will almost certainly be partially non-aligned or offset with the region to be contacted, hence, will contact an adjacent, differently doped region. This is generally undesirable. This situation is commonly referred to as "line-on-line" features, i.e. features that require minimum line on minimum line registration. Whether the alignment is visual or automatic it is not practical to make precise line-on-line features. This difficulty can be overcome by implanting an appropriate impurity into the contact window prior to formi the contact. Thus, even if the contact window overlaps and improperly exposes a surface portion of an adjacent, differ ently doped region, that portion is converted to the conductivity type of the region to be contacted, and shorti between this latter region and its adjacent region is avoid As useful as this solution is, it suffers when applied to CMOS technology. In that case two implants and additional masks are required because p-type impurities are required for the p-channel device windows and n-type impurities for the n-channel device windows. A need remains for a technique for producing line- on-line contacts that can be applied to CMOS as well as other technologies. Statement of the Invention The present invention is based on the recognition that, with careful choice and control of the doping levels in the device, it is possible to dope non-selectively all the contact window exposed portions on the chip in a single implant step, hence convert the conductivity type of the improperly exposed regions of one conductivity type without converting the conductivity type of properly exposed regions of the same conductivity type. A key to achieving this is to have the properly exposed region more heavily doped than the improperly exposed regions. Brief Description of the Drawings PIG. 1 is a schematic representation of a cross section of portions of an integrated CMOS chip at the point in processing showing the problem introduced by offset line-on-line contacts; FIG. 2 is a schematic view similar to that of FIG. 1 further illustrating the proposed solution according to the invention; FIG. 3 shows the device of FIG. 2 with the contact layer in place. Detailed Description The problem that the invention addresses is illustrated in connection with FIG. 1. In FIG. 1, sections of an exemplary CMOS integrated circuit chip are shown. The device on the left of the broken chip section is exemplary of an n-channel device. An n-channel device contact (e.g., to a n + source region 21 of the device) is made line-on-line, which means that the width W of the contact region 21 is approximately the same as the width W' of contact window 22. As shown, the contact window is offset by an amount Δ, which corresponds in this exemplary showing to the alignment tolerance set by the lithographic design rules. The contact may be misaligned less the Δ, so shown here is the worst case. The consequence of the offset is the potential connection between the contact layer 14 that will subsequently be applied (see FIG. 3) and the adjacent region, shown at 23. That results in a short of the p region 23 to the n region 21. In a conventional CMOS pair of devices, the other device of the pair, the p- channel device, is of comparable size, and will exhibit a similar short if the contact (e.g., to a p source region 21') is made line-on-line. It is evident that a non- selective implant through the window W* using impurities of one conductivity type, or the other, for the purpose of converting the conductivity type of the exposed adjacent region will cure one shorting problem but not the other. The solution according to this invention is to design one type of device, in this case the p-channel devices, with contact widths of the order of W + 2Δ. This insures that the offset that creates the shorting problem with the n-channel devices will not create the same problem with the p-channel devices. Shown in FIG. 1 is a p + type region 21* with width W + 2Δ. The contact window, having a width approximately equal to W, will not expose the adjacent n type region 23' and no shorting problem exists. At this point a non-selective ion implant, as shown in FIG. 2, converts the exposed portion of the p type region 23 in the n-channel device contact window to n-type material, thus removing the potential for shorting of the contact to the substrate. Due to careful selection of the respective doping levels between the p-type region 23 and the p-type region 21 ' , and the non-selective implant dose applied at the step represented by FIG. 2, the exposed portion of the region 23 is converted to n-type as desired, but the contact region 21' remains p-type as desired. Because the doping levels of certain of the regions shown in FIGS. 1-3 are especially important, further details are now given. In FIG. 1, the substrate 10 is typically silicon having a resistivity of the order of 3-100 cm. It is designated p- with respect to the other two regions shown, p-type region 23 which is relatively more heavily doped, and p-type region 21 • which is more heavily doped still and is designated p+. The doping level for region 23 can be 10 16 -10 19 /cm 3 , typically 10 16 -10 17 , and the doping level for region 21 ' can be 10 18 -10 21 /cm 3 typically 10 19 -10 20 /cm 3 . As indicated earlier the region 21 ' is more heavily doped than the region 23 to allow the latter to be converted to n-type by the non-selective n-implant without changing the conductivity type of the former. For the purposes of this invention it is recommended that this difference be of the order of factor of >10x, and preferably of the order of 100x. Referring back to FIG. 1 the conventional insulating means 11, 12 and 13, i.e. field oxide 11, regrown window oxide 12 and doped or deposited glass layer 13, are typically present as shown. Doping levels for the section containing the p-channel device are conventional. The region 21 ' is exposed to the non-selective n-type implant which forms region 25 (FIG. 2) and which remains p-type. To insure this, a typical implant dosage for the n-type implant would be 10 13-1014/cm2. The dose required in a given case will depend on a variety of factors all well known to those skilled in the art and it is not practical here to deal specifically with all variables. The objective is straightforward, i.e. apply a dose sufficient to convert the conductivity type of an inadvertently exposed region (e.g., 23) but insufficient to convert the conductivity type of a properly exposed region (e.g., 21 ' ) » If the initial doping difference between these regions is 10 a practical result would be to use a dose that would produce a doping level in region 24 (FIG. 2) that is of the order of ten times that of 23 (FIG. 1) resulting in a p-type doping level in region 25 that is of the order of ten times less than that of region 21 ' . Typically that implant will be arsenic or phosphorus. FIG. 3 shows the structure of FIG. 2 with metal contact layer 14 in place. Structures complementary to those shown here may be treated in similar fashion. Also, while this description uses as an example contacts in a CMOS structure the need for making contacts to both n-type and p-type regions using a single mask level may arise in a variety o other contexts. The foregoing description sets forth in detail a solution to problems arising from line-on-line contacts. In the specific case illustrated both the contact and the region to be contacted are of comparable size. A directly analogous offset problem arises when the contact region is large but circumstances dictate that the contact be placed as near as possible to the edge of that region. Therefore for the purposes of this invention line-on-line is defined as if to mean edge-on-edge, i.e. the edge of one feature, e.g. the left edge of contact window 22 in FIG. 1, aligned to the left edge of diffused region 21. The remaining edge or edges may or may not be interdependent.

Claims: Claim A method for forming contacts to first (21 ) and second (21') oppositely doped (n and p, respectively) semiconductor regions disposed at a surface of a substrate (10), the first region (21) being disposed adjacent to a third (23) region of a conductivity type (p) opposite thereto, said method comprising forming an insulating layer (11, 13) over each of said regions and providing openings (W') therethrough exposing surface portions of said first and second regions, the width of said first (21) region being so small as to give rise to the problem of the window thereover also exposing a surface portion of the adjacent third (23) region, characterized by forming said second regions (21') with a width sufficiently large that the window thereover normally does not expose any region adjacent thereto, forming said second region (21*) with a higher doping than that of said third (23) region, implanting impurities through said windows at a level sufficient to reverse the conductivity type (p) of the exposed portion of the third (23) region to that of the conductivity type (n) of the first (21) region, but insufficient to reverse the conductivity type (p) of the second region (21'), and depositing conductive material (14) into said windows to form contacts to said first and second regions.

Inventors: LANCASTER LOREN THOMAS, LANCASTER, LOREN, THOMAS
Applicants: AMERICAN TELEPHONE & TELEGRAPH, AMERICAN TELEPHONE & TELEGRAPH COMPANY
Application year: 1985
Main classification: H01L21
IPC codes: H01L21, H01L27