Method for forming doped regions within a semiconductor substrate

A method for forming a first doped region (24) and a second doped region (26) within a substrate (12). A masking layer (14) overlies the substrate (12). A first region (20) of the masking layer (14) is etched to form a first plurality of openings. A second region (22) of the masking layer (14) is etched to form a single opening or a second plurality of openings different in geometry from the first plurality of openings. A single ion implant step or an equivalent doping step is used to dope exposed portions of the substrate (12). The geometric differences in the masking layer (14) between region (20) and region (22) results in the formation of the first and second doped regions (24 and 26) wherein the first and second doped regions (24 and 26) vary in doping uniformity, doping concentration, and doping junction depth.

Field of the Invention 
The present invention relates generally to semiconductor technology, and 
more particularly, to the formation of doped regions. 
BACKGROUND OF THE INVENTION 
Integrated circuits are continually becoming more complex. Because of 
increased complexity, integrated circuit substrates are required to have 
several doped regions of varying size and doping concentration. The doped 
regions may function as transistor current electrodes (i.e. source, drain, 
emitter, collector), transistor control electrodes (i.e. base and/or 
gate), interconnects, well regions, isolation regions, contact areas, and 
the like. If various integrated circuit technologies, such as 
complementary metal oxide semiconductor (CMOS) technologies, bipolar 
technologies, analog circuitry, high-power devices, memory elements (such 
as erasable programmable read only memory {EPROM}), are combined onto a 
single integrated circuit, an increased number of doped regions are 
usually required. 
The formation of each doped region is performed sequentially. For example, 
an integrated circuit having a memory array of EPROMs, and supporting both 
bipolar and CMOS devices may require up to four well regions (one for the 
memory, two for the CMOS devices, and one for the bipolar devices) and may 
require numerous buried layers. Each well region may have a different 
optimal junction depth, optimal doping profile, and geometric shape. For 
these reasons, in order to optimize all of the four well regions, the four 
well regions are often formed independently and individually in a 
sequential fashion. Due to the independent formation, the alignment of 
well regions to each other is not always consistent. In addition, several 
photolithographic masking steps and diffusion steps (or implant steps) are 
required. The sequential process is therefore not time optimal or resource 
optimal. 
SUMMARY OF THE INVENTION 
The previously mentioned disadvantages are overcome and other advantages 
achieved with the present invention. In one form, the present invention 
comprises a method for forming a first doped region and a second doped 
region of a semiconductor integrated circuit. A substrate is provided. A 
masking layer is formed overlying the substrate. The masking layer has a 
first region and a second region wherein the first region of the masking 
layer has a feature defined by a first geometry and the second region of 
the masking layer has a feature defined by a second geometry. A single 
doping step is used to form both the first doped region and second doped 
region within the substrate. The first doped region has a first doping 
concentration and is formed under the first region of the masking layer. 
The second doped region has a second doping concentration and is formed 
under the second region of the masking layer. The first doping 
concentration is different from the second doping concentration. 
The present invention will be more clearly understood from the detailed 
description below in conjunction with the accompanying drawings.

DESCRIPTION OF A PREFERRED EMBODIMENT 
Illustrated in FIG. 1 is a structure suitable for formation of a doped 
region 10. A substrate 12 is provided. Substrate 12 may be made of 
silicon, gallium arsenide, silicon on sapphire, epitaxial formations, 
germanium, germanium silicon, polysilicon, diamond, silicon on insulator 
(SOI), and/or like substrate materials. Preferably, the substrate 12 is 
made of silicon. A masking layer 14 is formed overlying the substrate 12. 
In some cases, the masking layer may be made of a hard mask material or a 
dielectric material such as silicon dioxide or silicon nitride. The 
masking layer 14 may also be a composite masking material such as 
oxide-nitride-oxide (ONO), oxide-polysilicon-oxide, composite dielectrics, 
or the like. In a preferred form, the masking layer 14 is a layer of 
photosensitive material such as photoresist. Many forms of photoresist 
exist in the art and all are applicable to masking layer 14. 
In FIG. 2, a plurality of rectangular features is lithographically defined 
in masking layer 14. Other methods, such as removable spacers, exist which 
will allow an opening formation without the use of lithography. The 
openings in masking layer 14 are typically less then two microns in width 
and spaced apart by no more than roughly two microns. A diffusion process, 
an ion implantation process, or a like doping step is used to dope the 
substrate 12 through the openings in masking layer 14 to form doped 
regions 16. Doped regions 16 are usually doped with dopant atoms such as 
boron, phosphorus, arsenic or a combination of these elements. 
In FIG. 3, a thermal cycle or heating step is used to drive the dopant 
atoms out of doped regions 16 to form a continuous well of doped material 
18 within the substrate 12. The masking layer 14 is usually removed prior 
to the thermal cycle, but is illustrated in FIG. 3 and in subsequent 
figures for conceptual reasons. For example, removal of the masking layer 
14 prior to the thermal cycle is recommended when the masking layer 14 is 
made of photoresist. The required heating time and the amount of 
distribution of the dopant atoms is a function of both the size of the 
openings in masking layer 14 and the spacing between the openings in 
masking layer 14. 
In FIGS. 4-6, a method for forming two doped regions within the substrate 
12 is illustrated. By using the method illustrated in FIGS. 1-3 along with 
a conventional well implanting and masking approach, two doped regions may 
be formed from a single implant wherein the two doped regions each have 
different doping profiles and different doping depths. This method of 
forming two wells with one implant step reduces processing time, process 
cost, and process complexity by reducing processing steps and reducing 
masking steps. The wells which are formed in FIGS. 4-6 are also 
self-aligned to one another without the need for accurate lithographic 
alignment. 
FIG. 4 illustrates the substrate 12 and the masking layer 14. In FIG. 5, 
the masking layer is etched to form a first region 20 of the masking layer 
14. In the first region 20, the masking layer 14 is etched to form a 
plurality of rectangular regions as illustrated. A second region 22 is 
formed wherein the masking layer 14 has one big opening as in conventional 
well formation technology. An ion implant step or a like doping step is 
used to form doped regions 15 within the substrate 12. The doped regions 
contain dopant atoms such as boron, arsenic, and/or phosphorus. 
In FIG. 6, the doped regions 15 from FIG. 5 have been heated to thermally 
drive the dopant atoms to form a first well region 26 within region 20 and 
a second well region 24 within region 22. The well region 24 has a higher 
doping concentration and a deeper doping profile due to the differences in 
implant dose resulting from the differences in masking layer 14 between 
regions 20 and 22. Because region 20 is more protected by masking layer 14 
than is region 22, region 20 receives less total dopant atoms per surface 
area than region 22. The amount of dopant atoms received by the substrate 
12 within region 20 is a function of the width "X" of the openings of 
masking layer 14 and the spacing "Y" of the openings in masking layer 14. 
In summary, a twin well substrate is formed via well regions 24 and 26 
using a single mask and single implant technique. 
FIG. 7 illustrates that the method illustrated in FIGS. 4-6 and discussed 
above can be used with a field oxide region 28. Field oxide region 28 may 
also be replaced by other known isolation methods such as trench isolation 
and the like. 
In FIG. 8, the substrate 12 and the masking layer 14 are illustrated. A 
checker board pattern is etched into the masking layer to expose even less 
substrate area than that exposed in FIG. 2. An ion implant step is used to 
form doped regions 30 within the substrate as illustrated. 
In FIG. 9, a thermal drive step is used to drive all of the doped regions 
30 into a single doped well 32. As stated before, the masking layer 14 is 
usually removed prior to thermal drive, but is illustrated in FIG. 9 for 
conceptual reasons. The checker board method taught in FIG. 9 may be used 
in conjunction with FIGS. 3-6 to form a third distinct well with a single 
mask and implant step. In general, N distinct wells, where N is a positive 
integer, may be formed by forming N distinct patterned areas of masking 
layer 14. Each of the N areas of masking layer 14 expose a different 
percentage of substrate surface area. If the ratio of exposed substrate 
surface area to protected substrate surface area is high, then a high 
concentration well is formed having a deeper junction depth. If the ratio 
of exposed substrate surface area to protected substrate surface area is 
low, then a low concentration well is formed having a shallower junction 
depth. 
In FIGS. 10-11, a method for forming a twin doped substrate is used to form 
complementary metal oxide semiconductor (CMOS) regions and bipolar regions 
in a single ion implant step. In FIG. 10, a substrate 12 is provided. A 
buried collector 33 is formed. The masking layer 14 is used to form a 
region 36 and a region 38. Region 36 has a small opening in masking layer 
14 and will therefore allow fewer dopant atoms into the substrate 12 than 
region 38. Region 38 has a larger opening in masking layer 14 and will 
therefore allow more dopant atoms into the substrate 12 than region 36. An 
ion implant step is used to form doped regions 37 and 39. 
In FIG. 11, field oxide regions 34 are formed. A metal oxide semiconductor 
(MOS) transistor is formed within region 38 and a bipolar transistor is 
formed within region 36. The doped region 37 is thermally driven to form a 
doped region 42. The doped regions 39 is thermally driven to form a doped 
region 40. Source and drain regions 58 are formed within the region 38. A 
gate electrode 54 is formed within the region 38. The doped regions 42 and 
44 form a base region. A doped region 46 forms an emitter region within 
region 36. Conductive layers 48 and 50 make electrical contact to the 
bipolar transistor electrodes. The collector contact to buried collector 
33 is not illustrated. 
By forming region 42 from region 37, region 42 is formed as a lightly doped 
while a heavier doped region 40 is formed for CMOS devices. Regions 40 and 
42 are formed by using a single ion implant step. FIGS. 10-11 also 
illustrate the fact that a plurality of openings in a masking layer is not 
required. The size of a single opening may determine the doping 
concentration and the doping junction depth for a device. In some cases, 
multiple openings (see FIG. 2 or FIG. 8) in the masking layer 14 may be 
used to form the well region 42. Reduced parasitic capacitances are 
achieved in the bipolar transistor by using the method illustrated in 
FIGS. 10-11. 
In FIG. 12, a graph illustrates the doping concentration (phosphorus is 
used as an example) and the doping depth of a doped region as a function 
of opening size (for an illustration of opening size see "X" in FIG. 6 or 
FIG. 10). In FIG. 12, a 2.0 micron-wide window will create a doped region 
having a surface concentration (i.e. concentration at 0.0 micron depth) of 
1.13.times.10.sup.17 dopants/cm.sup.3 and a doping depth of roughly 1.75 
microns. Doping depth is arbitrarily defined in FIG. 12 as being the depth 
at which the doping concentration drops below 1.times.10.sup.15. In FIG. 
12, a 1.0 micron-wide window will create a doped region having a surface 
concentration (i.e. concentration at 0.0 micron depth) of 
1.08.times.10.sup.17 dopants/cm.sup.3 and a doping depth of roughly 1.70 
microns. A 0.5 micron-wide window will create a doped region having a 
surface concentration of 9.0.times.10.sup.16 dopants/cm.sup.3 and a doping 
depth of roughly 1.59 microns. A 0.4 micron-wide window will create a 
doped region having a surface concentration of 8.0.times.10.sup.16 
dopants/cm.sup.3 and a doping depth of roughly 1.55 microns. A 0.3 
micron-wide window will create a doped region having a surface 
concentration of 6.0.times.10.sup.16 dopants/cm.sup.3 and a doping depth 
of roughly 1.50 microns. 
As graphically illustrated in FIG. 12, as the opening of the window in mask 
layer 14 decreases, the doping concentration of the doped region decreases 
and the doping junction depth of the doped region decreases. Generally, 
FIG. 12 illustrates the vertical phosphorus profile at a center of a 
window implant region. 
FIG. 13 illustrates a graph of surface doping concentration versus window 
width "X". In FIG. 13, as window width is decreased, the surface 
phosphorus concentration decreases in a logarithmic manner toward zero. 
More specifically, FIG. 13 illustrates the surface phosphorus 
concentration at the center of an implant window as a function of the 
window width. 
FIG. 14 graphically illustrates the lateral dopant uniformity of a doped 
region as a function of opening width and opening spacing. FIG. 14 
illustrates a masking layer 14 having a plurality of openings. Each of the 
openings in the plurality of openings are roughly 0.4 micron in width and 
are spaced apart by roughly 0.8 micron. FIG. 14 illustrates that 0.4 
micron openings separated by 0.8 micron spaces forms a doped region which 
varies laterally in surface doping concentration from roughly 
3.0.times.10.sup.16 to 9.times.10.sup.16 in a lateral direction after a 
brief thermal cycle. Three different vertical depths are used to 
illustrate the doping concentration in FIG. 14. 
FIG. 15 graphically illustrates the lateral dopant uniformity of a doped 
region as a function of opening width and opening spacing. FIG. 15 
illustrates a masking layer 14 having a plurality of openings. Each of the 
openings in the plurality of openings are roughly 0.4 micron in width and 
are spaced apart by roughly 0.4 micron. The pattern of masking layer 14 is 
continuos to the left of the X axis in FIG. 15. FIG. 15 illustrates that 
0.4 micron openings separated by 0.4 micron spaces forms a doped region 
which does not vary significantly in doping concentration in a lateral 
direction after a brief thermal cycle. Three different vertical depths are 
used to illustrate the doping concentration in FIG. 15. 
FIG. 16 graphically illustrates the lateral dopant uniformity of a doped 
region as a function of opening width and opening spacing. FIG. 16 
illustrates a masking layer 14 having a plurality of openings. Each of the 
openings in the plurality of openings are roughly 0.6 micron in width and 
are spaced apart by roughly 0.6 micron. The pattern of masking layer 14 is 
continuos to the left of the X axis in FIG. 16. FIG. 16 illustrates that 
0.6 micron openings separated by 0.6 micron spaces forms a doped region 
which varies only slightly in doping concentration in a lateral direction 
after a brief thermal cycle. Three different vertical depths are used to 
illustrate the doping concentration in FIG. 16. 
FIGS. 14-16 collectively illustrate that spacing dimensions and spacing 
separation significantly influence lateral doping uniformity, doping 
concentration, and doped region junction depth. The mask and implant 
techniques taught herein may therefore be exactly constructed to form one 
or more doped regions or well regions each having a specific/different 
doping depth, a specific/different doping concentration, and a 
specific/different doping uniformity. 
FIGS. 17-18 illustrate that the inventive mask and implant technique taught 
herein may be used to form two or more buried layers having different 
doping concentrations. In FIG. 17, two or more doped regions (i.e. well 
regions 24 and 26) are formed within the substrate 12 as illustrated in 
FIGS. 4-6. FIG. 17 is identical to FIG. 6. Once the doped regions have 
been formed, FIG. 18 illustrates that the masking layer 14 is removed. An 
epitaxial/selective growth step is used to form buried layers from the 
doped regions 24 and 26 via the formation of an epitaxial growth layer 
100. The epitaxial growth layer 100 may be insitu doped, undoped, ion 
implanted, or the like. The buried layers may be formed, for example, as 
an N+ buried layer and an N buried layer combination or a P+ buried layer 
and a P buried layer combination. In a preferred form, the epitaxial 
growth layer 100 is a semiconductive material and may be selectively grown 
or formed via a blanket growth process (i.e. a growth region is formed 
across the entire integrated circuit). 
While the present invention has been illustrated and described with 
reference to specific embodiments, further modifications and improvements 
will occur to those skilled in the art. For example, both phosphorus and 
arsenic may be implanted to form diffusion regions with different 
diffusion characteristics. The masking layers taught herein are usually 
always cut in rectangular, checker board, or square patterns, but other 
geometries exist. Triangles, circles, grids, crossing lines, crosses, and 
other geometries may be used to pattern the masking layers taught herein. 
Other forms of lithography such as phase shifting, X-ray lithography, 
E-beam lithography, and the like may be used to form features within the 
masking layers. The doped regions formed herein may be used to form BiCMOS 
devices, microcontroller circuits, combined digital/analog circuits, 
memory arrays, and the like. In FIG. 2 a dielectric layer (not 
illustrated) may optionally be formed between the masking layer 14 and the 
substrate 12. An implant may then be performed through the dielectric 
layer (not illustrated) for an improved doping profile. Doping profile and 
doping concentration are both affected by the process described herein. It 
should be understood that this invention may be used to implant non-dopant 
atoms such as oxygen and nitrogen. This technique of implanting non-dopant 
atoms may be useful for selective implantation of oxygen (SIMOX) or 
nitrogen when different oxide or nitride thicknesses are useful. It is to 
be understood, therefore, that this invention is not limited to the 
particular forms illustrated and that it is intended in the appended 
claims to cover all modifications that do not depart from the spirit and 
scope of this invention.