U.S. patent application number 12/150417 was filed with the patent office on 2009-10-29 for club extension to a t-gate high electron mobility transistor.
Invention is credited to Yaochung Chen, Robert Coffie, Po-Hsin Liu, Carol Osaka Namba, Ioulia Smorchkova, Michael Wojtowicz.
Application Number | 20090267115 12/150417 |
Document ID | / |
Family ID | 41211076 |
Filed Date | 2009-10-29 |
United States Patent
Application |
20090267115 |
Kind Code |
A1 |
Namba; Carol Osaka ; et
al. |
October 29, 2009 |
CLUB EXTENSION TO A T-GATE HIGH ELECTRON MOBILITY TRANSISTOR
Abstract
A method of fabricating a T-gate HEMT with a club extension
comprising the steps of: providing a substrate; providing a
bi-layer resist on the substrate; exposing an area of the bi-layer
resist to electron beam lithography where the area corresponds to a
T-gate opening; exposing an area of the bi-layer resist to electron
beam lithography where the area corresponds to the shape of the
club extension wherein the area corresponding to the club extension
is approximately 1 micron to an ohmic source side of a T-gate and
approximately 0.5 microns forward from a front of the T-gate;
developing out the bi-layer resist in the exposed area that
corresponds to the T-gate opening; developing out the bi-layer
resist in the exposed area that corresponds to the club extension;
and forming the T-gate and club extension through a metallization
process.
Inventors: |
Namba; Carol Osaka; (Walnut,
CA) ; Liu; Po-Hsin; (Anaheim, CA) ;
Smorchkova; Ioulia; (Lakewood, CA) ; Wojtowicz;
Michael; (Long Beach, CA) ; Coffie; Robert;
(Camarillo, CA) ; Chen; Yaochung; (Rancho Palos
Verdes, CA) |
Correspondence
Address: |
Carmen Patti Law Group , LLC
ONE N. LASALLE STREET, 44TH FLOOR
CHICAGO
IL
60602
US
|
Family ID: |
41211076 |
Appl. No.: |
12/150417 |
Filed: |
April 28, 2008 |
Current U.S.
Class: |
257/194 ;
257/E21.173; 257/E29.246; 438/172 |
Current CPC
Class: |
H01L 29/1608 20130101;
H01L 29/66462 20130101; H01L 29/42316 20130101; H01L 29/2003
20130101; H01L 21/0277 20130101; H01L 21/0272 20130101; H01L
29/7786 20130101 |
Class at
Publication: |
257/194 ;
438/172; 257/E21.173; 257/E29.246 |
International
Class: |
H01L 29/778 20060101
H01L029/778; H01L 21/338 20060101 H01L021/338 |
Claims
1-10. (canceled)
11. A system comprising: a T-gate high electron mobility transistor
(HEMT); a club extension positioned on an ohmic source side of a
proximate front of the T-gate HEMT and approximately 0.03 to 0.5
microns forward from a front of the T-gate HEMT; and wherein the
club extension is metallically affixed to the T-gate HEMT and the
T-gate HEMT is affixed to a substrate.
12. The system of claim 11 wherein the substrate is one of GaN,
Sapphire, SiN, SiC, and a III V substrate.
13. The system of claim 11 wherein the club extends to the
substrate.
14. The system of claim 13 wherein the club approximates a circle
with an approximate diameter of 0.4 to 2 microns.
15. The system of claim 13 wherein the club approximates a square
wherein: a length of a side of the approximate square is
approximately 0.4 to 2 microns; and at least one side of the
approximate square is substantially parallel to a wing of the
T-gate HEMT.
16. The system of claim 13 wherein the club approximates a
parallelogram wherein: a height of the approximate parallelogram is
approximately 0.4 to 2 microns; a width of the approximate
parallelogram is approximately 0.4 to 2 microns; and a longer side
of the approximate parallelogram is parallel to a wing of the
T-gate HEMT.
17. The system of claim 13 wherein the club approximates a polygon
wherein a diameter of the approximate polygon is approximately 0.4
to 2 microns.
18. The system of claim 11 wherein the club does not extend to the
substrate.
19. The system of claim 18 wherein the club approximates a circle
wherein the approximately circular club has an approximate diameter
of 0.4 to 2 microns.
20. The system of claim 18 wherein the club approximates a square
wherein: a length of a side of the approximately square is
approximately 0.4 to 2 microns; and at least one side of the
approximately square club is parallel to a wing of the T-gate
HEMT.
21. The system of claim 18 wherein the club approximates a
parallelogram wherein: a height of the approximate parallelogram is
approximately 0.4 to 2 microns; a width of the approximate
parallelogram is approximately 0.4 to 2 microns; and the longer
side of the approximate parallelogram is parallel to a wing of the
T-gate HEMT.
22. The system of claim 18 wherein the club approximates a polygon
wherein: a diameter of the approximate polygon is approximately 0.4
to 2 microns.
Description
TECHNICAL FIELD
[0001] The invention relates generally to a T-gate High Electron
Mobility Transistor and, more particularly, to a club extension to
a T-gate High Electron Mobility Transistor.
BACKGROUND
[0002] As demands on wireless and other electronic devices evolve
there is an increased need for electronic devices that can provide
higher performance at high frequency. One way of meeting these
requirements is to create devices using T-gates. The T-gate is a
gate conductor structure for a semiconductor device, such as a
Gallium Nitride High Electron Mobility Transistor (GaN HEMT). For
high performance such as a high operating frequency and a high
transconductance, the stem of the T-gate is narrow. For high
switching speeds the wings (or top) of the T-gate are wide. The
result is a gate conductor structure that provides the high
performance and high frequency demanded in electronic devices such
as high performance commercial communications and military
systems.
[0003] The demand for higher performance conductor structures leads
to a more demanding semiconductor fabrication process. Particularly
in the area of fabricating T-gates using bi-layer resists, there
cannot be any spurious material extending from a T-gate to a source
or drain ohmic contact. Electron beam exposure and development may
cause stress cracks in a bi-layer resist. Fabricating a T-gate
using a cracked resist may lead to spurious material extending from
these cracks. Such spurious material may cause the T-gate to short
to an ohmic contact. Even if the spurious material does not cause
the T-gate to short, the spurious material may cause electrical
breakdown of HEMT devices.
[0004] Therefore, there is a need in the art for an improved method
and system for fabricating T-gates such that electron beam exposure
and development does not cause stress cracks in a resist, and
spurious material does not extend from a T-gate to a source or
drain ohmic contact.
SUMMARY
[0005] One embodiment of a method and system is a method of
fabricating a T-gate HEMT with a club extension comprising the
steps of: providing a substrate; providing a bi-layer resist on the
substrate; exposing an area of the bi-layer resist to electron beam
lithography where the area corresponds to a T-gate opening;
exposing an area of the bi-layer resist to electron beam
lithography where the area corresponds to the shape of the club
extension wherein the area corresponding to the club extension is
approximately 1 micron to an ohmic source side of a T-gate and
approximately 0.03 to 0.5 microns forward from a front of the
T-gate; developing out the bi-layer resist in the exposed area that
corresponds to the T-gate opening; developing out the bi-layer
resist in the exposed area that corresponds to the club extension;
and forming the T-gate and club extension through a metallization
process.
[0006] Another embodiment of the method and system encompasses a
system. The system may comprise: a T-gate HEMT; a club extension
positioned on an ohmic source side of a proximate front of the
T-gate and approximately 0.03 to 0.5 microns forward from a front
of the T-gate; and wherein the club extension is metallically
affixed to the T-gate and the T-gate is affixed to a substrate.
DESCRIPTION OF THE DRAWINGS
[0007] The features of the embodiments of the present method and
apparatus are set forth with particularity in the appended claims.
These embodiments may best be understood by reference to the
following description taken in conjunction with the accompanying
drawings, in the several figures of which like reference numerals
identify like elements, and in which:
[0008] FIG. 1a is a front view of a T-gate, FIG. 1b is a side view
of the T-gate;
[0009] FIG. 2 is a front view of a bi-layer resist on a
substrate;
[0010] FIGS. 3a-d are overhead views of areas of bi-layer resists
that are exposed to electron beam lithography in order to form
T-gates with a club extension;
[0011] FIG. 4a and FIG. 4b are front views of bi-layer resists on
substrates after electron beam lithography exposure and
development;
[0012] FIG. 5a and FIG. 5b are front views of T-gates and club
extensions on substrates after metallization and before
lift-off;
[0013] FIG. 6a and FIG. 6b are front views of T-gates and club
extensions after metallization and lift-off; and
[0014] FIGS. 7a-d are overhead views of T-gates with a club
extension after lift-off.
DETAILED DESCRIPTION
[0015] Embodiments of the present method and system fabricate a
T-gate HEMT without spurious metal extending from the T-gate to an
ohmic contact.
[0016] T-gates are typically formed on a substrate that is covered
with a resist. The resist may be a bi-layer resist. Electron Beam
Lithography (EBL) is a technique used to form fine patterns used in
integrated circuits. The patterns are typically formed in the
resist. The resist may be an electron sensitive polymer that forms
a coating on the substrate. The resist is exposed to an electron
beam and the resist is chemically treated to form a pattern in the
resist. The pattern formed may comprise an area where a T-gate and
club extension is ultimately created. Resting on the substrate may
be ohmic contacts. Ohmic contacts serve the purpose of carrying
electrical current into and out of the semiconductor.
[0017] Turning to FIG. 1, a typical T-gate 110 is shown. As shown
in FIG. 1a, the T-gate 110 may have a stem 115 and wings 120. The
wings 120 of the T-gate 110 may be wider than the stem 115 of the
T-gate 110. A part of the T-gate that sits above the stem may be
considered a top of the T-gate. Although in FIG. 1a the T-gate 110
is shown with a top that comes to an approximate apex, the top of
the T-gate 110 may form an apex, or the top of the T-gate 110 may
form an irregular shape.
[0018] Herein, a "front" view of the T-gate 110 provides the viewer
with the widest view of the wings 120 of the T-gate 110. Thus the
view of the T-gate 110 as seen in FIG. 1a is a front view. FIG. 1b
illustrates a side-view 122 of FIG. 1a. As seen from a side 122,
the T-gate 110 may appear as two rectangles 124, 126 stacked on top
of each other. A lower rectangle 124 may be a side-view 122 of the
stem 115 of the T-gate 110. An upper rectangle 126 may be a view of
the wing 120 of the T-gate 110. The T-gate 110 may also have a
length 128.
[0019] The T-gate 110 may have a first end 130 and a second end
132. Either end 130, 132 of the T-gate 110 may be referred to as a
front or a back. For example, the first end 130 may be referred to
as a front end 130 of the T-gate 110, and the second end 132 may be
referred to as the back end 132 of the T-gate. It is equally true
that the second end 132 may be referred to as a front end 132, and
the first end 130 may be referred to as a back end 132. Each end
130, 132 may have a position that is forward from that end 130,
132. The forward position from a front end may a direction that is
perpendicularly away from the end 130, 132 of the T-gate 110. Thus,
if the first end 130 were a front end, the forward 134 direction
would be perpendicularly away from the first end 130. On the other
hand, if the second end 132 were a front end, the forward 136
direction would be perpendicularly away from the second end
132.
[0020] Turning to FIG. 2 that depicts a structure 200 that may be
used to form the T-gate 110. The structure 200 may consist of a
bi-layer resist 210 resting on a substrate 220. Resting on the
substrate 220 may be ohmic contacts 240, 250. As seen in FIG. 2 a
left side ohmic contact 240 may be a source ohmic contact. A right
side ohmic contact 250 may be drain ohmic contact. Among other
materials, the substrate 220 may be comprised of Gallium Nitride
(GaN), Silicon Carbide (SiC), SiN, Sapphire, or any III V
substrate. The bi-layer resist 210 may be comprised of two layers
of materials. A bottom layer 260 of the resist 210 may be comprised
of a polymethyl methacrylate (PMMA). A top layer 270 of the resist
210 may be a copolymer of methacrylic acid and methyl
methacrylate.
[0021] The top of the resist 210 may be exposed to an electron beam
280. This is the electron beam lithography (EBL) process. The
electron beam 280 may form a pattern in the resist 210. The pattern
in the resist 210 may correspond to an opening where the T-gate 110
may reside. Thus the pattern may approximate a rectangle. Another
pattern in the resist 210 may also correspond to a club extension.
The T-gate 110 and club extension patterns may be formed using two
or more passes of the electron beam 280 or by using one pass of the
electron beam 280.
[0022] FIG. 3 is an overhead view of the structure illustrated in
FIG. 2. FIG. 3 illustrates an area of the resist 210 that may be
exposed to the electron beam 280 to form an opening that may
contain the T-gate 110 and an opening that may contain a club
extension. FIG. 3 is broken down into four separate figures. In
each figure, a large rectangle 300 illustrates a view from above
the structure 200. The area exposed to the electron beam 280 that
may correspond to an opening where the T-gate 110 may rest is shown
by a smaller rectangle 310. An area exposed to the electron beam
280 that may correspond to an opening where a club extension may be
formed is shown by one of four example shapes 320, 330, 340,
350.
[0023] Turning to FIG. 3a, an approximately circular shape 320
illustrates the area exposed to the electron beam 280 that may
correspond to an approximately circular shaped club extension.
Although the shape 320 shown in FIG. 3 is a circle, shapes that are
not perfectly circular may be exposed to form an area where an
approximately circular club extension may reside. For example, the
side of the example circular shape 320 may be jagged, irregular, or
misshapen. A misshapen circular shape may approximate an oval or
ellipse. The example shape 320 has an approximate diameter of 0.4
microns to 2 microns. There is a gap 355 between the approximately
circular shape 320 and a side of the exposed area 310. The gap 355
may be approximately greater or equal to 0.1 microns. The
approximately circular shape 320 may rest a distance 360 forward
from a front 365 of the exposed area 310. The distance 360 may be
approximately 0.3 to 0.5 microns from the center of the
approximately circular shape 320.
[0024] FIG. 3b illustrates an approximately square shape 330
exposed to the electron beam 280 that may correspond to an
approximately square shaped club extension. Although the shape
shown in FIG. 3b is a square, shapes that do not form a perfect
square may be exposed to the electron beam 280 to form an area
where an approximately square club extension may reside. For
example, the sides of the approximately square shape 330 do not
have to be of equal length. The angles that form the approximately
square shape 330 do not have to be 90 degrees. Sides of the
approximately square shape 330 may be irregular, curved or jagged.
To form an approximately square club extension, a side of the
approximately square shape 330 may be approximately 0.4 microns to
approximately 2 microns long. There is a gap 370 between a side of
the approximately square shape 330 and the exposed area 310. The
gap 370 may be approximately 0.1 micron or more. The approximately
square shape 330 may rest a distance 375 slightly forward from a
front 365 of the exposed area 310. The distance 375 may be
approximately 0.3 to 0.5 microns from the center of the approximate
square shape 330.
[0025] FIG. 3c illustrates an example approximately parallelogram
shape 340 exposed to the electron beam 280 that may correspond to
an approximately parallelogram shaped club extension. Although the
area shown in FIG. 3c is a parallelogram, areas that do not form a
perfect parallelogram may be exposed to form an area that may
contain an approximate parallelogram shaped club extension. For
example, the sides of the approximately parallelogram shape 340 may
be curved or jagged. Furthermore, the opposite angles of the
approximate parallelogram shape 340 may be incongruent or the
opposite sides may be unparallel. A height 394 of the parallelogram
shape 340 may be approximately 0.4 microns to 2 microns. A width
396 of the parallelogram shape 340 may be approximately 0.4 microns
to approximately 2 microns. A longer side 398 of the shape 340 may
be parallel to the exposed area 365. There is a gap 380 between a
side of the parallelogram shape 340 and the exposed area 310. The
gap 380 may be approximately equal to or greater than 0.1 micron.
The approximate parallelogram 340 may rest a distance 385 slightly
forward a front 365 of the exposed area 310. The distance 385 may
be approximately 0.3 to 0.5 microns from the center of the
approximate parallelogram 340.
[0026] FIG. 3d illustrates an approximate polygon shape 350 exposed
to the electron beam 280 that may correspond to an approximately
polygon shaped club extension. Herein a polygon is a figure that
has at least two sides that forms an enclosure. Although the area
shown in FIG. 3d is a polygon, areas that do not form a perfect
polygon may be exposed to EBL to form an area where an
approximately polygon club extension may reside. For example, the
sides of the polygon shape 350 may be curved or jagged. The
approximate polygon shape 350 may have an approximate diameter of
0.4 microns to 2 microns. Although a polygon does not have a radius
per se, an approximate radius of a polygon may be calculated by
taking the average distance from an approximate center of the
polygon to each vertex. Alternatively, an approximate radius of a
polygon may be calculated by taking an average distance of a
plurality of distances between an approximate center of the polygon
and an edge of the polygon. Measuring a circumference of the
polygon and dividing the circumference by twice pi may also provide
an approximate radius of a polygon. There is a gap 390 between the
approximate polygon shape 350 and exposed area 310. The gap may be
approximately greater than or equal to 0.1 microns. The approximate
polygon shape 350 may rest a distance 392 slightly forward from a
front 365 of the exposed area 310. The distance may be
approximately 0.3 to 0.5 microns from the center of the approximate
polygon 350.
[0027] After the resist 210 is exposed to the electron beam 280,
the resist 210 may be developed, or developed out. Developing the
resist 210 may entail immersing the resist 210 in a solution
comprised of a methyl isobutyl ketone or a combination of methyl
isobutyl ketone and isopropanol. After immersion, resist 210 that
was exposed to the electron beam 280 is developed out. Developing
out the resist may entail removing parts of the resist that were
exposed to the electron beam 280. The result is an opening in the
resist where the T-gate 110 and the club extension may sit. The
resist 210 may develop stress cracks in the process of electron
beam 280 exposure and development. Developing an area of the resist
210 where a club extension may sit may alleviate stress cracks
formed during electron beam 280 exposure and development.
[0028] The combination of the size of the area exposed and ebeam
conditions on the ebeam 280 may affect the final three dimensional
club shape obtained in the resist profile. By modifying the ebeam
conditions on the ebeam 280 and the area exposed, some resist may
remain 260 and the upper portion of the resist may be developed out
270. Modifying ebeam conditions on the ebeam 280 and area exposed
may result in the development of the resist 270, 260 (or 210) to
the substrate 220. For example, the type of ebeam conditions used
on the ebeam 280 and the area exposed may result in exposure
through the resist 210 to the substrate 220. The area of resist 210
exposed to ebeam 280 may be developed out to the substrate 220. On
the other hand, if a different area of the resist 210 is exposed
and the ebeam conditions is re-modified on the ebeam 280, the area
of the resist exposed to the ebeam 280 may not be developed out
fully to the substrate 220. In this case, there may be resist 220
remaining under the exposed area after the exposed area is
developed out.
[0029] An example of the developed resist 210 is illustrated in
FIG. 4. FIG. 4a illustrates the bi-layer resist 210 developed such
that a portion of the bi-layer resist 405 remains on the substrate
220 under the area of the resist 210 that was exposed to create a
space for a club extension. After the resist 210 is developed,
there is an opening where the T-gate stem may rest 415. There is
also an area where the wings of the T-gate 420 and an area where
the club extension may reside 425. In this particular case, a
portion of the lower layer of the bi-layer resist 405 remains on
the substrate 220 after the resist 210 is developed. In this
example, the beam conditions of the ebeam 280 used on an exposed
area results in the lower layer 260 of the bi-layer resist 210
remaining. In other examples, by using other types of electron beam
conditions, the developed area may extend partly through the upper
layer 270 of the resist 210. Alternatively, modifying electron beam
conditions on the ebeam 280 may result in the upper layer 270 of
the resist 210 being completely exposed and partly exposed through
the lower layer 260 of the resist 210. The electron beam 280,
depending on the conditions used, may expose the resist 210
anywhere between a portion of the top layer 270 of the bi-layer
resist 210 to a depth through both layers 260, 270 of the resist
210 to the substrate 220.
[0030] FIG. 4b illustrates the bi-layer resist 210 developed such
that all the resist 210 is removed in the area where the club
extension may reside 440. There is an area where the T-gate stem
may reside 430. There is an area where the wings of the T-gate may
reside 435. There is also an area where the club extension may
reside 440. In this particular example, the exposed area
corresponding to the club extension 440 may be large. The resist
210 is completely removed where the club extension may reside 440
depending on condition used on the ebeam 280. After the resist is
developed, it is possible that small portions of resist 437 may
remain between the space for the T-gate stem 430 and the location
the club extension may reside 440.
[0031] After the resist 210 is developed, a T-gate and club
extension may be formed using a metallization process. During the
metallization process electrically conductive material such as
gold, titanium, nickel or tantalum is used to form the T-gate and
club extension. After the T-gate and club extension are formed, any
resist 210 remaining on the substrate 220 is removed during a
lift-off process. After the resist 210 has been lifted off, the
T-gate and club extension may remain on the substrate 220.
[0032] Turning to FIG. 5, a T-gate 505, 535 and club extension 510,
520 are shown after metallization and before lift-off. FIG. 5a,
illustrates a club extension 510 that does not extend to the
substrate 220. In this example, resist 415 remains under the club
extension 510. The club extension 510 rests on the resist 210 on a
side of the T-gate 505 nearest the source ohmic contact 240. The
club extension 510 may be affixed to a T-gate wing 515.
[0033] FIG. 5b illustrates a club extension 520 that extends to the
substrate 220. In this example, the club extension 520 extends to
the substrate 220. As discussed, a small portion of resist 437 may
remain between a base 540 of the club extension and the stem of the
T-gate 547. It is also possible that after development no resist
210 remains between the club extension 520 and the T-gate stem 537.
The base of the club extension 540 may rest on the substrate 220 on
a side of the T-gate 535 nearest the source ohmic contact 240. The
club extension 520 may be affixed to the T-gate 535 at a T-gate
wing 545.
[0034] Turning to FIG. 6 that illustrates the T-gates 505, 535 and
club extensions 510, 520 of FIG. 5 after the resist 210 has been
lifted off. In FIG. 6a the club extension 510 is affixed to the
wing 515 of the T-gate 505. There may be a space 605 between a
bottom 610 of the club extension 510 and the substrate 220. The
size of the space 605 between the bottom 610 of the club extension
510 and the substrate 220 may vary depending on the results of
electron beam 280 exposure. In other words, the depth 612 of the
club 510 may vary depending on the results of electron beam 280
exposure. The club 510 also extends a distance 614 from the T-gate
505. The distance 614 may vary depending on the results of electron
beam 280 exposure.
[0035] FIG. 6b is an illustration of the T-gate 535 and club
extension 520 of FIG. 5b after the resist 220 has been lifted off.
The base 540 of the club extension 520 is affixed to the substrate
220. The club extension 520 is also affixed to a wing 545 of the
T-gate 535. There may be a gap 615 between the base 540 of the club
extension 520 and the stem 620 of the T-gate 535. The size of the
gap 615 may vary depending on the results of electron beam 280
exposure.
[0036] Turning to FIG. 7 that is an overhead view of club
extensions 705, 710, 715, 720 and T-gates 722, 724, 726, 728 after
development and lift off. The club extensions 705, 710, 715, 720
and T-gates 722, 724, 726, 728 illustrated in FIG. 7 correlate to
the example shapes depicted in FIG. 3. Thus FIG. 7a is an example
illustration of a club extension 705 that may be created from the
exposed shape 320 as shown in FIG. 3a. FIG. 7b is an example
illustration of a club extension 710 that may be created from the
exposed shape 330 as shown in FIG. 3b. FIG. 7c is an example
illustration of a club extension 715 that may be created from the
exposed shape 340 as shown in FIG. 3b. FIG. 7d is an example
illustration of a club extension 720 that may be created from the
exposed shape 350 as shown in FIG. 3d.
[0037] Depending on conditions used during electron beam exposure,
the example club extensions 705, 710, 715, 720 shown in FIG. 7 may
be larger than the shape 320, 330, 340, 350 exposed to create the
club extension 705, 710, 715, 720. The club extensions 705, 710,
715, 720 depicted in FIG. 7 are affixed to the T-gate 722, 724,
726, 728. The club extensions 705, 710, 715, 720 may be affixed to
a side 730, 732, 734, 736 of the T-gate 722, 724, 726, 728 as well
as a front 738, 740, 742, 744 of the T-gate 722, 724, 726, 728.
Although the edges extending from the club extension 705, 710, 715,
720 to the T-gate 722, 724, 726, 728 are depicted using straight
lines, in practice the edges that extend from the club extension
705, 710, 715, 720 to the T-gate 722, 724, 726, 728 may be jagged,
curved, or some other non-linear shape.
[0038] The present method and apparatus are not limited to the
particular details of the depicted embodiments and other
modifications and applications are contemplated. Certain other
changes may be made in the above-described embodiments without
departing from the true spirit and scope of the present method and
apparatus herein involved. It is intended, therefore, that the
subject matter in the above depiction shall be interpreted as
illustrative and not in a limiting sense.
* * * * *