Non-destructive method for gauging undercut in a hidden layer

A method for non-destructively determining the amount of undercutting in a hidden layer of material disposed on a substrate after device patterning by etching. The method involves forming at least two lines of etch resistant material of increasing width over the hidden layer of material of the substrate and inspecting the lines after etching for a given time period to determine how many lines have been removed. The width dimension of the largest removed line corresponds approximately to the amount of undercut for two sides in the hidden layer of material after etching for the given time period.

FIELD OF THE INVENTION 
This invention relates to the fabrication of optical and electronic 
devices, and in particular, to a test pattern for non-destructive, gauging 
of undercutting in a hidden layer. 
BACKGROUND OF THE INVENTION 
Etching processes are used for selectively removing metal and dielectric 
films in the fabrication of optical and electronic devices. These etching 
processes usually exhibit either anisotropic or isotropic etching 
behavior. Anisotropic etching involves etching along only one crystal axis 
of the material or along each of the crystal axes of the material at 
substantially different rates. This type of etching typically produces 
vertical rather than lateral etching. Dry etching processes such as ion 
beam milling, reactive ion etching and plasma etching, generally exhibit 
anisotropic etching behavior. 
Isotropic etching behavior involves etching rates which are generally the 
same along plural crystal axes of the material. Isotropic etching 
typically produces both vertical and lateral etching of material. The 
lateral etching usually undercuts the mask layer. Wet chemical etching 
generally exhibits isotropic etching behavior, and some dry etching 
processes can also exhibit isotropic etching. 
Gauging the amount of undercutting during isotropic etching is important 
for maintaining repeatability and consistency of device fabrication 
processes. The amount of undercutting can be determined visually in some 
cases if the material being etched involves a single layer or multiple 
transparent layers. However, where the material involves opaque multiple 
layers, the undercutting of the lowest layer or layers may be visually 
hidden by the layers on top. This inability to determine undercut is 
especially troublesome when the etchant is selective to the lower hidden 
layer, i.e., where the hidden layer etches at a significantly faster rate 
than the overlying layer. 
Conventional methods such as cross-section SEM, TEM and additional 
selective etching have been used for gauging the amount of undercutting of 
visually hidden layers. Unfortunately, all of these methods involve costly 
destruction of the device. 
Accordingly, a non-destructive method for determining the amount of 
undercutting of visually hidden layers is needed for maintaining 
repeatability and consistency in isotropic etching processes used in 
optical and electronic device fabrication. 
SUMMARY 
A method for non-destructively determining the amount of undercutting in a 
hidden layer of material disposed on a substrate after device patterning 
by etching. The method comprises forming at least two lines of etch 
resistant material of increasing width over the hidden layer of material 
of the substrate and inspecting the workpiece after etching to determine 
how many lines have been removed. The width dimension of the largest 
removed line provides a measure of the undercut in the hidden layer.

DETAILED DESCRIPTION 
FIG. 1 is a sectional view showing a typical substrate 10, such as a 
semiconductor wafer, to be evaluated after etching using the test pattern 
and method of the present invention. The substrate 10 includes a hidden 
layer 12 of metallic, dielectric or semiconductive material deposited 
directly on a principle surface of the substrate 10 followed by a 
non-transparent upper layer 14 of metallic, dielectric or semiconductive 
material. 
FIGS. 2A and 2B are respective sectional and top views of a non-device area 
of the substrate 10 of FIG. 1 after the deposition of a test pattern 16 
for gauging the amount of undercut of the hidden layer after etching. The 
test pattern 16 comprises an inline array of two or more lines 18a-18g of 
increasing width W. The lines 18a -18g are typically made from photoresist 
or any other suitable masking material such silicon dioxide. This permits 
one or more test patterns 16 to be easily fabricated in non-device areas 
of the substrate 10 using conventional photolithographic methods. 
FIG. 3 is a sectional view showing the substrate 10 after etching for a 
time (T.sub.1) sufficient to completely etch through the unmasked portions 
of the upper layer 14 and etch partially into the underlying portions of 
the hidden layer 12. Etching produces stacked island-like structures 20 
beneath the lines consisting of portions of the upper and hidden layers 
14, 12. The isotropic etching behavior in the hidden layer 12 removes 
material both laterally and vertically. The lateral etching component 
produces an undercut region 22 of width U.sub.1 in hidden layer portion of 
each island 20 (the undercut region 22 extends along the periphery of the 
hidden layer portion). 
FIG. 4 is a sectional view showing the substrate 10 after etching for a 
time (T.sub.2) sufficient to eliminate line 18a. As the time of etching 
increases, the width of the undercut regions 22 increase until the hidden 
layer portions of the islands 20 are sequentially eliminated starting with 
the hidden layer portion under the thinnest line. The remaining portion of 
the island 20 (the upper layer portion and the corresponding line) becomes 
dislodged from the substrate. Increasing or decreasing the time duration 
of the etching process respectively increases or decreases the number of 
hidden layer portions which are eliminated under the lines 18a-18g and 
thus, the number of lines which are dislodged from the substrate 10. 
If the etching process is by wet chemical etching, the etchant will wash 
away the dislodged islands 20 and their corresponding lines. Dry etching 
processes will typically require a separate washing step to remove the 
unsupported upper layer islands 20. 
FIG. 5 is a top view of the substrate 10 shown in section in FIG. 4. FIG. 5 
shows the test pattern 16 viewed with an optical microscope. The test 
pattern 16 provides a visual indication of the elimination of line 18a. 
The width dimension W of the largest removed line will approximately 
correspond to the amount of undercut for two (2) sides in the hidden layer 
12 after etching for time T.sub.2. In the case of FIG. 5, the largest 
removed line is 18a. Thus, the amount of undercut for two sides in the 
hidden layer 12 after etching for time T.sub.2 is measured by the width 
dimension of line 18a. 
Undercut information can be advantageously used for evaluating undercut 
uniformity both intra and inter substrate. This type of information also 
permits statistical process control techniques to be used to address 
undercut issues in the etching process. 
FIG. 6 is a sectional view showing the substrate 10 after etching for a 
time (T.sub.3) sufficient to eliminate lines 18a and 18b. Since line 18b 
is the largest removed line, the amount of undercut in the hidden layer 12 
after etching for time T.sub.3 is approximately equal to the width 
dimension of line 18b. 
FIG. 7 is a top view of the substrate 10 shown in section in FIG. 6. In 
FIG. 7 the test pattern 16 provides a visual indication of the elimination 
of lines 18a and 18b. The width dimension W of the removed line 18b 
approximately corresponds to the amount of undercut for two (2) sides in 
the hidden layer 12 after etching for time T.sub.3. 
It is understood that the above-described embodiments illustrate only a few 
of the many possible specific embodiments which can represent applications 
of the principles of the invention. Further modifications and changes can 
be made by those skilled in the art without departing from the spirit and 
scope of the invention.