Apparatus and method for measuring the orientation of a single crystal surface

An apparatus and method for measuring the misorientation of a polished surface of a single crystal wafer with respect to a set of low index crystal planes utilizes both an optical beam and an X-ray beam that are directed along the same axis at the wafer. The optical beam is reflected by the polished surface of the wafer, but the X-ray beam penetrates the surface and is diffracted by the low index crystal planes located below the surface. The separation between the diffracted and reflected beams is measured with a detection device. The separation between the reflected and diffracted beams is an indication of the magnitude and sense of the misorientation between the surface and the crystal planes.

FIELD OF THE INVENTION 
The apparatus and method of the present invention relates to the processing 
of a monocrystalline substrate, and more particularly an apparatus and 
method for measuring the orientation of the surface of the substrate with 
respect to one or more low index planes. 
BACKGROUND OF THE INVENTION 
Monocrystalline substrates, such as silicon wafers, typically comprise the 
building blocks of most microelectronic devices. In practice, silicon 
wafer manufacturers dice wafers from a single crystal ingot such that each 
wafer has its upper surface slightly tilted, or misoriented by a magnitude 
often approximately 3 degrees. The direction, or sense of the tilt from 
the wafer's vicinal or true (100) crystal plane is often along either the 
(011) or (011) plane after polishing. Those skilled in the art have 
discovered that in many cases, purposely misorienting the polished surface 
of the wafer enhances and improves subsequent wafer oxidation processes 
and other surface deposition steps. 
While purposely misorienting the polished surface of a wafer often improves 
many semiconductor processes for fiber optic devices, misorientation above 
certain magnitudes may create problems in optical fiber alignments. In the 
case of V-grooves precisely formed in a monocrystalline wafer by 
anisotropic etching, substantial misorientation of the V-grooves relative 
to a low index crystal plane may lead to misalignment of one or more of 
the optical fibers held by the assembly. This problem has been described 
and addressed in commonly owned U.S. patent application Ser. No. 
08/408,800 filed Mar. 23, 1995. 
As a result of the positive and negative impacts of wafer surface 
misorientation, those skilled in the art have realized the importance of, 
and need for, screening of individual wafers for appropriate surface 
orientations for particular applications. This involves measuring the 
surface orientations before allowing subsequent processing. Failure to do 
so may result in the costly scrapping of fully fabricated but defective 
fiber optic components. 
A variety of conventional techniques exist for establishing the 
crystallographic orientation of the wafer. One such technique involves 
chemical etching of the wafer surface to produce etch-pits, whereby the 
shape of the etch-pits is usually indicative of the wafer orientation. One 
proposal, by Townley, is discussed in "Optimum Crystallographic 
Orientation For Silicon Device Fabrication", Solid State Technology, Jan. 
1993 (pp. 43-47). (See also, "The Chemical Polishing Of Semiconductors", 
Journal of Materials Science, 1975 (pp. 321-339); "Silicon As A Mechanical 
Material", Proceedings of the IEEE, May 1982 (pp. 420-457). These 
technologies utilize visible light reflected from the etched surface of 
the wafer to produce a set of detectable lobes having a prescribed size 
relationship with each other depending upon the orientation of the wafer. 
Unfortunately, the etching of the wafer destroys the wafer for all 
practical purposes, thereby characterizing this procedure as destructive. 
Because of the costs involved in physically destroying the structures, 
wafers used in the orientation measurement are typically taken from stock 
already removed from production as defective for any number of reasons. 
The individual wafer is traceable to the particular originating ingot such 
that wafers from the same lot may be identified as having the particular 
measured orientation. While this method works well for its intended 
purposes, by destructively testing defective wafers the accuracy of the 
resulting orientation measurement becomes subject to a plurality of 
unknown variables. 
Another conventional method for determining surface orientation involves 
using X-rays, as proposed by Laue, see chapter 8 of Elements of X-ray 
Diffraction, B.D. Cullety (Addison Wesler 1956). In the Laue method, 
X-rays are directed to the surface of the crystal and reflected therefrom. 
The reflected X-rays follow a path to a sheet of photographic film to 
expose a complex image representing the crystal surface orientation. After 
the film develops, skilled technicians interpret the exposed image to 
determine the orientation information. Although this method avoids 
destruction of the wafer, the time and labor involved to extract the 
orientation information from the film makes such a process undesirable in 
automated high production environments. Moreover, because of the 
"indirect" manner of taking an X-ray diffraction pattern, the 
predictability and accuracy of the surface orientation of the test wafer 
are only approximate. 
Therefore, those skilled in the art have realized the need for an apparatus 
and method for accurately measuring the orientation of a monocrystalline 
substrate surface without destroying the substrate and which is applicable 
for use in automated high speed production operations. The apparatus and 
method of the present invention satisfy these needs. 
SUMMARY OF THE INVENTION 
The apparatus and method of my present invention conveniently provide 
nondestructive measurements of monocrystalline substrates to determine the 
orientations of the substrates and their surfaces. By directly measuring 
the substrate surface orientation, my invention offers a substantially 
higher degree of accuracy than indirect measuring means and methods. As a 
result, the costs involved in proceeding with the full fabrication of 
semiconductor devices from wafers that were initially defective due to 
misorientation, can be substantially minimized. 
The method of the present invention comprises a technique for measuring the 
misorientation of a polished surface of an immobilized crystal substrate 
with respect to a set of low index crystal planes of the crystal which 
exists beneath the polished surface. The method includes the steps of 
establishing a reference beam of radiation (e.g. x-ray radiation) along a 
predetermined path to non-destructively penetrate the surface and diffract 
from the set of crystal planes at a predictable angle to define a 
diffraction beam; and guiding an optical (e.g. light) beam along the 
predetermined path co-linear with the reference beam of radiation to 
reflect from the surface of the substrate. If there is misorientation 
between the polished surface and the crystal plane, there will be a 
relative separation between the position of the diffraction radiation beam 
and the reflected optical beam which is directly related to the 
misorientation. 
To realize the advantages of this method, the apparatus of my present 
invention comprises a beam detection device for use with a measurement 
system having an energy beam source (e.g. an X-ray source), an optical 
beam source (e.g. a light source), and a substrate fixture. The detection 
device includes a multi-axis platform and a scale mounted on the platform. 
The platform includes a track formed along a track path disposed radially 
equidistant from a predetermined point. A multi-detector mechanism is 
mounted slidably to the track and includes respective energy beam and 
light beam detectors disposed in measurably adjustable relative relation 
along the track. 
During use, as the energy and optical beams are directed towards the 
substrate located at the predetermined point, the energy beam will 
diffract from the crystal plane and the optical beam will reflect from the 
substrate surface. The detectors are moved along the track to allow them 
to align with the respective diffracted and reflected beams. The aligned 
detectors define a relative spacing therebetween corresponding to the 
relative angular separation between the respective beams, which represents 
the sense and magnitude of misorientation of the substrate surface with 
respect to its vicinal low index crystal plane.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In practice, wafers of the type used in the microelectronic and 
opto-electronic industries, such as Si, Ge, ZnS, GaAs, InP, CdTe, to name 
a few, are diced from a single crystal ingot 10, as shown in FIG. 1A. The 
cuts are made such that each wafer 12, such as shown in FIG. 1B, has its 
upper surface 14 slightly tilted (typically about 3 degrees) from the 
wafer's vicinal or true (100) crystal plane along either the (011) or 
(011) plane after polishing. Low index crystalline planes 16, (FIGS. 2A 
and 2B) underlying the polished surface 14 are aligned in parallel stacked 
relation according to the vicinal plane. Accurately aligning the surface 
14 at the proper angle with respect to the planes 16 facilitates improved 
control over wafer oxidation methods and other surface deposition steps 
performed during the processing of the wafer. 
With reference to FIG. 1B, during processing, a reference flat 18 is ground 
into the wafer 12 parallel to the 011! direction. Although all wafer 
manufacturers grind one or two flats in each wafer, there is no standard 
regarding the orientation of the flat. For different manufacturers, the 
flat may be oriented differently. Further, it should be pointed out that 
unlike Si and Ge, the 011 ! and 011! directions in III-V (GaAs, InP, and 
others) and II-VI semiconductors (Cd Te, ZnS, and others) are not 
equivalent. Thus, it is typically not possible to determine the 
orientation of the wafer, and more particularly, the orientation of the 
tilt axis, simply by reference to the location of the reference flat 18. 
Referring to FIG. 2A, it should be understood that processing of the 
monocrystalline wafer surface during wafer fabrication typically involves 
purposely misorienting, or tilting, the polished wafer surface 14 with 
respect to the plurality of vicinal low index parallel crystalline planes 
16. In order to measure the magnitude and sense of the surface 
misorientation, one must be able to determine the relative angle .theta. 
between the vector n (normal to the polished surface 14), and the vector 
ghkl (normal to the low index crystalline planes 16). 
It has long been known that if an X-ray beam, such as Xo, is directed at a 
polished substrate surface, the beam will propagate through the surface 
and, at a suitable angle which is determined by the Bragg condition, the 
beam will diffract off of one of the crystalline planes to form a 
diffraction beam, such as X'. Under such conditions, the angle of the 
diffracted beam will equal the angle of the incident beam and comprise a 
predictable reference beam representing the orientation of the low index 
vicinal plane 16. As is well known in the art, not every low index crystal 
plane satisfies the Bragg condition. 
While an X-ray beam comprises radiation capable of non-destructively 
passing through the polished wafer surface 14, optical radiation cannot. 
Instead, an optical beam, such as 0, reflects from the polished surface at 
a reflection angle, and is directed towards O'. I have discovered that 
when both beams are initially incident on the same point P on the 
substrate 12 (FIGS. 2A, 2B and 3), the angular relationship between a 
Bragg diffracted X-ray beam, X', and a reflected optical beam, O', is 
2.theta.. Therefore, by measuring this relationship, the misorientation, 
0.theta., of the crystal surface 14 with respect to the vicinal plane 16 
can be easily calculated. 
FIGS. 3 and 4, show apparatus for implementing the concepts of the 
invention, i.e. the measurement of the magnitude and sense of the 
orientation of the surface 14 with respect to the planes 16. According to 
this embodiment of the present invention, a measurement system 20 includes 
an energy beam source 22, an optical beam source 26, a substrate fixture 
28, and a detection device 30. 
The energy beam source 22 typically comprises an X-ray source which 
produces an X-ray beam Xo (FIGS. 2A and 2B) along a controllable path 24 
in the direction of the wafer 12 so as to non-destructively penetrate the 
polished surface 14 of the wafer 12 and diffract off a lower index plane 
16 to form a diffracted beam X'(see FIGS. 2A and 2B). It is anticipated 
that other sources of radiation capable of penetrating the polished 
surface such as electron beams and ion beams, may be utilized in place of 
the X-ray beam. 
An important consideration in the energy source used involves the 
nondestructiveness and predictability of the diffracted energy beam. In 
the case of X-rays, by satisfying the Bragg condition, as is well known in 
the art, the angle of diffraction from the set of low index planes will 
equal the angle of incidence. In this manner, the diffracted beam X' 
serves as a convenient reference beam for marking the relative orientation 
of the lower index plane 16. 
The optical beam source 26 typically comprises any form of visible, 
infrared or ultraviolet light beam source capable of directing a light 
beam O (FIGS. 2A and 2B) along the same path 24 taken by the energy beam 
Xo. However, unlike the surface penetrating characteristic of the beam 
from the energy source 22, the key consideration for the optical radiation 
implemented by the optical beam involves its ability to reflect off the 
misoriented polished surface 14 to form a reflected beam O'. In the case 
of a surface misoriented wafer, O' will be displaced relative to the 
diffracted beam X'. 
Further referring to FIG. 3, the wafer fixture 28 is formed in a 
conventional configuration, as is well known in the art, to immobilize the 
wafer 12 in a controllable position. Means are typically provided to 
adjust the tilt of the wafer such that the Bragg condition may be quickly 
satisfied with respect to the energy beam Xo. Of course, satisfying the 
Bragg condition may be accomplished by adjusting the energy beam, the 
wafer fixture position, or both. 
One illustrative embodiment of a beam detector device 30 in accordance with 
my invention is depiction in FIG. 4. The optical detector device 30 is 
disposed substantially opposite the respective energy beam and optical 
beam sources 22 and 26 to detect the diffracted and reflected beams X' and 
O' from the wafer 12. The device includes, generally, a multi-axis 
platform 32, a circular track scale 46, and a multi-detector mechanism 56 
to efficiently and accurately determine, from the detected beams, the 
magnitude and sense of orientation of the surface 14 of the substrate 12 
with respect to the planes 16. 
With particular reference to FIG. 4, the multi-axis platform 32 includes an 
elongated rectangularly formed base 34 having a reduced cross-sectional 
bridge 36 extending along a horizontal axis. The bridge slidably carries 
an adjustable support 38 having a clamping element 40. This bridge 36 is 
formed with an upwardly projecting hollow cylindrical riser 42 defining a 
vertical axis. The riser includes a distal open end 43 opening upwardly to 
telescopically receive a cylindrical extender 50 attached to the circular 
scale 46. An adjustable fastener 44 selectively immobilizes the arm in 
place to enable vertical and rotational positioning of the scale. 
The scale 46 comprises a formed circular ring 48 vertically carried by the 
extender 50 and includes a plurality of spaced apart graduations 52 
disposed around the ring for measuring angular displacement. The ring 48 
defines a narrow circularly shaped band or track 47 disposed about a 
central reference point for guiding the multi-detector mechanism 56 
therearound. 
With continued reference to FIG. 4, the multi-detector mechanism 56 
includes a narrow bar 58 having an adjustment end formed with a guide 60 
to slidably engage the circular track 47 of ring 48. The bar includes an 
alignment end 62 disposed radially inwardly from the track to align with a 
central reference point. The alignment end mounts a fixed X-ray beam 
detector 64 to sense a diffracted X-ray beam X' according to one 
embodiment of the present invention. The bar slidably carries an optical 
beam detector 66 and includes a plurality of formed graduations 68 for 
measuring the displacement between the X-ray beam detector 64 and the 
optical beam detector 66. 
Operation of the system and apparatus of the present invention is best 
described with reference to one embodiment of the method of the present 
invention, shown in FIG. 5. 
Implementing these general physical concepts, the method of the present 
invention includes establishing the reference beam of radiation Xo along a 
predetermined path to diffract from the near-surface crystalline planes of 
the wafer, at step 200 as shown in FIG. 5, and guiding an optical beam O 
along the predetermined path to reflect from the polished surface 14, at 
step 202. The relationship between the resulting diffracted and reflected 
beams represents the polished surface misorientation magnitude and sense. 
More particularly, and further referring to FIG. 5, the method begins by 
first immobilizing the substrate 12, at step 204, such that the polished 
surface 14 is exposed and can be easily targeted by the energy beam Xo and 
the optical beam O. In accordance with the system of the present invention 
20, this step may be conveniently carried out by mounting the substrate in 
the substrate fixture 28. 
Following immobilization of the substrate, the respective energy and 
optical beams Xo and O are generated at step 206. The generated energy 
beam is then directed along a predetermined path, at step 208, toward the 
substrate surface 14. The X-ray beam, because of its nature, passes 
through the polished surface 14 and continues through to one of the set of 
underlying crystalline planes 16 (FIG. 2A), where the beam is diffracted 
at step 210 because the planes satisfy the Bragg condition. In satisfying 
the Bragg condition, the angle of diffraction matches the angle of 
incidence with respect to the crystalline planes, and thus serves as a 
convenient reference marker for the crystalline plane orientation. 
To take advantage of the reference marker represented by the diffracted 
energy beam X', the method continues, at step 212, by guiding the optical 
beam along the predetermined path followed by the energy beam Xo. The 
optical beam O reflects off the substrate surface at a reflection angle 
equivalent to the angle of incidence, at step 214, to define the 
reflection beam O'. The resulting reflection beam serves as a measurable 
quantity to gauge the relative orientation of the polished surface 14 with 
respect to the crystalline planes 16. 
The surface misorientation is then determined by detecting the respective 
diffracted and reflected beams and the relative angular relationship 
therebetween, at step 216. To identify the sense of misorientation, the 
relative beam positions are determined, at step 218. 
Referring now to FIGS. 4 and 6, the detecting and determining steps, 216 
and 218, are conveniently carried out by implementing the beam detection 
device 30, according to one embodiment of the apparatus of the present 
invention. As shown in FIG. 3, the detection device is disposed 
substantially opposite the respective energy and optical beam sources 22 
and 26 to define a detection plane, at step 220 (FIG. 6). This detection 
plane is located a measurable distance from the substrate and is arranged 
to intersect the diffracted and reflected wavefronts, or beams X' and O'. 
The measurable distance that defines an optical lever R is measured at 
step 222 and is used to calculate the misorientation, step 224. 
Once positioned, the circular scale 46 of the device 30, FIG. 4, is 
adjusted to align the x-ray beam detector 64 with the diffracted energy 
beam X', and the multi-detector mechanism bar 58 is placed substantially 
perpendicular to the diffracted beam path, as shown in FIG. 2. The bar is 
then rotated along the track 48, and the optical beam detector 66 then 
slid along the bar 58 until aligned with the reflected optical beam O'. 
The relative spacing between the detected beams may be easily identified 
by the spaced apart graduations 68 so as to represent the quantity AX. 
This quantity is then used with the optical lever, R, to calculate the 
relative angular separation, or misorientation .theta., according to the 
equation: 
EQU .theta.=.DELTA.X/2R 
Moreover, by merely noting the relative position of the respective 
diffracted and reflected beams X' and O', as indicated by the angular 
position of the multi-detector mechanism 56 along the scale track 54, one 
may easily determine the sense of misorientation. 
While the steps above have been described with reference to using the 
aforedescribed multi-detector mechanism 56, it is anticipated that a flat 
or spherical scale may be used that eliminates the detectors 64 and 66, 
and instead includes a readable surface such that the relative angular 
spacing of the beams X' and O' may be visually determined through, for 
instance, visual inspection. 
As noted previously, the failure to fully screen wafers according to the 
magnitude and sense of misorientation raises the potential for unnecessary 
waste and costs due to defects which form during subsequent processing 
operations because the misorientation is incorrect. For example, FIG. 7 is 
a photograph showing the effect of surface misorientation on the quality 
of an InP layer grown by liquid phase epitaxy on an InP substrate having a 
formed spherical surface. The exact (100) surface orientation produced a 
highly flat facet (the central planar area), whereas the terraced growth 
occurred at the misoriented surface all around it. 
Those skilled in the art will appreciate the elegant straightforwardness of 
the detection apparatus and measuring method described above for the many 
advantages afforded. Especially significant is the minimal time required 
to make the misorientation measurement. For this reason alone, the 
apparatus and method of the present invention readily lend themselves to 
high production semiconductor processing operations. By having the 
capability of screening wafers in a high production environment during the 
early stages of device fabrication, substantial reductions in the number 
of defective devices may be realized resulting in corresponding reductions 
in cost. 
Additionally advantageous is the non-destructive nature of the invention 
which further minimizes costs by eliminating any additional etching 
processes and chemicals. This allows measurements of actual production 
wafers which dramatically reduces unknown variables typically associated 
with defective wafers set aside for destructive testing purposes. 
The improved accuracy and repeatability of the invention will also be 
appreciated by those skilled in the art. These features are realized 
because of the "direct" measurement approach utilized by the invention, 
which eliminates any interpreting steps characteristic of indirect 
methods. 
The process may in most respects be automated if the fixture adjustments 
are automated and the motions of support 38, riser 42 and guide 60 are 
achieved, e.g, by stepper motors under the control of a computer operated 
according to a program as set forth in the flow charts of FIGS. 5 and 6. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and detail may be 
made therein without departing from the spirit and scope of the invention.