Height measuring system

The invention of this disclosure is a profiling and testing system that uses an air probe to determine the contour of a wafer containing a plurality of dies so that an electrical sensor may automatically step from die to die and test the completed dies in the wafer.

FIELD OF THE INVENTION 
This invention relates to the manufacture of microelectronic circuits and 
more particularly to a method and apparatus for probing and testing 
microelectronic circuits. 
DESCRIPTION OF THE PRIOR ART 
The use of microelectronic circuits have greatly increase during the past 
decades a period of general economic inflation, while the costs of 
manufacturing microelectronic circuits have been decreasing. A 
microelectronic circuit is a complete electronic circuit contained on a 
single layer of silicon comprising transistors and perhaps diodes, 
resistors and capacitors, along with their interconnecting electrical 
connectors. Hundreds of microelectronic circuits or dies may be 
manufactured on a single wafer of silicon. 
In order to reduce manufacturing costs, the prior art tested the dies to 
determine which dies are defective. The defective dies were marked with a 
dab of paint so that people would know not to continue processing these 
dies. The prior art utilized a plurality of probes which were positioned 
around a ring or probe card to perform the electronic tests. The ring or 
probe card was lowered so that the probes would be positioned over the 
input and output pads of individual dies. The probes must make good 
electrical contact with the pads in order to have an accurate electrical 
test. Often times the wafer may not be flat and there may be local 
variations in the surface geometry of the wafer so that when the probe was 
stepped from one die to the next die, the probe would not exert the same 
pressure on the dies' test pads and poor electrical contact was obtained 
resulting in erroneous test data. The prior art attempted to solve the 
foregoing problem by utilizing another probe, in conjunction with the 
prior probe. This additional probe had a small switch or contact on its 
side so that when the wafer was moved in the z direction the contact on 
the probe would determine when the wafer was at the proper height for 
electrical tests to be performed. One of the disadvantages of the prior 
art is that the contacts of the additional probe are very susceptible to 
contamination and oxidation thereby limiting the reliability of the entire 
probe system. Another disadvantage of the prior art is that the 
contact-type probe was delicate and was easily damaged. An additional 
disadvantage of the prior art was that painting the defective dies with a 
dab of paint sometimes damaged dies that were not defective. A further 
disadvantage of the prior art was that it was difficult to know if an 
incomplete or partial die was near the edge of the wafer. When this 
happened, electrical probes would be testing the die while other probes 
were hanging over the edges of the die resulting in time consuming 
electrical tests being performed on obviously defective dies. 
SUMMARY OF THE INVENTION 
This invention overcomes the disadvantages of the prior art by providing an 
improved probing system that is used to determine the height to test the 
dies on a wafer and determine the edge of the wafer so that time consuming 
electrical tests will not have to be performed on incomplete dies. Another 
advantage of this invention is that the aforementioned probing may be used 
in conjunction with a computer to determine the center of the wafer and 
the test points of the dies so that the testing sensor will be able to 
automatically step from die to die. The foregoing is possible because the 
location of the dies' test pads were calculated by the computer. A further 
advantage of this invention is that it is no longer necessary to mark 
defective dies with a dab of paint, since the computer can store the 
location of the defective die in its memory and the die may be 
automatically removed after the wafer is diced. The computer may also 
record the testing information pertaining to each die on many wafers. The 
aforementioned testing information may be analyzed to determine the large 
number of defective dies at specific wafer locations. If a large number of 
dies were defective at a particular wafer location, it would be advisable 
to check prior manufacturing steps and determine what is causing dies to 
fail at specific wafer locations. The apparatus of this invention achieves 
the foregoing by utilizing a probe that senses differences in air pressure 
between the die and probe to adjust the height of the chuck that is 
holding the wafer. 
It is an object of this invention to provide a new and improved system and 
apparatus for testing dies on wafers. 
It is another object of this invention to provide a new and improved system 
for determining the geometric profile of a wafer so that an electronic 
sensor may automatically move from the test pads on one die to the test 
pads on another die. 
It is a further object of this invention to provide a new and improved 
system that automatically tests dies and records their test performance.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to the drawing in detail, the reference character 11 represents a 
probe that has a stream of air 12 that exits from one end of probe 11. The 
other end of probe 11 is coupled to pressure sensor or transducer 13. The 
output of pressure sensor 13 is coupled to one of the two inputs of 
comparator 14. The other input of comparator 14 is a calibration input 
that is adjusted to a specific voltage that represents a specified height 
of probe 11. Comparator 14 has two outputs. A high output and a low 
output. The high output is coupled to one of the two inputs of exclusive 
OR gate 15 and one of the two inputs of AND gate 16. While the low output 
of comparator 14 is coupled to one of the two inputs of exclusive OR gate 
15 and one of the two inputs of AND gate 17. The output of gate 15 is 
coupled to the second input of AND gate 16 and the second input of AND 
gate 17. The output of AND gate 16 is coupled to the first input of 
computer 18 and the output of gate 17 is coupled to the second input of 
computer 18. Computer 18 has two outputs, the first output is coupled to 
the input of Z drive 19 and the second output is coupled to the input of 
X, Y position driver 20. The output of Z drive 19 is coupled to one of the 
inputs of chuck 21 and the output of driver 20 is coupled to one of the 
inputs of chuck 21. A wafer 22 having a plurality of dies (not shown) 
rests on chuck 21. Wafer 22 is usually circular and for purposes of this 
description, we will assume that wafer 22 is circularly shaped. A 
plurality of sensors 25 perform electrical tests on the dies that comprise 
wafer 22. Sensors 25 are attached to probe 11 and coupled to a tester (not 
shown) which sends test results to the memory of computer 18. The 
apparatus of this invention determined the positions of the aforementioned 
sensors relative to wafer 22 by having probe 11 direct a stream of air 12 
at the dies that comprise wafer 22. Air stream 12 will have turbulent flow 
near wafer 22 and be reflected back toward probe 11. The aforementioned 
air stream will create a differential pressure. This differential pressure 
will be the difference between the pressure of air stream 12 and the 
ambient atmospheric pressure on the surface of wafer 22 with no air being 
emitted from probe 11. This differential pressure will be used to 
determine the height of electrical testing sensors 25. Sensor 13 produces 
an electrical output signal or output voltage that represents the 
differential pressure. 
During the calibration of this system a reference voltage will be placed on 
the calibration input of comparator 14. The above-referenced voltage will 
be determined by placing a feeler gauge (not shown) between probe 11 and 
wafer 22. The thickness of the feeler gauge will depend upon the specified 
working distance of electrical testing sensors 25. If the testing sensors 
are either too close or too far away from the dies' test, the testing 
sensors will not have the correct sensitivity and erroneous test results 
will be obtained. The referenced voltage will represent the height of 
wafer 22 plus the height of the feeler gauge. Comparator 14 will be used 
as a null sensing device i.e., is wafer 22 above or below the reference 
height. If wafer 22 is above the reference height, comparator 22 will have 
an output that will be transmitted on the high voltage output line of 
comparator 14. The aforementioned output would represent the distance that 
wafer 22 is above the reference height. In the event wafer 22 is below the 
reference height, comparator 14 will have an output on its low voltage 
line. This output would represent the distance that wafer 22 is below the 
reference height. If wafer 22 is at the reference height (the correct 
height for the performance of electrical test), then the input voltage to 
comparator 14 will be the same as the reference voltage and comparator 14 
will have no output. 
When probe 11 indicates that wafer 22 is above the reference height, 
comparator 14 will transmit a pulse on its high voltage line to one of the 
inputs of AND gate 16 and one of the inputs of exclusive OR gate 15. Gate 
15 will be enabled, which will cause gate 16 to be enabled. Gate 16 will 
notify computer 18 that the height of wafer 22 is too high. Computer 18 
will tell Z drive 19 to reduce the height of chuck 21 and wafer 22. When 
probe 11 indicates that wafer 22 is below the reference height, comparator 
14 will transmit a pulse on its low voltage line to one of the inputs of 
AND gate 17 and one of the inputs of the exclusive OR gate 15. Gate 15 
will be enabled, which will cause gate 17 to be enabled. Gate 17 will 
notify computer 18 that the height of wafer 22 is too low. Computer 18 
will tell Z drive 19 to increase the height of chuck 21 and wafer 22. The 
foregoing process will continue until a null is reached and wafer 22 is at 
the reference height. 
The surface of wafer 22 may be profiled using the above circuit in 
conjunction with X, Y position driver 20. The surface of wafer 22 is 
profiled so that the contour of wafer 22 may be known and electrical test 
sensors 25 automatically driven to the correct height and test pad 
location of individual dies on wafer 22. This is done to permit the 
automatic testing of the dies on wafer 22 and the determining of the edges 
of wafer 22. Hence, the height and location of the test pads on each 
individual die would not have to be measured or test performed on 
incomplete dies. The program contained within computer 18 to perform the 
foregoing is 
______________________________________ 
SUBPROGRAM profiler, 
FUNCTION zprof --height :INTEGER; 
VAR 
zz :INTEGER; 
x, y, zx, zy, zx y :REAL; 
BEGIN 
x := xcurrent - xcenter ; y := ycurrent - ycenter; 
IF x &gt; 0.0 
THEN zx := (z[1] + z[4]) / 2.0 { first or fourth quad } 
ELSE zx := (z[2] + z[3]) / 2.0;{ second or third quad } 
IF y &gt; 0.0 
THEN zy := (z[3] + z[4]) / 2.0 { third or fourth quad } 
ELSE zy := (z[1] + z[2]) / 2.0;{ first or second quad } 
zxy := (zx * ABS(x) + zy * ABS(y)) / (radius*0.8/SQRT(2.0)); 
zz := round ( zxy ) + zz0; 
zprof --height := zz; 
END; 
PROCEDURE z --up --profiled; 
VAR 
z :INTEGER; 
BEGIN 
z := zprof --height + zdifference + r --parm.z --overtravel; 
movez ( z ); 
END; 
PROCEDURE z --down --profiled; 
VAR 
z :INTEGER; 
BEGIN 
z := zpos - r --parm.z --cvertravel - r --parm.z --clearance; 
movez ( z ) 
END; 
PROCEDURE profile ; 
CONST 
diam --offset = 200; 
scan --number = 2120; 
VAR 
ix, iy, i, q :INTEGER; 
pt2rxy, pt3rxy, sc --rxy, xdiam, 
xC, yC :XINTEGER; 
x,y :ARRAY [1..4] OF XINTEGER; 
mxy, nxy, rxy :REAL; 
newdiameter :REAL; 
PROCEDURE measure --z ( x, y :XINTEGER; VAR z -:INTEGER ); 
BEGIN 
movez ( zsafe ); 
movexy ( x, y ); 
xcurrent := x; ycurrent := y; 
{ . . . start jogging if below ref height } 
IF hsense &lt; 0 
THEN BEGIN 
REPEAT 
jogzup; 
UNTIL hsense &gt;= 0 ; 
END; 
z := zpos; 
END; 
PROCEDURE tincecce ( VAR xe, ye :XINTEGER; dx, dy -:INTEGER ); 
CONST 
one --step = 5; 
VAR 
1, ze :INTEGER; 
x, y :XINTEGER; 
BEGIN 
xcurrent := x0 + sc --rxy * dx; 
ycurrent := y0 + sc -- rxy * dy; 
movexy ( xcurrent, ycurrent ); 
WHILE hsense &lt; 0 DC 
jogzup; 
ze := 4 * r --parm.z --scale; {continue 4 mils higher} 
FOR i := 1 TO ze DO jogzup; 
x := xcurrent + pt2rxy * dx; 
y := ycurrent + pt2rxy * dy; 
move ( x, y ); 
xcurrent := x: ycurrent := y; 
{ . . . wait until chuck is below reference height . . . } 
REPEAT UNTIL hsense &lt; 0 ; 
stop --xy; 
read --xy(xe,ye); 
{ . . . jog back and find exact edge . . . } 
REPEAT 
xe := xe - one --step * dx; 
ye := ye - one --step * dy; 
move ( xe, ye, ); 
xcurrent := xe; ycurrent := ye; 
{ . . . wait until edge sensor changes to plus . . . } 
UNTIL hsense &gt;= 0; 
movez ( zsafe ); 
END; 
BEGIN 
pt rxy := rcure ( radius * 0.3 / SORT(2.0) ); 
ptZrxy := pt8rxy DIV Z; 
sc --rxy := rol ( radius / SQRT(2.0) ) - tar --number, 
hsensor (TRL ; { turn sense air . . . } 
movez ( z200 ), 
movexy ( xsen, ysen ); 
xcurrent := xsen; ycurrent := ysen; 
x0 := xsen; y0 := ysen; 
zsafe := z200; 
q := 4; 
{ . . . measure height at the wafer center . . . } 
measure z ( xsen, ysen, zz0 ); 
zsafe := zz0 - 12 * r --parm.z --scale; 
FOR i := 1 TO 4 DO 
BEGIN 
IF (1 &lt; q) AND (q &lt; 4 ) THEN 
ix := -1 ELSE ix := 1; 
IF q &lt; 3 THEN 
measure --z(xsertix*pt8rxy,ysertiy*pt8rxy,z[q]); 
z[q] := z[q] - zz0; 
IF i &lt;&gt; 2 THEN 
findedge(x[i],y[i],ix,iy); 
IF i = 3 THEN 
BEGIN 
x0 := (x[3] + x[1]) DIV 2; 
y0 := (y[3] + y[1]) DIV 2 
END; 
q := q - 1; 
END; 
mxy := (abs(x[1]- x[3]) + abs(y[1] - y[3])) / 4.0; 
nxy := (abs(x[4] - x0) + abs(y0 - y[4])) / 2.0; 
rxy := (((mxy * mxy) / nxy) + nxy) / 2.0; 
xdiam := round(rxy); 
xron --con := [4] + xdiam; 
xcenter := xcen - xser + xnor --con; 
ynon --con := y[4] - xdiam; 
ycenter := ycen - ysen + ynor --con; 
xdiam := xdiam * 2 + diam --offset; 
rsq := xdiam * xdiam DIV 2; 
newdiameter := xdiam; 
newdiameter := newdiameter * SQRT(2.0); 
movez ( z200 ); 
hsensor ( FALSE ); 
END 
______________________________________ 
Thus, wafer 22 is profiled by having probe 11 measure heights at several 
points on wafer 22 so that computer 18 may make a surface approximation of 
wafer 22. 
Typically, wafer 22 is profiled by the operator of this invention by 
placing the nominal center of wafer 22 point (X.sub.1 Y.sub.1) under probe 
11. The real center of wafer 22 is determined by having driver 20 move 
chuck 21 and wafer 22 in X and Y until probe 11 goes off the edge of wafer 
22. During the foregoing movement, the height measurement will be 
overdriven so that sensor 13 will always read high. Thus, when a low 
reading is sensed by sensor 13 and comparator 14, probe 11 may be over the 
edge of wafer 22. When probe 11 goes off the edge of wafer 22, computer 18 
will note the X, Y position detected by detector 20 just before probe 11 
went off the edge. For purposes of discussion we will call the 
aforementioned edge point (X.sub.2 Y.sub.2). Now driver 20 will cause 
chuck 21 to move wafer 22 under probe 11 in such a manner that the 
previous line X.sub.1 Y.sub.1 ; X.sub.2 Y.sub.2 will be retraced and the 
above-mentioned line continued until probe 11 senses another edge of wafer 
22. This edge of wafer 22 will be called point (X.sub.4 Y.sub.4). When 
line X.sub.2 Y.sub.2, X.sub.3 Y.sub.3 is bisected and a perpendicular line 
X.sub.4 Y.sub.4, X.sub.5 Y.sub.5 drawn through line X.sub.2 Y.sub.2, 
X.sub.3 Y.sub.3 the perpendicular line will be the diameter of wafer 22. 
Points (X.sub.4 Y.sub.4) and (X.sub.5)(Y.sub.5) will be the end points of 
the aforementioned perpendicular line on the circumference of wafer 22. 
Computer 18 will determine the mid-point of line X.sub.4 Y.sub.4, X.sub.5 
Y.sub.5 which will be the center of wafer 22 point X.sub.6 Y.sub.6. Once 
computer 18 determines the center of wafer 22, it will be able to 
calculate the location of all of the test pads on each individual die on 
wafer 22 as well as the location of incomplete dies. The reason for the 
above is that the sizes of the dies are known by the computer and are 
unique for a particular type of wafer. In order to adequately map the 
surface of wafer 22 and determine an approximation of the surface of wafer 
22, probe 11 will measure the height of three or four points in each 
quadrant of wafer 22. The three or four points measured in each quadrant 
will define a plane. Computer 18 will know that in any plane the Z height 
measured by probe 11 will be a function of X and Y. Computer 18 will note 
the X, Y and Z measured readings and compute a Z surface profile (assuming 
that wafer 22 is planar within a quadrant) wherein Z is a function of X 
and Y. Thus, whenever electrical sensors probes 25 test the pads of a die 
computer 18 will direct driver 20 to move chuck 21 to the test pads of 
that die and computer 18 will direct Z drive 19 to move chuck 21 to the 
proper height for performing that test. Hence, the apparatus of this 
invention will be able to automatically step from die to die on wafer 22 
without individually sensing the location and height of each die's test 
pads. 
Computer 18 knows the X Y location of each die on wafer 22 thus it also 
knows which die is being tested at a given time. Hence, the results of the 
test for each die may be stored in the memory of computer 18 so that the 
test results of the dies that failed particular tests may be compared with 
dies on other wafers at the same location to check if something is wrong 
with the wafer manufacturing process. It is no longer necessary to mark 
defective dies with a dab of paint and risk damaging other dies, since 
computer 18 knows which dies are defective, it may send a map of these 
dies to a die attach machine (not shown) to pick up the good dies in the 
dicing process. 
The above specification describes a new and improved wafer height measuring 
system that may be used to test dies. It is realized that the above 
description may indicate to those skilled in the arts additional ways in 
which the principals of this invention may be used without departing from 
its spirit. It is, therefore, intended that this invention be limited only 
by the scope of the appended claims.