[0001] This invention relates to methods of trimming thin metal film resistors.
[0002] The invention is particularly, but not exclusively, applicable in manufacturing the
graphic tablet disclosed in US 4,087,625, DE 2,753.968, FR 2,376,399 and GB 1,546,317.
This tablet is particularly adapted to be mounted on the face of a display (such as
a gas panel or a cathode ray tube) because it requires only a single layer of thin
transparent metal electrodes in contrast to the construction in which there are two
layers of orthoganally extending electrodes (and an intervening layer of insulation)
for sensing X and Y co-ordinates of the position of the pen that is held on the surface
of the display. The single metal layer is shaped to provide elongated rectangular
resistive voltage dividers across the top and bottom edges (or across the side edges)
of the active area of the tablet. Complementary shaped triangular electrodes extend
along only one dimension from the two voltage dividers. These triangular electrodes
are given an alternating voltage that is capacitatively coupled to a pen that is held
to a selected point on the display by a user of the display. The pen voltage is proportional
to the position of the pen in the X - Y co-ordinate system of the tablet.
[0003] It is desirable for the two voltage dividing resistors to be closely linear to establish
a linear relationship between the position of the pen along the X dimension and the
amplitude of the pen signal. Resistor linearity is less significant for tablet operation
in the Y dimension. This linearity can be achieved by forming the resistors and the
triangular electrodes on the glass substrate by any technique that is suitable for
economical large scale production and by then trimming the resistors individually.
A general object of this invention is to provide a new and improved method and apparatus
for trimming such a resistor.
[0004] US 4,184,062 teaches a resistor made of a strip of film material with a trimming
cut made along one edge of the resistor by means of a laser. As the laser trim operation
proceeds from one end of the resistor to the other, the resistance to ground of the
point of the trimming operation is measured. The slider of a precise master resistor
is moved in step with the trimming operation. Where the resistor that is being trimmed
is accurate, the voltage at the slider of the master resistor equals the voltage at
the point of the trim. Circuits sense any difference between the two voltages and
move the cutting operation inward as necessary to raise the resistance of the resistor
being trimmed or outward to decrease the resistance of the resistor being trimmed.
[0005] This invention seeks to provide a method for trimming a resistor which avoids an
ohmic contact probe on the surface of the resistor. It also seeks to trim a resistor
to be linear independent of the absolute value of resistance. This invention further
seeks to trim a resistor to a linear shape in a way that removes the least amount
of the conductive layer and thereby maintains the lowest resistance with linearity.
The invention also seeks to provide a new resistor trimming method that is simple
to use for making a second trim for increased linearity over a resistor that has had
at least an initial trim.
[0006] In another prior art technique, small notches are formed along the edge of the resistor
to increase the resistance. This technique is accurate only if the measurement and
the trim are made together. This invention seeks to provide an improved method for
trimming a resistor in which resistance measurements are made independently of the
trim operation. This feature is-an advantage in a tablet operation because the information
about the residual nonlinearity of the tablet is used in aligning the tablet with
the display face for a minimum mismatch.
[0007] According to the invention, a method of trimming a thin metal film resistor, comprises
applying an alternating voltage across the resistor, capacitatively measuring the
voltage of the resistor at discrete points along the length of the resistor, storing
the measured values, calculating a resistance profile from the measured values, and
trimming the resistor along an edge according to the resistance profile.
[0008] In a method according to the invention, the resistance of a tablet resistor is first
measured at discrete points using a capacitative voltage probe that is similar to
the pen used in normal tablet operation. The capacitative probe "sees" an area of
the resistor that is large enough to avoid any pin holes in the resistor film affecting
the resistance reading. The resistance measurements are stored in a data processing
system and are used to calculate a profile for a resistor trim oepration. The trim
is performed by conventional apparatus such as electro-erosion apparatus. The resistance
is measured again and if necessary, a second or subsequent trim is made.
[0009] The scope of the invention is defined by the appended claims; and how it can be carried
into effect is hereinafter particularly described with reference to the accompanying
drawings, in which :-
FIGURE 1 shows a tablet in which resistors to be trimmed according to the method of
this invention;
FIGURE 2 shows an edge view of the tablet and apparatus for measuring resistance and
calculating a resistance profile in one part of the method;
FIGURE 3 is a graph illustrating the resistance measurements;
FIGURES 4 and 5 are graphs illustrating steps in the calculation of the resistor trim
profile;
FIGURE 6 is a top view of a resistor of the tablet of Fig.l illustrating the trim
operation; and
FIGURE 7 is a view similar to Fig.2 illustrating the resistor trim operation and apparatus.
[0010] The invention will be described as applied to the trimming of a thin film resistor
in a graphic tablet disclosed in US-4,087,625, DE-2,753,968, FR-2,376,399 and GB-1,546,317.
The tablet shown in Fig.l, could be in a substantially vertical plane for use with
a display device or in a substantially horizontal plane for use in an application
such as providing handwritten input to a computer system. The terms "Top", "Bottom",
"Sides" and "Up", "Down", "Right", "Left" will be used to designate positions and
directions in the plane of the drawing. Motions in a direction orthogonal to the plane
of the drawing will be called "Raising" and "Lowering" as is conventional for describing
positions of a hand-held pen.
[0011] The tablet has a substrate 12 of glass. A film of a transparent conductor such as
indium-tin oxide is vacuum deposited on the surface of the glass and is then etched
to form a patten of electrodes identified as 14, 15, 16, 17, 18, 19, 20, 21, 22 and
23. Elongated rectangular electrodes 14 and 15, which act as resistors, extend along
opposite edges of the tablet. Small connector electrodes 16 and 17 extend at fixed
intervals from the electrodes 14 and 15, respectively. Elongated triangular electrodes
18 and 19 extend in interdigitated formation from the electrodes 16 and 17, respectively,
and form the active area of the tablet. These electrodes are energized to establish
a voltage that can be sensed capacitatively by a conventional capacitative pen (not
shown). The pen voltage is proportional to the position of the pen in either the X
or Y dimension, depending on the way in which the electrodes are energized. Screened
wide metal tabs 26 and 27 connect to the left ends of the electrodes 14 and 15, respectively.
Similar tabs 28 and 29 connect to the right ends of the electrodes. The tabs connect
the electrodes to circuits (not shown) for normal operations that establish an alternating
voltage gradient across one dimension of the tablet. These tabs are also used to apply
test voltages to the electrodes for the manufacturing steps of this invention.
[0012] In the preferred orientation, the electrodes extend horizontally and the resistors
are located along the sides because the active area is ordinarily made wider than
it is high and this orientation of the electrodes permits a smaller area to be used
for the resistors.
[0013] In subsequent stages of the manufacturing process, additional transparent layers
are formed over the layer of metal electrodes. These layers space the pen slightly
from the triangular metal electrodes to produce an averaging effect on the voltage
of the discrete nearby electrodes that is sensed by the pen. Consequently, the pen
does not sense the discrete voltage steps that are established by connector electrodes
16 and 17 when a voltage is applied across the electrodes 14 and 15.
[0014] . The normal operation of the tablet will be described as an introduction to the
related resistance measuring operations.
[0015] For sensing the pen position in the Y dimension, the upper resistor electrode 14
(or the lower resistor electrode 15) is held at ground potential all along its length
by grounding its two tabs 26 and 28 (or 27 and 29) so that each of the associated
triangular electrodes 18 (or 19) is also at ground potential. The lower resistor electrode
15 (or the upper resistor electrode 14) is given an alternating voltage uniformly
across its length by connecting both tabs 27 and 29 (or 26 and 28) to an alternating
voltage point so that each of the associated triangular electrodes 19 (or 18) also
receives this alternating voltage. When a pen is positioned on the tablet, it receives
an alternating voltage by capacitative coupling to the nearby triangular electrodes
18 and 19. This capacitative coupling is proportional to the area of the electrodes
in the field of view of the pen. When a pen is positioned towards the bottom (or top)
of the tablet, is receives nearly the full alternating voltage of the resistor electrode
15 (or 14). As the pen is moved towards the top (or bottom) of the tablet, the capacitative
coupling to electrodes 18 (or 19), which are at ground potential, becomes greater
and the pen voltage falls in proportion to its Y dimension position.
[0016] When the tablet is operated in the mode just described, the movement of the pen horizontally
across the tablet without a change in position of the Y dimension should produce no
change in the pen voltage. To achieve this tablet linearity, the voltage at each connector
electrode 17 (or 16) must be kept essentially identical in spite of the fact that
the capacitative charging current for triangular electrodes 18 and 19 flow in resistor
electrodes 14 and 15 and are greater toward the outside ends of the resistor electrodes
than toward the middle. Accordingly, for accuracy in the Y dimension operation, the
resistor electrodes 14 and 15 should have a low resistance value. The thickness of
resistor electrodes 14 and 15 is limited by the requirement that the associated triangular
electrodes 18 and 19 must be thin in order to be transparent. The thickness of the
electrodes is of the order of 1300 Angstroms. The width of the resistor electrodes
14 and 15 is limited by the requirement to make the active area of the tablet as large
as possible within the limited overall dimensions of the glass substrate 12.
[0017] For sensing the position of the pen in the X dimension, the left (or right) ends
of both resistor electrodes 14 and 15 are held at ground potential and the right (or
left) ends are given the same alternating voltage. The two resistor electrodes 14
and 15 are made linear so that the potential at a conector electrode 16 of a triangular
electrode 18 is identical to the potential at the connector electrode 17 of the corresponding
triangular electrode 19. A pair of triangular electrodes 18 and 19 then has the effect
of a single rectangular electrode at the potential of the associated connector electrodes
16, 17. When a linear tablet is operated for sensing the position of the pen in the
X dimension, the pen can be moved up and down the tablet at a constant X dimension
position without changing the voltage that is sensed by the pen. When the pen is moved
from left to right across the tablet, the pen voltage increases uniformly (or decreases,
if the connections are reversed). For this operation both resistor electrodes 14 and
15 must be linear so that the alternating voltage will be identical at corresponding
points on the two resistor electrodes. However, it is not necessary for the total
resistances of the two resistor electrodes to be identical. Thus, an object of this
invention is to provide a resistor trim operation that maintains the lowest resistance
possible for each individual tablet (in contrast to trimming the resistors of each
tablet to a specified resistance value).
Measuring Resistor Linearity
[0018] A resistance measuring probe 34 (Fig.2) has a metal electrode 35 and a dielectric
body 36 preferably of a plastics material having a high dielectric constant to enhance
the capacitative coupling. The electrode 35 is spaced by dielectric from a tablet
electrode to establish a capacitative circuit so that the probe voltage is related
to the voltage of the tablet electrode as in normal tablet operation using a pen.
[0019] The tablet comprising the substrate 12 and the electrodes is positioned in a vacuum
check or other suitable fixture (not shown) and the probe 34 is connected by conventional
means, represented by line 38, to an X-Y stepper positioning mechanism 39. The probe
is first positioned at one end of resistor electrode 15 approximately on the centre
line (to avoid edge effects) and then stepped along the resistor electrode in increments
of about 12.7mm (.50 inch). During this resistance test, an oscillator 40 supplies
an alternating voltage to the tab 29 located at the right end of resistor 15 and the
tab 27 at the left end of the resistor is connected to ground. Thus, the voltage of
oscillator 40 appears between tabs 29 and 27 and resistor electrode 15 resistively
divides this voltage so that an alternating voltage gradient appears along the length
of the resistor. This alternating voltage is linear with respect to the length of
the resistor according to the uniformity of the resistance. The voltage of the area
of resistor electrode 15 under probe 34 is capacitatively coupled to the electrode
35 and this voltage is transmitted to a conventional analog-to-digital converter 44.
Digital values of the voltage are supplied to a conventional data processor 45 which
processes and stores the incremental values. The processor 45 is also connected to
control the operation of the stepper mechanism 39.
[0020] It will be understood that the alternating voltage may be applied to opposite ends
of the electrode 15, and that the operation is repeated for the electrode 14.
[0021] This capacitative measurement technique avoids several problems that can be encountered
with ohmic contact probes. For example, the spacing between electrodes 15 and 35 produces
an averaging effect as has been explained for the operation of the tablet. The spacing
is made great enough to average out the effect of pin holes that may occur in resistor
electrodes 14 and 15, because the probe electrode "sees" an area of the electrode
that is suitable in size. Otherwise, the spacing between electrode 35 and electrode
14 or 15 is made small to produce good capacitative coupling for a strong signal and
to avoid effects from any nearby edge of the resistor electrode so that the resistor
electrode appears as an essentially infinite plane.
The Resistance Data
[0022] Figure 3 is a graph which illustrates the data obtained from the resistance measuring
apparatus just described. The numbers O to 10 along the horizontal axis represent
discrete sample points. The vertical axis represents the voltage measurement, equivalent
to the resistance to the grounded tab 27. A line connecting points on the graph illustrates
that the resistor is slightly nonlinear. The non-linearity is measured by calculating
a least mean squared error linear function by linear regression analysis (a conventional
statistical function) and by comparing the actual voltage or resistance values with
this linear function. If the points are within a suitable percentage (preferably 0.25%),
the resistor is accepted for product use without the trim operation.
[0023] Figure 4 is a graph which illustrates the next stage in processing the data represented
in Fig.3. In Fig.4, the left-hand vertical axis represents the incremental resistance
or difference between the sample point identified on the horizontal axis and the preceding
sample point obtained by subtracting one resistance value from the next. The height
of a point in Fig.4 corresponds to the slope of the line in Fig.3.
[0024] The highest incremental resistance, in this case at points 5 and 6, is obtained by
comparison and a normalized resistance for each point obtained by dividing each value
by the highest value, which occurs at points 5 and 6. The right-hand vertical axis
indicates the normalized resistance values. Thus, points 5 and 6 have normalized values
of 1 and the other points have smaller normalized values. The normalized values are
used to determine the absolute width of the-trim as explained hereinafter. The normalized
resistance values are subtracted from 1 to give the graph in Fig.5, a mirror image
of Fig.4. At points 5 and 6, which have maximum incremental resistance values and
normalized resistance value of 1, a zero value occurs.
[0025] The graph of Fig.5 is a suitable profile for the resistor trim operation. No trim
should be required at points 5 and 6 of highest incremental resistance. Similarly,
the greatest trim should be made at point 2, where the lowest incremental resistance
is measured.
The Trim Operation
[0026] The trimming cut is preferably made by a spark erosion apparatus as shown in Fig.7,
but similar metal cutting operations can be made by a laser, and the invention is
applicable to various metal cutting techniques. The data processor 45 supplies the
appropriate commands to the X-Y stepper positioning mechanism 39 to position an electrode
50 of a conventional spark erosion device along the required path. The electrode 50
is connected in circuit with a current source 51 for the spark erosion apparatus and
is in contact with the resistor electrode 15.
[0027] The mechanism 39 moves the electrode 50 along the length the X dimension of the resistor
electrode in steps of a fixed distance preferably 2.54mm (0.1 inch) or one fifth of
the distance between sample points. At the end of a step, the mechanism 39 under the
control of the processor 45 moves the electrode sideways (up or down in the Y dimension)
by an amound equal to one fifth of the difference between the normalized resistances
at adjacent sample points. Thus, the trim profile of Fig.5 is followed in a step fashion-by
the electrode 50 and the actual trim approximates a continuous function of resistance
with respect to position on the resistor electrode.
[0028] This has been exaggerated for illustration in Fig.6, where the incremental resistance
at points 7 and 8 is only two thirds of the maximum value which occurs at points 5,
6 and 10. Because the incremental resistance is approximately proportional to the
width of the resistor, reducing the width between points 7 and 8 to two thirds the
total width raises the incremental resistance to the value at the maximum points.
[0029] The electrode 50 makes a cut 53 that produces a selvedge 15' which is not electrically
connected to the main body of the electrode 15.
[0030] After the resistors have been trimmed, the resistance measurement and data processing
is repeated. In many cases, a single trim brings the resistance with the desired linearity.
If not, a second trim is made. This operation is similar to the steps already described
except that the width of the second trim is calculated from the line 53 of the first
trim.
[0031] The invention is useful in other applications where a metal film resistor is to be
trimmed to be linear but the actual resistor value can be within a specified range.
Although the capacitative probe and the associated alternating voltage circuits have
been found particularly useful with metal film resistors because the probe avoids
the problems that are caused by pinholes when only contact probes are used, the invention
may also be useful in applications in which the film is electrically insulated or
otherwise cannot be suitably contacted by an Ohmic probe. The invention can be implemented
with various kinds of mechanical stepping apparatus and with a variety of metal cutting
techniques. The statistical and mathamatical techniques for calculating the trim profile
from the resistance samples are well known and can be implemented readily in many
programming languages. The analysis of the resistance measuring technique has been
presented in terms of resistance measurements, but it can be presented equivalently
in terms of conductance (the reciprocal of resistance) or from other viewpoints.
1 A method of trimmming a thin metal film resistor, comprising applying an alternating
voltage (40) across the resistor (15), capacitatively measuring the voltage of the
resistor at discrete points along the length of the resistor, storing the measured
values, calculating a resistance profile from the measured values, and trimming the
resistor along an edge according to the resistance profile.
2 A method according to claim 1, in which the calculation of the resistance profile
includes calculating incremental resistance values between discrete points (Fig.4).
3 A method according to claim 2, in which the calculation of the resistance profile
includes normalizing the incremental resistance values to the maximum incremental
resistance value (Fig.4).
4 A method according to claim 3, in which the resistance profile is derived from the
normalized incremental resistance values by deducting them from the maximum normalized
incremental resistance value (Fig.5).
5 A method according to any preceding claim, in which additional points on the resistance
profile are calculated by interpolation between values at adjacent discrete points
(Fig.6).
6 A method according to any preceding claim, in which the capacitative voltage measurement
is effected by positioning a capacitative probe (34) at a sufficient distance above
the resistor to avoid the effects of any pinholes in the resistor.
7 A method according to claim 4 or any claim appendant thereto, in which the resistor
is not trimmed at the point of maximum normalized incremental resistance value, whereby
a minimum increase in total resistance is achieved.
8 A method according to claim 5 or any claim appendant thereto, in which the resistor
trimmer is stepped in a plurality of steps between points according to the profile
which approximates to a continuous function.
9 A method of trimming the thin metal film resistors of a graphic tablet of the type
having two resistors (14,15) located along opposite edges of the tablet and having
interdigitated triangular elements (18,19) extending from the resistors in the active
area of the tablet, comprising the steps of trimming each resistor along an outside
edge by a method according to any preceding claim.