Probe for Determining P or N-Type Conductivity of Semiconductor Material

a. Field of the Invention

The invention relates to a probe for determining the "p" or "n" type conduction characteristic of a semiconductor.

b. Prior Art

In semiconductors, conduction of electricity is explained in terms of majority and minority carriers of electric charge. In n-type semiconductor materials, electrons are the majority carriers and holes, i.e. the spaces left by electrons, are the minority carriers. In p-type semiconductor materials the opposite is true: the holes are the majority carriers and the electrons are the minority carriers.

Semiconductor integrated circuits are often fabricated on a semiconductor wafer substrate, usually by methods similar to those described in U.S. Pat. No. 3,025,589 to J. A. Hoerni. It is necessary at the outset to know the p or n-type conduction characteristic of the wafer substrate.

Conduction type is determined by the kind of impurities, or dopants, added to pure silicon or germanium or the like to transform the pure material into a semiconductor. For example, if a trivalent impurity, such as boron, is added to a pure silicon crystal, a p-type semiconductor material results. On the other hand, if the chemical impurity, or dopant, has a valence of five, such as phosphorus, an excess electron would result with respect to each silicon atom. Phosphorus thus forms an n-type semiconductor material because electrons are the majority carrier.

Although manufacturers of semiconductor substrates, such as silicon and germanium wafers, specify p or n-type, sometimes the specification for a wafer is not known because of mix-ups in handling of the wafers, or other reasons. If integrated circuit manufacturers fail to test each wafer to be fabricated into a batch of hundreds of integrated circuits, one or more bad batches may occur. The time and expense for processing a batch is considerable in view of the relatively large amount of capital equipment required for wafer processing. Therefore, for quality control purposes, many manufacturers have a need for testing the p or n-type of each wafer to be processed.

Previously, manufacturers have used the "hot probe" technique to measure p or n-type. A hot probe consists of two electrodes, one of which is hot. The electrodes are placed in contact with a wafer, which is locally heated, increasing the mobility of majority carriers. Carriers flow away from the heated spot, causing the spot to acquire electrical charge opposite to the majority carriers. While the hot probe is widely used it is destructive because of thermal shock to the material tested.

Another test for p or n-type involves forming a contact diode with a wafer by means of a probe. The direction of current flow, either d.c. or a.c., through the diode indicates conduction type. The wafer usually sits on a conductive support to complete the circuit from the probe. A problem is that the metal support can readily contaminate large areas of the wafer.

An object of the present invention is to devise a non-destructive test method and apparatus for more quickly determining the p or n-type conduction characteristic of semiconductor materials, without contamination of the materials.

The above object has been achieved by using a conductive probe to form a p-n junction with a semiconductor, electrically charging the probe with radio frequency current through a capacitor and then sensing the direction of charge on the capacitor as an indirect indication of the polarity of the p-n junction. If the capacitor charges one way, the semiconductor is p-type; if the capacitor charges oppositely, the semiconductor is n-type. The capacitor is charged by an r.f. oscillator and the capacitor is sensed by at least one voltage comparator referenced to a fixed voltage level. The semiconductor is supported on a non-diffusing dielectric coating over a conductive carrier, such that r.f. current passes through the coating of the carrier, thereby completing the circuit. A display device or data recording equipment receives the output indication signal for use.

An advantage of the invention is that the p or n-type conduction characteristic of a semiconductor may be very rapidly and simply determined without destruction or contamination of the material.

FIG. 1 is a simplified electrical schematic of the semiconductor probe of the present invention.

FIG. 2 is a more detailed electrical schematic of the semiconductor probe of FIG. 1.

FIG. 3 is a perspective view of a housing for the probe of FIG. 1.

FIG. 4 is an alternate housing for the probe shown in FIG. 1.

FIG. 5 is another alternate housing for the probe shown in FIG. 1.

FIG. 6 is a mechanical detail of a probe of the present invention mounted for automated wafer handing.

FIG. 7 is a first operational side sectional view of the mechanical detail of FIG. 6.

FIG. 8 is a second operational side sectional view of the mechanical detail of FIG. 6.

FIG. 9 is a perspective view of an alternate housing for the probe circuit shown in FIG. 1.

With reference to FIG. 1 a probe or V-shaped conductive wire 11 is shown to be in physical and electrical contact with wafer W. Wafer W is resting on an anodized aluminum carrier, not shown, which is non-conductive relative to the wafer, but there is capacitive coupling between the aluminum of the carrier and the wafer W through the anodized coating. Wire 11 is energized by a radio frequency oscillator 13 which operates at approximately 10 MHz. The r.f. energy from oscillator 13 is fed through a very small capacitor 15 having a value of approximately 50 pf.

In operation, the wire 11 and wafer W act as a diode at their contact point. Energy is delivered through capacitor 15 into wire 11 and a charge is built up on capacitor 15, with the direction of charge depending on the polarity of the diode junction formed between wire 381 and wafer W. The circuit is completed through a dielectric coated conductive wafer carrier, not shown, which is capacitively coupled to wafer W with the capacitive value being approximately 10 times the capacitive value of the capacitor 15. If the wafer W has one polarity, i.e. p or n-type, the capacitor 15 will be charged in one direction; if the water W has another polarity, the capacitor 15 will be charged in the opposite direction. The charge on capacitor 15 is sensed by an operational amplifier 17, having a high input impedance, acting as a voltage comparator which puts out a positive or negative signal at output node 19 with respect to a reference voltage which is supplied to the amplifier. The positive or negative polarity characteristic of the wafer indicates its semiconductor type, p or n by means of a corresponding signal at node 19.

To discharge the dielectric coating of the carrier before or after a measurement, a single grounded contact touches the bottom surface of the wafer. Because the wafer being tested has conductivity and the carrier is conductive, the grounded contact between them discharges the charge on the dielectric coating of the carrier in the same way a bleeder resistor operates. During a measurement, the r.f. path through capacitor 15 and through the dielectric coating of the wafer carrier is the major current path and the path through the grounded contact is not significant. A wafer carrier is not required as discussed below with reference to FIG. 4.

In FIG. 2, the electrical configuration of FIG. 1 may be viewed in more detail. The r.f. oscillator 13' enclosed within dashed line 23 may be seen to comprise a transistor 31 connected as an oscillator whose frequency is controlled by a crystal 33 connected between the base and collector thereof. Crystal control is not necessary, but merely preferred. Suitable load members 35 are used to complete the oscillator circuit. The frequency of the oscillator is not critical, but a radio frequency having a wavelength which is long compared to the probe dimension is preferred in order to limit r.f. interference. The selected frequency should also be able to couple through the dielectric coating to the carrier, if one is used, and to ground so that a complete circuit can be formed. Sine waves are preferred, but clipped waves or square waves may be used as long as r.g. interference can be contained.

A transistor amplifier, within the dashed line 25, of the emitter follower type, has input d.c. blocking capacitors 27 and a load resistor 29. The amplifier includes transistor 28 which amplifies the output of oscillator 23 and sends the amplified r.f. signal through capacitor 15 to the first conductive wire 11 which makes contact with wafer W. Contact can be very light because r.f. current is easily transferred to the wafer.

All materials in this instrument, including the carrier 29 and first wire 11, which contact wafer W, are made of materials which will not readily diffuse into, or otherwise contaminate, the wafer. For this reason, first wire 11 is made of tungsten and the carrier 39 is made of anodized aluminum with the anodized coating 41 forming a dielectric barrier between the aluminum base portion 43 of the carrier and the wafer W which is placed on top of the anodized layer and in contact with wire 11. Electrostatic charge built up on the wafer is discharged through a second conductive wire 45, connected to ground at one end and positioned so that its free end contacts wafer W, but not first wire 11. The grounding contact 45 acts as a bleeder resistor to discharge wafer W before or after a measurement, as mentioned previously. Grounding contact 45 is not essential, but serves to discharge the semiconductor material to be measured so that the same measurement can be accurately repeated, or that the initial measurement will be more reliable, since handling a wafer can induce static charge. Note that the wire 11 and grounding contact 45 need not be V-shaped, but such a shape helps to make physical and electrical contact with wafer W. Contact can be very light because r.f. current is easily transferred to the wafer.

When p or n type of a wafer is measured, the capacitor 15 is charged with a polarity depending on the positive or negative polarity type of the wafer because of the polarity of the point contact diode formed between the wire 11 and the wafer itself. This diode formed between the wire 11 and wafer W rectifies the radio frequency current coming through capacitor 15 and thus charges the capacitor to a positive or negative polarity at charging node 47. The charge at node 47 is sensed by one or more operational amplifiers connected as voltage comparators which with ancillary components are enclosed by the dashed lines 51, 81. To make a measurement an r.f. current pulse of about 100 milliseconds is used, although that time is not critical. After a measurement the charge of capacitor 15 at node 47 bleeds into the input impedance of operational amplifier 61 or 83, as well as into wafer W and to ground via wire 45.

A first comparator surrounded by dashed line 51 includes an integrated circuit operational amplifier 61 which may be for example, type CA 3140, which has an inverting input terminal 63 connected to a negative supply so that the amplifier output at terminal 67 is usually positive, except when an n-type wafer is detected. An n-type wafer causes a negative signal from capacitor 15 to be detected through resistor 49 at the non-inverting input 65. The negative signal is amplified and transmitted to the output terminal 67. Resistor 69 established a proper working point for the operational amplifier 61, while diodes 71 form a large resistance for biasing the operational amplifier input with respect to ground. Resistor 62 and capacitor 64 provide feedback for the operational amplifier.

A second comparator surrounded by dashed line 81 includes an integrated circuit operational amplifier 83, which may be identical to integrated circuit operational amplifier 61. The operational amplifier has an inverting input terminal 85 connected to a positive supply so that the amplifier output at terminal 87 is usually negative, except when a p-type wafer is detected. A p-type wafer causes a positive signal from capacitor 15 to be detected through resistor 49 at the non-inverting input 89. The positive signal is amplified and transmitted to the output terminal 87. Resistor 90 establishes a proper working point for the amplifier 83, while diodes 71 form a large resistance for biasing the operational amplifier input with respect to ground. Resistor 91 and capacitor 93 are feedback devices for making the operational amplifier 83 into a voltage comparator, with hysteresis, similar to resistor 62 and capacitor 64 connected across operational amplifier 61.

When an n-type wafer is detected a negative going pulse, such as pulse 95 is detected at terminal 67 while the output at terminal 87 remains essentially constant. On the other hand, when a p-type wafer is detected the output at terminal 67 remains constant, but the output at terminal 87 goes positive, as indicated by the pulse 97. In the event that a wafer triggers both a negative pulse at output node 67 and a positive pulse at node 87, an error has occurred in the system. Such an error may be detected by an anti-coincidence detector which looks for two output pulses occurring at the same time. When such an event is detected, an error is signalled. Under normal conditions, only one output pulse from either terminal 67 or 87 should be detected at one time.

A battery 32 may be connected to positive and negative voltage terminals V+ and V- to power the apparatus of the present invention. A switch, not shown, may be provided for connecting battery 32 to the circuit of FIG. 2 when a measurement is required. Such a switch may be a button as shown in FIG. 3 or a trigger as shown in FIG. 4.

In FIG. 2, two voltage comparators 51 and 81 have been shown to be used for the purpose of detecting the charge on capacitor 15. For purposes of simplicity, only one comparator 51 needs to be used. If only one comparator is used, a wafer is assumed to be p-type, unless a negative pulse is received at terminal 67. Besides using voltage comparators, other types of electronic devices could be used to sense the charge across capacitor 15, such as oscilloscopes or other devices.

Exemplary housings for such a separate probe instrument are shown in FIGS. 3 and 4. With respect to FIG. 3, the housing is a box-like chassis 100 having the first wire 11 projecting from a slot 101 such that a wafer W on a wafer feeding member 103 moving in the direction of arrow H will come into contact with the first wire 11. The second wire 45 projects through a separate aperture 105 with the first and second wires 11, 45 making contact with the underside of wafer W. A measurement of wafer type is made by keying the transistor oscillator by manual means, such as a push button 107. A visual indication of wafer type may be read on a visual display 109. It will be realized that the wire 45 is grounded to the anodized aluminum chassis 100 which is in capacitive relation to the wafer W since the anodized coating forms a dielectric between the wafer and the aluminum layer below it. The chassis 100 need not be anodized aluminum, but could be an insulative material, such as plastic.

In FIG. 4, a hand held probe is shown for measuring wafer p or n type, either while a wafer is supported on a carrier or on wafer shipping trays or other holders, including support by hand. The hand held unit resembles a soldering gun in which forwardly projecting wire 11 is a fine wire made of a material which will not diffuse into the wafer. Like a soldering gun, the body 111 has a trigger or button 113 for energizing the circuit shown in FIG. 2 and a pistol grip or handle 119. The wire 11 is intended to make contact with one side of a wafer, while second wire 112 also makes contact, the wafer supported on a non-conductive surface, such as the insulative transporting device 115. In this case the conductive wire 112 provides a capacitive path to ground so that a dielectric coated conductive wafer carrier is not required to make a measurement. In order to make good contact and to be in a capacitive relation to wire 11, wire 112 has a circular shape surrounding wire 11 in the region where wafer contact is expected. The circular shape is smaller than a wafer to be measured. Both wires 11 and 112 emerge from gun barrel 117 in a parallel, mutually non-contacting relation, but both wires are adapted to contact a wafer. The measurement is made by keying the transistor oscillator by means of the trigger 113 and obtaining a determination of the p or n type in a visual display, for example two light emitting diodes 118 which may indicate either wafer type. The apparatus may be powdered by battery supplies, as is customary for many integrated circuit products.

One of the advantages of the hand held probe of FIG. 4 is that silicon wafers can be tested while stacked in parallel, spaced apart alignment in a wafer carrier or tray.

FIG. 5 shows a housing similar to that shown in FIG. 3. In FIG. 5 there is a chassis 130 having a mesa or table region 132 which is generally rectangular in shape. The upper portion of mesa 132 is adapted to fit between tracks or rails or conveyor belts of a wafer handling system. In such a system, opposed rails orbelts are provided which are less than the width of a semiconductor wafer. These rails or belts, straddle mesa 132 and bring a wafer over the top of the mesa. The wafer passes over the first wire 11 projecting from a hole in the top of the mesa such that the wire contacts the wafer so that a point contact diode can be formed. A second wire 45 also projecting from the mesa may contact the wafer in order to discharge any charge thereon. The mesa surface is preferably anodized aluminum so that the surface of the mesa forms a dielectric between the underlying aluminum base and the water itself. A visual display 134 is provided for reading wafer type and a push button 136 is provided for activating the circuit. Alternatively, the circuit may be activated automatically by a sensor, such as an optical detector, when the wafer is directly over the first wire 11 as well as the second wire 45.

FIG. 6 shows a wafer transport of a type adapted to be used in automated wafer handling. A wafer carrier 141 has pins 143 which fix the position of a wafer, indicated by the dashed lines W. The carrier 141 is driven along a base 145 from one position to another. In the process of being moved, the wafer is tested for conductivity type. This is achieved by mounting the first wire 11 of the probe of the present invention on a lever arm 147 which is pivoted at end 148. The first wire 11, mounted at the end opposite to end 148 is kept away from the wafer W except at a test point where the wire is pushed into contact with the wafer by a cam which causes upward rotation of the level 147.

This mechanical action is illustrated with reference to FIG. 7. In that figure, the lever arm 147, made of insulating material, is shown to be in a down position relative to base 145. The wafer carrier 141 is moving over base 145 in the direction indicated by arrow A. The wire 11 is connected to a conductive cam follower 149 by means of an electrical feed-through 151. Intended to make contact with cam follower 149 is a cam 153 which is stationary and connected through base 145 to an electrical terminal 155. The cam 153 may be the rounded head of a conductive bolt having an insulated sleeve over the shank of the bolt so that the bolt is not shorted to the base 145. A nut 157 may then be threaded on the end of the shank to keep electrical terminal 155 mechanically in place.

FIG. 8 shows the lever 147 in a raised position as the cam follower 149 is raised by the cam 153 forcing wire 11 into contact with wafer W. The stiffness of wire 11 is such that the wafer W is not raised. Instead, the wire 11 is bent slightly downwardly. This is achieved by using thin wire having a relatively long linear extended portion supporting an inverted V-shaped contact, similar to the wire configuration indicated in FIGS. 1 and 3.

Contact between the wire 11 and wafer W is only momentary. The lever arm 147 continues its motion, clearing the cam 153. The circuit of FIG. 2 is activated only for the brief instant, by sensors, not shown, when the wire 11 is making contact with cam 153 by means of the cam follower 149, as shown in FIG. 8. At other times, the electrical circuit of FIG. 2 may be in a standby mode.

With reference to FIG. 9, a perspective view of a slightly modified housing for the circuit of FIGS. 1 and 2 is shown. FIG. 9 is similar to FIG. 3, except that a wafer is automatically transported over wire 11 which is supported near a guage head. The automatic transport of a wafer W is by means of a wafer transport onto which wafers are loaded, measured and then removed. Carrier 163 has pins 164, 165, 166 for centering the wafer W in a desired location. Carrier 163 has an elongated cutout region 171 which provides clearance for a turntable 169 to be elevated within the carrier and come into contact with wafer W in order to reposition the wafer, if desired. The elongated cutout region 171 has a length greater than an article to be measured, whereby carrier 163 encompasses the measurement head 173 supported by arm 174 at all times as the carrier moves back and forth. This is important because it enables the full diameter of a wafer, positioned on the carrier, to pass between the measurement head near where wire 11 of FIG. 1 is located in a fixed position. The upper surface of the carrier is slightly higher than the lower portion of the measurement head, so that an article on the carrier passes midway between the head. The motion of carrier 163 is programmed by a film desk 175 which is mounted on a hub by means of a removable screw 177. Film disk 175 is opaque except for a pattern of radial transparent stripes which are read by a photodetector.

Photo diodes are mounted on each side of head 173. For example, photo diode 181 is aimed angularly downward at photo detector 183 mounted within a hole in base member 185 which supports carrier 163. Another photo detector 187 is mounted in base member 185 on the opposite side of head 161 to receive light from a photo diode, not shown, near head 173.

The photo diodes serve to indicate the position of the wafer W within the gauge head so that wire 11 of FIG. 2 can be momentarily energized and wafer type determined.