Abstract:
Methods and apparatus for non-contact electrical probes are described. In accordance with the invention, non-contact electrical probes use negative or positive corona discharge. Non-contact electrical probes are suited for testing of OLED flat panel displays.

Description:
BACKGROUND 
   There are a number of techniques for measuring voltages on a flat panel display which comply with the requirement that there be no electrical contact on the active area of the flat panel display to avoid contamination of the electrode surfaces although the electrical contact may be made at the edges of the flat panel display. For example, an electron beam may be used to image the surface with voltage differences appearing as contrast differences. However, testing of the thin film circuitry for Organic Light Emitting Diode (OLED) flat panel requires measuring the current because the OLED pixel brightness is controlled using a current signal as opposed to a voltage signal used to control brightness for Liquid Crystal Display (LCD) pixel 
   Typically, it is more difficult to measure currents. One typical technique is to incorporate an additional capacitor per pixel on the OLED display circuit and to measure the charging of this capacitor. This technique typically adds complexity to the circuit that will not be used once testing is complete. Another typical technique uses an electron beam as a non-contact probe but this technique requires placing the flat panel under test into a vacuum chamber which adds cost and time to the test procedure. 
   SUMMARY 
   In accordance with the invention, a non-contact probe permits electrical current to flow through a small gas gap to the surface of a device under test at atmospheric pressure. The non-contact probe typically includes a sharp electrode and a flat electrode where current flowing from the sharp electrode passes through a hole in the flat electrode and is captured by the device under test. The device under test is typically located beneath the aperture in the flat electrode and the voltage drop between the flat electrode and the device under test controls the current flowing to the device under test. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  shows an embodiment in accordance with the invention. 
       FIG. 1   b  shows an embodiment in accordance with the invention. 
       FIG. 1   c  shows a close up of the sharp electrode positioned with respect to the flat electrode. 
       FIG. 2  shows an embodiment in accordance with the invention. 
       FIG. 3  shows an embodiment in accordance with the invention with a thin wire electrode. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1   a  and  1   b  show non-contact probe  100  in accordance with the invention. An atmospheric pressure plasma forms around the tip of sharp electrode  105  where the electric field is sufficiently high due to geometric effects.  FIG. 1   c  shows sharp electrode  105  and flat electrode  120  separated by distance D, typically in the range of about 1 mm to 10 mm with the tip of sharp electrode  105  having radius of curvaturer. An electrode is “sharp” for the purposes of this application when the tip radius of curvature r of sharp electrode  105  is less than about D/5. Typical materials for sharp electrode  105  include steel, copper, platinum, nickel titanium alloy and chrome or sharp electrode  105  may be a Spindt tip, see U.S. Pat. No. 3,755,704 incorporated by reference. The high curvature ensures a high potential gradient around sharp electrode  105  for the generation of the atmospheric pressure plasma. Typical potentials for sharp electrode  105  are greater than about 1000 V. Sharp electrode  105  is typically a needle-like electrode and is typically used to test a single device or a small number of devices. 
   Flat electrode  120  captures most of charged species  125  that travel across the gap between sharp electrode  105  and flat electrode  120 . Flat electrode  120  is typically made from a material chosen for manufacturability such as nickel, stainless steel or silicon. Aperture  101 , typically about 10 μn to 300 μm in diameter, in flat electrode  120  allows a portion of charged species  125  that have traveled across the gap from sharp electrode  105  to flat electrode  120  to travel past flat electrode  120  to device under test  115 . The gap between flat electrode  120  and device under test  115  is sufficiently small to ensure that the current flows to device under test  115  and not to adjacent devices  116  not under test. A typical size range for this gap is between about 0.1 mm to about 1 mm and is typically on the order of about 100 μM. Gap sizes smaller than about 100 μm are typically discouraged because the probability that contamination on flat electrode  120  may be transferred to device under test  115  is considerably increased and also contamination trapped between device under test  115  and sharp electrode  105  may cause damage to either device under test  115  or sharp electrode  105 . For sharp electrode  105 , aperture  101  is typically square or round in shape as shown in  FIG. 2 . Flat electrode  120  is typically kept at or near ground potential. The typical impedance between flat electrode  120  and device under test  115  is typically less than about 100 KΩ and no greater than about 10 MΩ. 
   Device under test  115  may be an electrode on the surface of an OLED flat panel display and is electrically coupled to bias voltage supply  190  using device under test interface  116  (see  FIG. 2 ). Device under test  115  is typically biased relative to flat electrode  120  to control the amount of current, typically in the range from about 1 μA to 10 μA, flowing to the surface of device under test  115  from sharp electrode  105  as indicated in  FIG. 2 . Typical bias voltages are less than about 100 V. Other devices requiring contactless electrical probes may also be tested in accordance with the invention. 
   Steering structure  110  may be included in the gap region between sharp electrode  105  and flat electrode  120  as shown in  FIG. 1  a. Steering structure  110  functions to increase the portion of charged species  125  that pass through aperture  101 . Steering structure  110  may be electrostatic or electromagnetic in nature to control the path of charged species  125  from the plasma creation region to flat electrode  120 . If steering structure  110  is electrostatic in nature, such as one or more metal rings kept at a fixed voltage, the electric field is typically distorted to modify the path of charged species  125 . If steering structure  110  is electromagnetic in nature, such as a permanent magnet or electromagnet, the resulting magnetic field is typically used to focus charged species  125  with the resulting Lorentz force. 
     FIG. 3  shows an embodiment in accordance with the invention where sharp electrode  105  has been replaced by thin wire electrode  305  to measure a row of devices under test  315 . The radius of wire electrode  305  is typically less than 20% of the distance between wire electrode  305  and flat electrode  320 . Note that aperture  101  is replaced by slit-like aperture  301  in flat electrode  320 . Devices under test  315  are electrically coupled to bias voltage supply  390  using device under test interface  316 . Devices under test  315  are typically biased relative to flat electrode  320  to control the amount of current, typically in the range from about 1 μA to 10 μA, flowing to the surface of device under test  315  from thin wire electrode  305  as indicated in  FIG. 3 . Typical bias voltages are less than about 100 V. Other devices requiring contactless electrical probes may also be tested in accordance with the invention. 
   With reference to  FIGS. 1   a – 2 , operationally sharp electrode  105  is biased at a high voltage, typically greater than 1000 V with respect to flat electrode  120 . An atmospheric plasma is generated in the resulting large electric fields in the vicinity of sharp electrode  105 . Gas, such as, for example, argon mixed with hydrogen, argon mixed with a forming gas comprised of nitrogen and hydrogen, argon mixed with oxygen or nitrogen with amounts of ammonia typically less than about one percent, is flowed past sharp electrode  105  towards device under test  115  and to provide the local atmosphere at atmospheric pressure. Argon or nitrogen alone may also be used. The gas is typically flowed from the region of sharp electrode  105  or thin wire electrode  305  through aperture  101  or aperture  301  past device under test  115  or devices under test  315 , respectively. The mass flow of the gas also operates to enhance current flow through apertures  101  and  301 . A negative or positive corona may be used. If a negative corona is used, electronegative species are typically only a small portion of the gas because negative ions make the plasma noisy which is typically undesirable. Note that negative coronas can only be maintained in a gas with electronegative molecules. Charged species  125  are accelerated towards flat electrode  120  while an equal number of oppositely charged species are accelerated towards sharp electrode  105  creating a current between flat electrode  120  and sharp electrode  105  through high voltage supply  185 . In an embodiment in accordance with the invention, this current may be monitored and used as a feedback signal to high voltage supply  185  to reduce the current variation which results in measurement noise. 
   A portion of charged species  125  pass through aperture  101  in flat electrode  120 . After passing through aperture  101 , charged species  125  are accelerated towards device under test  115  because of the bias voltage that is maintained between flat electrode  120  and device under test  115  using bias voltage supply  190 . A portion of charged species  125  that passes through aperture  101  are captured by device under test  115  and produce a current. By adjusting the bias voltage, the number of charged species  125  that are drawn through aperture  101  can typically be increased. 
   Similarly for the embodiment in accordance with the invention shown in  FIG. 3 , the charged species (not shown) pass through aperture  301  in flat electrode  320  while an equal number of oppositely charged species are accelerated towards thin wire electrode  305  creating a current between flat electrode  320  and thin wire electrode  305  through high voltage supply  385 . After passing through aperture  301 , the charged species are accelerated towards devices under test  315  because of the bias voltage that is maintained between flat electrode  320  and devices under test  315  using a bias voltage supply  390 . A portion of the charged species that passes through aperture  301  are captured by devices under test  315  and produce a current. By adjusting the bias voltage, the number of charged species that are drawn through aperture  301  can typically be increased. 
   While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.