Patent Application
20080084224
The following sets forth a detailed description of the best contemplated mode for carrying out the invention. The description is intended to be illustrative of the invention and should not be taken to be limiting.
In general, photoionization probes permit the transport of current across a gas filled region or through a region in which gas is flowing. For nondestructive testing, photoionization probes serve to make electrical contact to sensitive surfaces where direct physical or mechanical contact is not desirable. Photoionization probes operate on the principle of photoionization, which is often used, for example, in photoionization detectors (PIDs) in gas chromatography devices. Photoionization uses a light source providing photons of the correct energy to ionize the target gas molecule. Various different light sources can be used (e.g., lasers, specialized lamps, light emitting diodes, etc.), but in the examples of the present application ultraviolet (UV) lamps will be illustrated.
If the energy of an incoming photon is high enough (and the molecule is quantum mechanically allowed to absorb the photon) photo-excitation can occur to such an extent that an electron is completely removed from its molecular orbital, i.e. ionization. The basic reaction can be illustrated as:
R+hv R
+
+e
−,
where R is the target gas molecule, hv is the photon energy of the light source photons having a frequency v, R+ is the resulting positively charged ion, and e− is the electron removed from the molecule. The ions or electrons produced by this process are collected by one or more suitable electrodes (e.g., as part of the device under test), and the current generated is therefore used to characterize the device. If the amount of ionization is reproducible for a given compound, pressure, and light source, then the current collected at the electrodes of the device under test is reproducibly proportional to the amount of that compound entering the probe. As will be discussed in greater detail below, the compounds used for photoionization probes are often aromatic hydrocarbons or heteroatom containing compounds (e.g., organosulfur or organophosphorus species) because these species have ionization potentials that are within reach of commercially available UV lamps. Typical UV lamp energies range from 8.3 to 11.7 eV. Examples of photoionization probes are disclosed in U.S. patent application Ser. No. 10/976,694, assigned to the assignee of the present application.
The photoionization probe illustrated in
In order to further increase the likelihood that ionization occurs within the aperture and between plates 120 and 125, the UV lamp can be further modified. For example, window 105 is typically formed from a highly UV transparent material, such as fused silica, CaF, BaF, or sapphire. Since the diameter of the aperture is typically smaller than that of window 105, a portion of window 105 can be masked (e.g., with a surface coating or an intervening optically absorbing mask) to present UV light only to the aperture area. In still other embodiments in accordance with the invention, one or more UV-quality lenses can be used to focus light from UV lamp 100 into the aperture. By increasing the amount of UV light in the aperture, photoionization can be increased an more easily controlled, producing larger and/or more stable photoionization currents. The distance between UV lamp 100 and the aperture assembly can also be adjusted to improve photoionization within the aperture. In some embodiments in accordance with the invention, lamp window 105 is located in close proximity to conducting plate 120, e.g., a few millimeters or less. The entire device can be designed such that one or both of UV lamp 100 and the aperture assembly can be moved with respect to each other so as to adjust this spacing. Similarly, one or both of UV lamp 100 and the aperture assembly can be adjusted to vary the separation between conducting plate 125 and the device under test. In still other embodiments in accordance with the present invention, the photoionization probe and/or the material holder for the device under test can be translated with respect to each other to achieve desired spacing. Numerous different material holders, support brackets, translation devices, and the like will be known to those skilled in the art.
The photoionization probes described in the present application can be used to test various different devices. In the example of
Here, a specific one of the array of driver circuits is under test. Thus, driver circuit 153 is on during the test, while driver circuits 151, 152, 154, and 155 remain off. In typical use, the bias voltage will be applied to a suitable contact (e.g., a data or bus line for a row of pixels) so as to conduct current through the desired portion of the device. The applied electrical field accelerates charge to electrode 160. By utilizing the switching present in the driver circuits and on the display device, an individual pixel can be singled out for measurement. This is useful because the size of the probe may be much larger than an individual pixel.
As noted above, aromatic hydrocarbons can be used as the ionizable vapor for the photoionization probe. Other examples of ionizable vapor sources include solvents such as acetone (propanone), butanone, toluene, ethanol, isopropanol, and the like. The ionizable vapor is generally selected based on its ionizability for a given light source and other factors, such as cost, ease of handling, safety, etc. The carrier gas used for the ionizable vapor (and potentially for the separately supplied carrier gas 110) is typically a relatively inert gas that will not otherwise interfere with probe operation or damage the probe or the device under test. Examples include air, nitrogen (N2), and noble gases such as argon. When used, additional carrier gas 110 is typically the same carrier gas used to supply the ionizable vapor, although this need not be the case.
The aperture assembly, including conducting plates 120 and 125, as well as insulating layer 130, can be constructed from a variety of different materials. For example, conducting plates 120 and 125 can be formed from thin sheet metal or metal foil. Various different metals can be used such as copper, gold, aluminum, and steel. The metal is selected based on its conductivity (higher conductivity is generally better), its machinability, and its compatibility with the ionizable vapor and carrier gas. Conducting plate material can also be selected to reduce the possibility of contaminating the device under test. Metallic meshes can also be used. In some embodiments, solid pieces of metal (or continuous metallic coatings) are used for the electrodes, but one or both of the aperture mouths (i.e., the side closest to the lamp and the side closest to the device under test) can be covered (or at least partially covered with metallic mesh to enhance probe operation. In other embodiments in accordance with the invention, the conducting plates are formed by electrically conductive material layers deposited on a substrate, e.g., a substrate formed by insulating layer 130. For example, conducting plates 120 and 125 can be formed from metallic thin films, conductive pastes, conductive adhesives, and the like. Numerous different electrically conducting materials will be known to those skilled in the art.
Insulating layer 130, can be similarly fabricated from various different materials such as glasses, ceramics, glass-ceramics, (e.g., Macor®), plastics, rubber, polymers, and even semiconductors. Depending on the size and shape of the assembly, and the manner in which channel 140 is provided, insulating layer 130 can be formed from a single piece of material or several pieces of material.
The size and shape of the aperture assembly can also vary. In general, the aperture assembly is disk-shaped, i.e., various components 120, 125, and 130 are themselves disk shaped, with a relatively small round aperture, e.g., 0.1 mm to 2 mm. However, other shapes can be used as desired, and each of the components need not possess the same shape. The overall thickness of the aperture assembly is typically on the order of several millimeters, but that too can vary depending on the size of the components and desired probe features. In embodiments where each of the components is a separate component, the aperture assembly can be held together using one or more of adhesives, mechanical fasteners, compression fittings, mounting brackets, and the like.
Additionally, the aperture assembly can be fabricated using semiconductor device and/or MEMS device fabrication processes and techniques. Examples include: photolithography techniques, thin film deposition and growth techniques, etching processes, and the like. These techniques can be used to fabricate a single aperture assembly, or multiple aperture assemblies, e.g., rows or arrays of aperture assemblies.
In order to introduce the ionizable vapor between conducting electrodes, a variety of different channel and inlet designs can be implemented.
Turning to
Analyzer 300 can be specially designed test equipment for providing precise bias voltages and performing specified device measurements. In other examples, analyzer 300 is a multipurpose semiconductor parameter analyzer for advanced device characterization. Such devices typically have high resolution for low-current and low-voltage measurements, and are often designed for quasi-static capacitance vs. voltage measurements, to extract process parameters, to measure leakage characteristics, and to perform on-wafer reliability tests with built-in stressing modes. For example, analyzer can provide a desired bias voltage, e.g., ±50V, or sweep through a range of bias voltages. Similarly it can measure photoionization currents through the device under test. Such currents are typically on the order of 1 μA, but may be larger or smaller depending on device and probe characteristics.
A carrier gas containing the ionizable vapor is provided to the aperture assembly by gas inlet 340. In this example, carrier gas source 370 provides a carrier gas such as N2 to acetone bubbler 350 where the carrier gas forces acetone vapor into inlet 340. As noted above, various different carrier gases and ionizable vapor sources can be used. The flow of carrier gas into inlet 340 is controlled by two mass flow controllers (360). One mass flow controller regulates the amount of carrier gas used to force vapor out of acetone bubbler 350, while the other regulates the additional carrier gas provided to inlet 340 via gas line 380. As noted above, additional carrier gas can be optionally provided (via gas line 390) to reduce the likelihood that ionizable vapor escapes into the area between the aperture assembly and the UV lamp. The flow of this carrier gas can be controlled by yet another mass flow controller (360). In one embodiment in accordance with the invention, the flow rate of carrier gas used to force vapor out of acetone bubler 350 is on the order of 5 standard cubic centemeters per minute (sccm), and the flow rate of additional carrier gas provided to inlet 340 via gas line 380 is on the order of 600 sccm. These values are merely illustrative, and numerous other values are possible depending upon a host of parameters and system features. While mass flow controllers are illustrated because they provide more accurate regulation of needed gas supplies, other gas flow regulation schemes can be used. For example; simple valves or pressure regulators can be used.
Basic aspects of various photoionization probes and test systems have been illustrated. Those skilled in the art will readily recognize that a variety of different types of components and materials can be used in place of the components and materials discussed above. Moreover, the description of the embodiments in accordance with the invention set forth herein is illustrative and is not intended to limit the scope of the invention as set forth in the following claims. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention as set forth in the following claims.
