1. Technical Field
The present invention relates generally to the nondestructive evaluation of surface contamination levels and more particularly to improvements to an apparatus and method of monitoring optically stimulated electron emission.
2. Discussion of the Related Art
It is often desirable to monitor the quality and conditions of a surface for various manufacturing processes. For example, the indication, identification and quantification of contaminants such as grease or dirt are vital in painting or coating processes, forming laminates, inspection for cyclic loading, maintaining the cleanliness of memory disc drive heads, etc. One area of particular interest is the placement of coatings or oxides having critical thicknesses onto the surfaces of semiconductor wafers to fabricate microelectronic components.
Several techniques are available to quantitatively observe and to quantitatively measure the condition of a surface and include direct or magnified observation, profilometers, ellipsometry, low energy electron diffraction, Augur electron spectroscopy and scanning electron microscopy. These techniques often require bulky equipment and are difficult for a relatively unskilled technician to operate.
A surface contamination monitor is commercially available from Photo Acoustic Technology, Inc. of Newbury Park, Calif. This surface contamination monitor is described in U.S. Pat. No. 4,590,376 to Tennyson Smith. An ultraviolet light is directed onto the surface of interest, causing photoelectrons to be emitted. These emitted photoelectrons are detected and compared to previously established values for surface conditions to determine acceptability based on criteria such as oxide thickness, contamination or fatigue. This technique is often referred to as Optically Stimulated Electron Emission, or OSEE. The prior OSEE monitor is described in greater detail in the Detailed Description of the present application.
This surface contamination monitor, while offering improved surface monitoring, has several drawbacks. The OSEE indications for a given measurement are time dependent, generally decreasing from an initial high value. For a given sample preparation, there is a large variability of initial values which can be obtained both among samples prepared the same way and from a single sample measured at different times over a period of several days. On a given extended sample measured by scanning, indications of contamination may persist in specific regions, even through several cycles of recleaning. This continued indication of contamination and subsequent good surface performance identify the indications as false positive contamination indications. Finally, oxidation is promoted on some metallic surfaces from exposure to the OSEE probes over extended times. Further drawbacks are discussed in the Detailed Description of the present application.