The present invention relates generally to a novel technique for limiting the accumulation of surface charge on a sample which is being bombarded by a primary beam and from which charged particles are being extracted. More particularly, the present invention relates to a method and apparatus for preventing the accumulation of surface charge beyond a predetermined limit by alternating during successive periods the extraction from the bombarded surface of charge of opposite signs.
Surface analysis of a material sample by processes such as secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES) or their equivalents involve bombarding a sample with a primary beam and removing charged particles from the sample.
Secondary ion mass spectrometry involves bombarding a sample material with a high energy primary beam of neutral and/or charged particles (positive or negative ions, atoms or molecules). These particles impact the surface of the sample causing the emission of secondary neutral and charged particles (e.g. electrons, positive and negative ions, atoms, and molecules) some of which are subsequently mass analyzed to determine the composition of the sample. Similarly, AES involves bombarding the surface of a sample with high energy electrons which then result in the emission of Auger electrons, which are then energy analyzed to provide information on the composition of the sample.
If the net flux of positive and negative charges, i.e. ions and electrons, entering and leaving the sample is not perfectly balanced the sample will tend to become charged. Sample charging can destroy primary beam characteristics, interfere with the extraction of charged particles, and shift the energy of the extracted charged particles, making analysis difficult or impossible.
In electrically conducting samples charge accumulation may be prevented by electrically grounding the sample. The ground assures that the net charge flux of the sample will always be zero and, consequently, no charge accumulation will occur. Preventing or limiting charge accumulation in electrically non-conducting or poorly conducting materials (such as polymers, ceramics, glasses, and biospecimens) presents a more difficult problem.
The charge cycles commonly encountered during SIMS analysis of an electrically nonconducting sample exemplify the conditions toward which the present invention is directed:
(a) The most common mode of SIMS analysis is to use a primary beam of positive ions which results in emission of secondary positive and negative ions and secondary electrons from the sample. For known materials, the positive primary beam creates fewer secondary positive ions than primary ions. Use of an electric field of negative potential to extract positive secondary ions consequently causes the sample to accumulate a positive charge. Use of an electric field of positive potential to extract negative secondary ions and electrons causes the sample to accumulate a positive charge at an even greater rate.
(b) A neutral primary beam for SIMS analysis causes charge accumulation at a rate that is significantly less than that of the positive primary beam, but the imbalance between positive ion and negative ion and electron production and extraction can still cause charge accumulation on the sample.
(c) A negative primary ion beam has been used for SIMS analysis to create positive and negative secondary ions. Extraction of positive secondary ions will cause the sample to accumulate negative charge. Charge accumulation on the sample due to extraction of negative secondary ions will depend upon the material and bombardment energy. The secondary negative charge yield under keV energy negative ion primary bombardment is greater than the charge flux of the primary beam for many materials. Extraction of negative particles will cause such materials to accumulate positive charge.
In the prior art, conducting grids and electron flooding have been used to reduce accumulation of positive charge in non-conducting samples during SIMS analysis.
U.S. Pat. No. 4,680,467 issued July 14, 1987, to Bryson III et. al. discloses the use of an electrically conductive grid comprised of intersecting wires or vanes positioned in closely spaced relationship to the surface of the sample for smoothing the gradients in the electrical potential in the region of the beam spot during electron spectroscopy. However, conducting grids are known to add a background signal to the mass spectrum of the sample. In addition, conducting grids have a high probability of introducing contaminants to the surface of the sample and may cause chemical changes in the surface of the sample.
U.S. Pat. No. 4,818,872 issued Apr. 4, 1989, to Parker et al. discloses an electron gun which floods the sample with low energy electrons neutralizing charge accumulation. Electron flood guns are effective in many cases but may damage the sample, particularly polymers. Electron stimulated desorption and resulting noise may also be caused by electron flood guns.
Therefore, in view of the above, it is an object of the present invention to provide an apparatus and method for maintaining a neutral surface potential for a sample of electrically nonconducting or poorly conducting material that is being analyzed by a method causing extraction of charged particles from the sample.
It is another object of the present invention to provide a non-intrusive apparatus and method for maintaining a neutral surface potential so that sample integrity is maintained.
A yet further object of this invention is to present a method for limiting surface charge accumulation within predetermined values.
It is another important object of this invention to present an apparatus and method which enables the near simultaneous collection of both positive and negative secondary ion spectra.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.