End Point Detection in Ion Beam Etching using SIMS Quadrupole Mass Spectrometry by Hiden Analytical

Topics Covered

Background

Hiden Analytical was founded in 1981 and is presently situated in a 2,130m² manufacturing plant in Warrington, England with a staff of over 50. As a privately owned company our reputation is built on creating close and positive relationships with our clients. Many of these customers are working at the forefront of new technology - in the fields of plasma research, surface science, vacuum processing and gas analysis. To maintain this reputation Hiden Analytical have, over the years, established exceptional levels of technical expertise in these areas within our company.

Summary

The use of SIMS Quadrupole Mass Spectrometry for End Point Detection during ion beam etch is demonstrated. The technique can be applied to a wide variety of materials and devices with depth resolution of just ± 5 Å and precise control of end point, or etch stop. This article describes the principle behind the technique and provides examples from the range of applications.

Introduction

Dry (ion beam) etching is used in numerous process steps during semiconductor device and magnetic recording media manufacture.

However, owing to the low selectivity in ion-beam processing, even when reactive gases are used, methods must be found to ascertain depth and location of the etch stop. Consequently test pieces of material are often processed to assess etch rates, which can be time consuming and costly. Metals and dielectrics are frequently patterned on semiconductor material and it is important not to etch or damage the underlying layer. It is therefore important to have a good selective etch or to be able to stop at, or near, the interface. These criteria are essential for a wide range of applications involving the presence of multilayers and interfaces.

Instrumentation

The technique of ion beam etching employs a broad ion beam to physically sputter and/or chemically etch the surface under consideration. During this process secondary ions are created at the surface with a mass to charge (m/z) ratio which is characteristic of the material at the surface. Mass spectrometry can be used to monitor the intensity of the secondary ion signal at specific m/z ratios, making it possible to detect when an interface has been reached and a new type of material exposed (figure 1). The ion beam etch can then be terminated if the new material represents the desired end point of the process. This approach is essentially Secondary Ion Mass Spectrometry (SIMS) - well established as the most sensitive of all UHV surface analysis technique.

Figure 1: Ion beam etching

The SIMS process involves the detection and identification of secondary ions formed at the surface of a sample by the impinging primary ion beam. The ion flux density is typically a few percent of the sputtered neutral flux density and the secondary ion energy distribution depends on the angle of primary beam incidence, secondary ion emissionangle and target material. Both positive and negative ions are formed at the surface along with electrons, sputtered neutrals and scattered primary particles.

Hiden’s SIMS End Point Detector (EPD) is a convenient, compact device and is appropriate for secondary ion mass analysis in this application. For adequate mass resolution the SIMS EPD requires an acceptance energy range of 0-5 eV. These considerations indicate that the ion transport system must reject high energy ions, neutrals and electrons, and accept positive ions with a kinetic energy range of 0-5 eV. Additionally the SIMS EPD can function as a residual gas analyser (RGA) so that chamber gas composition can be conveniently determined. This is particularly important when more than one gas is admitted to the chamber.

Two basic arrangements are shown schematically in figure 2.

Figure 2a: Schematic representation of the positioning of the primary ion source, target and SIMS EPD with axial acceptance

Figure 2b: Schematic representation of the positioning of the primary ion source, target and SIMS EPD with 30° acceptance configurations

Secondary ions are emitted from the surface with a wide range of angles and energies. The high transmission energy filter (A) either has an axial acceptance as in figure 1(a) or a 30o acceptance as shown in (b). The mass filter may be biased with respect to vacuum ground to select a range of secondary ion energies. A dual filament RGA electron-impact ion source (B) is positioned between the SIMS EPD energy filter and the quadrupole mass filter (C). The mass filter has pre- and post- r.f. filter stages which serve to enhance the mass transmission.

The air stable continuous dynode electron multiplier detector (D) is mounted in an off-axis position. Typical chamber pressures are in the 10^-4 mbar range during etching and the SIMS EPD is differentially pumped to maintain pressures low enough to enable the use of the detector (~5 x 10^-6 mbar). The system is designed for process operation and can be controlled as a standalone device or alternatively may be fully integrated with the host tool via a variety of digital I/O and software command sets. In production environments the SIMS EPD is tied into the production tool recipe select software for complete ‘hands free’ process integration. Layer by layer data is displayed in real time and the etch process is controlled by a variety of user selectable endpoint methods.

In typical applications such as MRAM manufacture, devices based on Giant Magneto-Resistant effect elements (GMR) require low primary ion energies because the multilayer structures can be just a few atomic layers thick. In addition to this the exposed area of pre-etched wafer devices may only be <3% of the total open area of a 150mm diameter wafer. Hiden’s SIMS EPD has been proven to be the only technique that produces consistently precise, accurate and reproducible end point control in an industrial setting for such demanding applications.

Results and Discussion

The elements to be monitored, corresponding to the layer materials are chosen from the drop-down periodic table (figure 3).

Figure 3: Software selection of layer materials.

In operation the intensities of the chosen elements of interest are plotted against time. Decisions can then be made on the end point.

Data acquired from a thin film structure used in the fabrication of magnetic film sensors is shown schematically in figure 4a and in more detail in figure 4b. The objective is to etch through the FeMn, NiFe and and Co/Cu/Ta layers stopping on the SiO2 substrate layer with no further etching. In this example 99% of the total surface area is masked. The profile shows each element from the various layers with excellent signal intensity and resolution. Etching is terminated at the end of the duration of the nickel and iron signal. Data was acquired from the aforementioned exposed sample area of only 1%, clearly demonstrating the high sensitivity of the technique.

Figure 4a: Multilayer SIMS EPD etch data

In figure 4b the periodicity in the signal is clearly seen and is due to the sample rotation.

Figure 4b: Multilayer SIMS EPD etch data - detail

This can in the worst case produce false endpoints but is completely dealt with in the advanced Hiden SIMS EPD endpoint methods by assigning the rotation period and collecting data matched with the EPD acquisition timing.

End point methods included with the Hiden SIMS EPD include rising and falling edge variables, the use of reference layer peaks and layer counting. End point control variables are comprehensive and shown in the screen shot below.

Figure 5: Endpoint variables

Conclusion

Hiden’s SIMS EPD has been proven as the industry standard end point detector. It is used by every major IBE tool manufacturer due to the demonstrated sensitivity, accuracy and reliability, and due to the excellent levels of after sales service and field support. In the most demanding IBE applications the Hiden SIMS EPD delivers depth resolution of +/- 5 angstroms.

Source: Hiden Analytical

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