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DRIE Etched Silicon MEMS

New applications for deep reactive ion etch require exacting specifications for high rates and precision with excellent uniformity. As more complex MEMS devices go into production exacting methods are required to achieve the required manufacturing methods. Surface Technology Systems (STS) Chief Technology Officer Dr Leslie Lea explores some of the issues.

Deep reactive ion etching (DRIE) of silicon for MEMS applications is steadily moving from the R&D phase to full-scale commercial production. In addition, there are an increasing number of opportunities for implementing the same plasma processing techniques in advanced packaging schemes in high volume applications. As this occurs, exacting requirements for high rate, high precision etching with excellent uniformity over 200mm wafers are expected, and are of paramount importance, in providing high throughput and yield and thus a cost effective end product.

Continuous developments at STS resulted in the introduction of a new generation of silicon etch system, known as Pegasus, in mid-2005. Pegasus employs a revolutionary new de-coupled inductively coupled plasma (ICP) source design.

The focus with the Pegasus system has been to reduce the cost per die by enhancing both the throughput and the yield of the DRIE process.

The Bosch time-multiplexed or DRIE process, as exemplified by the STS Advanced Silicon Etch (ASE) package, consists of repeated etch and passivation steps [1, 2]. As the cycle of etch and passivation steps is repeated, the feature is etched into the wafer until the required depth is reached. Plasma process equipment to operate the ASE process uses the inductive coupling of RF power into the plasma by an antenna which is usually situated on the atmospheric side of a dielectric window that forms part of the plasma source region.

ICP processing equipment may be configured so that the plasma is formed in the same chamber in which the wafer is processed. Alternatively the plasma may be formed in a separate chamber (decoupled plasma source) and allowed to diffuse into a second, usually larger chamber, in which the wafer is located.

The use of a de-coupled plasma source brings a number of benefits:

1. The reduced volume of the source allows more efficient breakdown of the precursor gas because of the higher power density that can be delivered from the RF power supply.

2. The geometry and volume of the source region can be tailored so that the ions and neutral radicals exiting it can diffuse down to the wafer with the desired profiles to obtain high etch uniformity.

3. Additional means may be included between the de-coupled plasma source region and the chamber in which the wafer is processed in order to alter the balance between numbers of ions and neutral radicals reaching the wafer to control selectivity to mask and/or ion damage to feature profiles.

For etching of silicon using their ASE process, STS manufactures both conventional ICP plasma processing tools for greater flexibility in less demanding applications and two different advanced de-coupled plasma sources for applications requiring enhanced etch rates, selectivity and manufacturing uniformity.

A conventional de-coupled source plasma processing tool forms the plasma in a smaller chamber by RF power coupled by an antenna through a dielectric window. The dielectric window is often a cylindrical tube located on the same axis as a cylindrical chamber in which the wafer is processed (Figure 1).

Ions and radicals diffuse from the smaller chamber into the chamber in which the silicon wafer is processed. Because of the differences in deflection or loss probability between ions, electrons and neutral radicals when encountering electric or magnetic fields and material surfaces such as the chamber walls, the radial profiles of the charged species may differ from the radial profiles of the neutral radicals in the vicinity of the wafer.

For the conventional de-coupled plasma processing tool, the spatial profile of both ions and neutral radicals in the vicinity of the wafer will usually be ‘centre-high', decreasing radially towards the walls of the chamber. The density profile variation of the ions will usually be more extreme than for the radicals, because on encountering the walls or an object within the chamber, ion and electron pairs have a high probability of recombining, while neutral radicals may survive a few reflections. When etching of silicon with a reasonable exposed silicon area, as defined by the mask, more radicals need to be present near the centre to achieve a near constant chemical etch rate across the wafer.

This is because in the centre each feature is surrounded by other features, while towards the edge there are less features outside of the radial position. The conventional de-coupled plasma processing tool can often quite reasonably satisfy this criterion.

Throughput improvement
With the introduction of Pegasus, throughput is improved by a significant enhancement in the etch rate of the DRIE process, coupled with integration of the process module to the latest wafer handling automation. Silicon etch rates exceeding 50µm/min for a 1% exposed area have been observed using the Pegasus source with the ASE process, and at a more realistic exposed area of around 10%, etch rates of more than 30µm/min can be achieved.

Improving Yields
Device yield is optimised by improving absolute etch depth uniformity and by reducing trench tilting artefacts that are observed with conventional de-coupled ICP plasma sources. As wafer size increases, the effects of any nonuniformity in the plasma become more pronounced.

For high etch uniformity it is necessary for the appropriate spatial profile of neutral radicals reaching the wafer to match the silicon's loading characteristics. In order to ensure precise control of the etch direction, the spatial density profile of ions above the wafer needs in general to be different from that of the radicals.

STS has developed a technique that controls the spatial profiles of the two species independently, enabling very high etch rate uniformity and feature profile control over 200mm wafers.

When combined with STS' patented technologies for ramping the magnitude of parameter values during an etch and for providing precise control of notching at silicon to insulator surfaces, the new uniformity control technique is extremely powerful.

Experiments show that while it is desirable to have a reasonably uniform thickness of the passivation layer across the wafer, it is not essential provided that the ion bombardment can remove the passivation from all areas in a similar time so that the overall etch rate determined by the chemical reaction of the neutral radicals with the silicon is similar across the wafer.

Figure 3 illustrates that the spatial profile of the ion density above the wafer for a conventional de-coupled plasma source tends to be quite ‘centre-high', which can cause the passivation layer to be sputtered away more quickly towards the wafer centre, increasing the risk of higher etch rate at the wafer centre than towards the edge. For a conventional de-coupled plasma source the ion density profile above the wafer is ‘centrehigh', thus the Debye length, and therefore the thickness of the sheath between the plasma and the wafer, will be less near the centre of the wafer than towards the edge. This non-uniformity of the sheath thickness across the wafer is likely to lead to a steering of ions as they are accelerated to the wafer. In the centre of the wafer they will move perpendicular to the wafer surface, but towards the edge of the wafer they may move in such a way that they impact the wafer at an angle to the perpendicular.

For features etched using the Bosch timemultiplexed etch process in conventional decoupled plasma sources, the feature may be etched perpendicular to the wafer surface near the centre of the wafer, but at a significant angle to the vertical near the edge of the wafer, with the feature tilting outwards (from wafer centre) as it is etched deeper into the wafer (Figure 4).

An ideal plasma processing tool should have the capability of delivering neutral radicals to the wafer surface with a density profile to match the loading effect of the exposed area of silicon so that the etch rate is very close to constant at all points on the wafer. The tool should also be capable of providing a near uniform ion density just above the wafer surface so that the sheath is of the same thickness across the wafer and ions are accelerated perpendicular to the wafer surface.

The Pegasus source has shown that its novel design combined with a number of techniques provides higher and spatially uniform silicon etch rates while minimising feature tilting.

[1] F. Laermer and A. Schilp., Patent DE4241045, granted to ROBERT Bosch GmbH.
[2] J.K. Bhardwaj and H. Ashraf, SPIE Micromachining and Microfabrication Process Technology, Vol 2639, pp224-233 (1995).

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