Selective etching of silicon oxides relative to silicon or photoresist is achieved when the concentrations of CF+ and CF2+ ions in the plasma are optimized relative to CF3+. Under these conditions, however, selectivity is achieved at the expense of a reduced etch rate. For etch applications where a metal mask can be used, the process conditions may conveniently be adjusted for maximum etch rate and high aspect-ratio capability without concern for selectivity.
Many mass-produced microelectromechanical systems (MEMS) are fabricated with processes derived from integrated circuit (IC) manufacturing techniques, including photolithography and dry etching. Emerging innovative MEMS devices, however, will require oxide etch processes that differ markedly from those used for fabricating ICs. In particular, silicon oxide films, or even bulk silica, will be etched to depths far exceeding those found in typical microelectronic applications, making it difficult to select and define an appropriate etch mask and control the etched sidewall profile in the oxide.
In microelectronic applications, oxide films rarely exceed 2µm in depth, and a photoresist mask typically is used to define the submicron structures. In recent years, however, many optical MEMS (MOEMS) devices have demanded anisotropic patterning of doped and undoped silicon oxide films to depths in the range of 6-50µm, with the finished structure having aspect ratios (ratio of structure depth to width) on the order of 10:1. These deep oxide etch processes are typically masked with photoresist where the depth is <~6µm, with polysilicon where the depth is ~6-50µm, and with metal films where greater depths are required.
MEMS-specific etching
Unlike silicon, undoped silicon dioxide films are not readily etched at high rates by plasma-generated neutral chemical species. Doped oxide films can be etched by neutral reactive species if the substrate temperature is raised, and this behavior has been exploited in IC manufacturing to etch “wine-glass” contact profiles to facilitate subsequent contact filling. Alternatively, oxide films can be etched in HF vapor, provided the reaction is catalyzed by addition of water vapor or other initiators. This process is wholly isotropic, making it extremely useful for releasing MEMS structures from a substrate after fabrication but inappropriate for high-precision pattern transfer.
In contrast, anisotropic oxide etch processes depend heavily on ion-driven chemical interactions between adsorbed fluorine-containing species and the material surface. They are most frequently implemented in plasma etch systems that permit independent control of the power input to the plasma and the bias voltage applied to the wafer to be processed. Such systems allow adjustment of the RF power input to the plasma to generate a sufficiently high density of reactive species and ions, while the bias voltage can be adjusted independently to control the average energy of ions impinging on the substrate surface. Magnetically enhanced reactive ion etching (MERIE) systems and inductively coupled plasma (ICP) systems both feature this facility; ICP systems, however, typically provide a higher plasma density than MERIE systems, potentially providing higher concentrations of both reactive species and ions in the process chamber.
Process optimization
Deep oxide etching for MEMS fabrication is typically implemented in an ICP etching tool; the advanced oxide etching (AOE) system is one example.
Since the oxide etch mechanism has a strong dependence on energetic ion interactions with the etching surface, it is helpful if the substrate is positioned close to (or within) the high-density plasma zone where the ion density is correspondingly high. This geometrical configuration is conveniently achieved in an ICP system by using a planar antenna that couples RF energy into the plasma through a dielectric window on the top of the process reactor. In the AOE system, the dielectric window is designed to ensure maximum inductive coupling of RF power into the plasma while minimizing capacitive coupling that can cause sputtering of the window material. In addition, it is helpful if the position of the substrate can be mechanically adjusted relative to the planar antenna to achieve the best balance between the gas dissociation rate and the ion flux at the etching surface.
Anisotropic oxide dry etch processes invariably employ fluorocarbon gases as the etching precursor material. These gases are dissociated in the plasma to create neutral reactive species such as F·, CF2, CF3, etc., and reactive ions such as CF+, CF2+, CF3+, etc. The relative numbers of neutral species and ions depend on the specific plasma operating parameters such as RF power input, gas residence time in the plasma zone, total reactor pressure, and partial pressure of the individual precursor gases, and also on reactor-specific attributes such as the material and geometry of the loss surfaces in the plasma zone.
In STS’s deep oxide etching process, the process gas of choice is octafluorocyclobutane (c-C4F8). This dissociates readily in an inductively coupled RF plasma to form a high density of radicals and ions such as those previously described. It is generally recognized that the highest oxide etch rates are achieved when the density of F and CF3+ species in the plasma is high. Conversely, selective etching of oxide relative to silicon or photoresist is achieved when the concentrations of CF+ and CF2+ ions in the plasma are increased relative to the CF3+ concentration - inevitably, selectivity is achieved at the expense of reduced etch rate.
Several groups have reported that the concentration of CF2+ relative to CF3+ in a c-C4F8 plasma depends on the degree of dissociation of the precursor gas. This, in turn, is partly determined by the residence time of the precursor gas fragments in the plasma zone for a fixed RF power input - the shorter the residence time, the less the degree of dissociation and the higher the concentration of CF2+ relative to CF3+. Therefore, we could expect to achieve most selective etching of oxide over silicon or photoresist when high total gas-flow rates (corresponding to short residence time) are used. In this context, inert gases are frequently used to increase the total gas flow without incurring the additional expense of large flow rates of c-C4F8, although it should be noted that the inert gas also will play a role in determining the dissociation processes of the precursor gas.
A cross-sectional SEM of a deep slot etched with an optimized process into silicon dioxide using polysilicon as an etch mask is shown in Fig. 1 - the etch depth is ~30µm. The highly anisotropic characteristic of plasma oxide etching is evidenced by the complete absence of undercutting beneath the polysilicon mask.
Should a metal mask (such as Al or Ni) be used for deep etches, the composition of the plasma is less critical. In these cases, the metal mask does not etch in the fluorocarbon plasma; therefore, the process can be tuned for maximum etch rate without concern for selectivity, provided that the concentration of F radicals is not so high and the concentration of polymerizing species so low that isotropic etching of doped oxide films occurs. The optimum maximum etch rate conditions are achieved when the residence time is increased by using a lower total gas-flow rate or when the partial pressure of the reactive precursor gas is increased.
This approach is particularly useful when etching high aspect-ratio oxide structures - ordinarily the etch rate would be expected to be lower in deep, narrow structures due to the lower diffusion rate of etching species into and reactant products out of the etched holes. If a metal mask is used, however, the process can be tuned for maximum etch rate to compensate for the aspect-ratio dependency of the application without fear of removing the mask [1].
An example of a high aspect-ratio submicron structure etched into silicon oxide is illustrated in Fig. 2. The nominal hole diameter is 0.7µm and etched depth is ~7.7µm. Figure 2a shows the Al mask structure prior to etching, while Fig. 2b shows the etched high aspect-ratio holes obtained with this process.
Conclusion
The basic characteristics of oxide dry etch processes were established many years ago when they were widely adopted in high-volume IC manufacturing. Processes employed in MEMS fabrication exhibit the same basic characteristics, but are adjusted to accommodate the considerably greater etch depths required by many emerging MEMS devices. In particular, the etch process can be made selective between oxide and photoresist or polysilicon masks by adjusting the residence time of gas in the plasma zone, although the selectivity is achieved as a trade-off against etch rate. Alternatively, metal masks permit the etch rate to be maximized without concern over excessive mask loss - this approach has been employed to etch vertical, cylindrical, submicron holes with an aspect ratio >10 and deep oxide structures with depths >100µm.
Reference
- L.A. Donohue, J. Hopkins, R. Barnett, A. Newton, A. Barker, “Recent Developments in Si and SiO2 Etching for MEMS based Optical Applications,” Micromachining Technology for Micro-optics & Nano-optics II, Photonics West, paper 5347-8, Jan. 2004.
Acknowledgments
Advanced oxide etching (AOE) is a trademark of Surface Technology Systems plc. The SEMs in Fig. 2 are included courtesy of Dalsa Semiconductor.
A.A. Chambers received his bachelors in mechanical engineering from Bristol Polytechnic and is technology director at Surface Technology Systems plc, Newport, NP10 8UJ UK; ph 44/1633-652400, e-mail andrew_chambers@stsystems.co.uk.
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