Table of Contents
SiO2 Etch Rate and Selectivity
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Various aspects related to dielectric etching are discussed in this paper. The two leading techniques for etching dielectric are diode RIE and high density based processes. The latest results for these techniques and the growing importance of nanoscale etching of dielectric films will be dealt with in this paper.
In recent years, dielectric etch processes have increasingly been carried out in a range of chambers, based on the customer’s etch requirements and cost constraints. In the case of dielectric etching where etch rate is not a major driver, with reasonable line widths (typically >1µm), conventional diode-type chambers are used. In cases where rate is a driver, with smaller line widths (typically <1µm), high-density plasma systems are used. Traditional diode or parallel-plate plasma chambers are commonly used in the industry.
There are two types of parallel plate systems that include the following:
Reactive Ion Etch (RIE) system
Plasma Etch (PE) system
In order to minimize sidewall losses and confine the plasma, magnetic enhancement has been added to these basic systems. The RIE type system is normally adopted for the etching of dielectric films. In the case of the RIE system, the plasma is typically generated at radio frequencies having an RF power in the range of a few hundreds of watts, through to kW.
For the driving frequency chosen, the electrons in the chamber are accelerated preferentially, whereas the ions are driven by the average electrostatic fields. The processed wafer resides on the powered electrode in order to enhance ion acceleration. The electron mean free path restricts the operating pressure. In case, the pressure is lowered near the level at which the electron mean free path approaches the gap between the electrodes, which is mostly several centimeters, the plasma is no longer self-sustaining. A typical RIE arrangement is shown in Figure 1.
Figure 1. RIE Schematic
High-density-plasma (HDP) chambers are designed in such a way that the plasma electrons are excited in a direction parallel to the chamber boundaries. The most common HDP source is the inductively coupled plasma (ICP) chamber utilized by OIPT. In this system, the plasma is driven by a magnetic potential set up by a coil wound outside dielectric walls as shown in Figure 2. The electron current direction is opposite to that of the coil currents, which are parallel to the chamber surfaces by design. The excitation of the plasma in this manner ensures that the electron mean free path is much greater than the chamber dimensions and the operating pressure is subsequently lowered. In most materials processing plasmas the electron heating is primarily resistive, and the impedance of the plasma is proportional with the density of neutrals available for inelastic collisions. As the impedance (pressure) is lowered so is the ability of the source to drive the plasma.
Figure 2. OIPT 300mm compatible source
High-density sources allow the wafer platen to be powered independently of the source, providing significant decoupling between the ion energy or wafer bias and the ion flux or plasma density driven primarily by source power. In a plasma-etching environment the anisotropy is offered by the acceleration of ions through the plasma sheaths, in a direction normal to the wafer surface. The anisotropic component is increased when the incoming ion flux is as normal as possible to the surface. The isotropic component of the incoming ion flux is either thermal, which is typically less than 0.1 eV. Operation in a lower-pressure/higher-density regime offers much thinner and less collisional sheaths, enabling it possible to obtain a more anisotropic etching component.
The primary processing advantages of ICP for dielectric etching are listed below:
Better CD control
Higher aspect ratios
Higher etching rates
Improved processing window
Dielectric patterning, especially silicon dioxide, is required for the manufacture of modern semiconductor devices, optical waveguides, RF ID’s, nanoimprint etc. Due to higher bond energies dielectric etching requires aggressive, ion- enhanced, fluorine-based plasma chemical systems. It is possible to obtain vertical profiles by sidewall passivation, typically by introducing a carbon-containing fluorine species to the plasma for instance, CF4, CHF3, C4F8). High ion bombardment energies are needed to remove this polymer layer from the oxide, as well as to mix the reactive species into the oxide surface to form SiFx products.
Dielectric etching applications mainly rely on the competing influences of polymer deposition and reactive ion etching to achieve vertical profiles, as well as etch-stopping on underlying layers. As hard-mask open-feature sizes shrink to 0.18 µm or less, for nanoimprint applications, aspect ratios are increasing to 4:1 or more. The ion and radical flux to the bottom of these features is minimized due to collisions with the feature sidewalls and other species present in the feature. Etch products for instance, SixFyOz and CxFy cannot diffuse out these features readily, resulting in excessive polymerization near the bottom of the feature which results in highly tapered features and poor mask transfer.
Traditional RIE type processes are based around CF4/CHF3 usually combined with either O2, He, Ar or a permutation. As the ion energy cannot be independently controlled increasing the RF power will eventually result in excessive photoresist damage. This limits the etch rate that can be achieved, which can be reduced to some degree by using better cooling by utilizing clamping and supplying of He to the backside of the wafer.
For the process performed in SEM1 it is possible to double the etch rate from 35 nm to 70 nm. Another way to increase the throughput is to increase the batch size. This is feasible for smaller wafer sizes, up to 100 mm, but for 150 mm and above, the system size becomes excessive, with the added issues of across batch uniformity etc. Diode chambers, are run at pressures of the order of 10’s of mT, in order to sustain the plasma (see earlier), this reduces the anisotropy and aspect ratios that can be etched.
SEM 1 RIE Waveguide etch
OIPT has developed high-density systems to address many of the issues related to etch rate, anisotropy and aspect ratio dependence. In a high-density system the operating pressure can be much lower (10 mTorr or less), and the diffusivity and mobility of the reactive species correspondingly higher. In addition the ion flux is independently tuneable by the source power, so that the total ion flux can be increased without as much of an increase in the ion energy, potentially reducing resist damage.
Due to their lower operating pressures (i.e. increased species diffusivities) chamber wall conditions play a more important role in ICP chambers. For example, to control polymer build-up the chamber wall temperature is controlled, pumping speed is increased, plus periodic plasma cleaning steps are used before processing a wafer. OIPT’s ICP based silicon dioxide etching system is based on C4F8 combined with O2 and/or noble gas He. Since C4F8 is a strained ring molecule, dissociation products are thought to consist of high levels of CFx (x ≤2) polymer precursors.
A simple L9 Taguchi matrix has been run at OIPT to ascertain the influences of the process parameters such as flow, ICP power etc., on the process. The trends are shown in Graph1
Utilising this information similar structures to those seen in SEM 1 have been etched, at more than three times the etch rate and with straighter sidewalls see SEM 2 and SEM 3.
SiO2 Etch Rate and Selectivity
By using an HDP source such as ICP, which operates at low pressures, ensures etching nanoscale features which are not possible in a traditional diode system. This necessitates accurate control of the ion flux to the surface to control the polymerization - too low, and the possibility is that the etch profile will taper or it will stop completely. Working closely with nano-centres such as those at Cornell and LBNL, OIPT has developed a range of processes capable of etching structures with line widths of the order of 100nm, examples of these are shown in SEMS 4, 5 and 6
Certain semiconductor equipment manufacturers have reported improved selectivity with the addition of hydrogen to the C4F8-based system. This hydrogen inclusion generates far greater levels of CxFy polymer compared with systems operating with none. OIPT have found that using such a process results in excessive polymer build up in the reactor, even if sophisticated chamber heating is utilised. This results in more frequent plasma cleaning as well as the possibility of more mechanical cleans - decreasing productive process time along with increasing cost of ownership. OIPT have found that by achieving the correct balance of process and hardware, whilst excluding the use of H2, that in excess of 1000 wafer µm can be etched prior to a plasma clean becoming necessary.
One process that shows the control that can be achieved, for dielectric etching, in the OIPT ICP system is the etching of micro-lenses into a SiO2-based material, such as quartz or glass. Control of the ion flux, plus gas chemistry, is required to achieve the desired micro-lens shape in the substrate material, as the carbon loading changes with time. SEM7 shows an example of a perfectly etched micro-lens.
Recent developments have shown that a trend towards deeper dielectric etches, of the order of more than 100µm, are being required. Normal photo-resist masks cannot be used to etch to this depth so metal masks, such as Cr and Ni, are being used which can offer selectivity’s of more than 100:1. This gives more latitude in the process chemistry that can be used, but control of the ion flux is still paramount. Too high, and the mask will be eroded due to sputtering before the desired depth is reached. SEM’s 8 and 9 show a deep quartz etch utilising a Cr mask. For SEM9 there was a masking issue which left residue, but it shows the capability to etch to substantial depths.
Both the diode and ICP processes, for dielectric etching, discussed have evolved over the years both in terms of hardware and process. The ICP based process offers higher etch rates, with better CD and anisotropy control, along with higher aspect ratios etc. Achieving these objectives, requires the use of larger turbomolecular pumps, which come at a high cost, but the advantages of the higher rates more than compensate for this. Also, by using these larger pumps and independent ion flux control, there is a possibility of etching nanoscale features. The diode system does offer a cost effective solution for etching of dielectrics with larger linewidths, but at a much slower rate, and cannot be used for etching of nanoscale features.
About Oxford Instruments Plasma Technology
Oxford Instruments Plasma Technology provides a range of high performance, flexible tools to semiconductor processing customers involved in research and development, and production. They specialise in three main areas:
RIE, ICP, DRIE, RIE/PE, Ion Beam
PECVD, ICP CVD, Nanofab, ALD, PVD, IBD
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma technology.
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