WO2001048789A1

WO2001048789A1 - Plasma processing methods

Info

Publication number: WO2001048789A1
Application number: PCT/GB2000/004927
Authority: WO
Inventors: Jyoti Kiron Bhardwaj; Leslie Michael Lea; Anthony James Barker; Jacques Andre Grange
Original assignee: Surface Technology Systems Plc
Priority date: 1999-12-24
Filing date: 2000-12-22
Publication date: 2001-07-05
Also published as: GB9930603D0

Abstract

A workpiece is treated in a vacuum chamber by a plasma struck by capacitative or inductive coupling of RF power. RF power is also supplied to an electrode supporting the workpiece at a high frequency well in excess of 13.56 MHz for all or part of the processing stage in order to control the ion energy distribution. Ideally the high frequency energy applied to the electrode will cause the energy distribution to peak around a single energy.

Description

"Plasma Processing Methods" This invention relates to the use of a plasma processing apparatus for particular applications. Plasma processing apparatus is used extensively in the fabrication of semiconductor devices and in many other applications in which the deposition of coatings or the etching of surfaces is required. In most circumstances, the process plasma is generated by the supply of energy to a gas or vapour introduced into a vacuum chamber. This energy may be introduced in any of a number of ways, such as by capacitive coupling of radio frequency power, inductive coupling of radio frequency power or by means of microwave power guided into the chamber.

For the inductive coupling of RF power into a process chamber it is preferred that the means for generating the plasma is located either outside of the chamber, or internal to the chamber and is suitably protected from the plasma. The plasma chamber may be surrounded by a coil as in US-A-4844775, or alternatively a spiral coil may be placed against one end of the processing chamber, as for example in EP-A-0379828. Alternative arrangements for the coil/antenna may be used. A dielectric window into the chamber will generally be required adjacent to the coil, and an RF impedance match unit will usually be located between the output of the RF power supply and the ICP coil/ antenna . In an Inductively Coupled Plasma process apparatus (ICP) , the plasma is generated by the inductive coupling of radio frequency power into the process gas supplied to the apparatus. The process gas typically fills the chamber to a chosen pressure in the range 1 - 100 microBar, although pressures outside of this range may be utilised. The workpiece is usually mounted on a support electrode within the chamber, and this electrode enables the workpiece to be electrically biased to attract ions from the plasma. A cooling fluid and gas may be supplied to the electrode in order to control the temperature of the workpiece while the processing operation is being carried out. A typical plasma processing apparatus will include automated means for loading and unloading the workpiece with respect to the support electrode. A schematic diagram of a plasma process chamber and major associated power supplies is shown in Figure 1.

If the workpiece is a good conductor, and is not coated by a non-conducting film during processing, it would be possible to bias the workpiece to attract positive ions simply by applying a DC negative potential to the support electrode. When the workpiece is an insulator or semiconductor, or is likely to have insulating material deposited on it, it is necessary to apply radio frequency, or pulsed DC power to the electrode, which is capacitively coupled through the workpiece. This results in the formation of a self-sustaining quasi DC potential difference between the plasma and the surface of the workpiece. Positive ions are then accelerated through this potential difference from the plasma to the surface of the workpiece. The quasi DC potential difference is a result of the difference in mobility between electrons and positive ions. When an RF voltage is applied to an object placed in a plasma, electrons will be attracted whenever the object is at a positive potential with respect to the plasma, and positive ions will be attracted whenever the object is at a negative potential with respect to the plasma. For an insulating object, during a period of time, the current reaching it due to positive ions must equal the current to it due to electrons (and negative ions if present) . Because of the greater mobility of the electrons than the positive ions, to balance the flow of currents to the object, the potential of the object with respect to the plasma must be positive for only a small fraction of the time that it is negative. Since a waveform which is sinusoidal, or close to sinusoidal, is supplied by an RF generator, in which positive and negative half cycles are of the same duration, the self consistent solution is for the object to adopt a bias comprising the RF waveform offset by a DC negative bias with respect to the plasma. The above description is somewhat simplistic, in that although the positive ions have a much lower mobility than the electrons, when the frequency of the applied RF power is reduced, the ions are more able to respond to the instantaneous potential difference. Conversely when the frequency is increased, the ions only feel the quasi DC bias and are unable to respond to the instantaneous applied RF potential. The energy and angular distribution of positive ions reaching the workpiece is therefore a function of the frequency of the RF power applied, as well as the magnitude of the power.

Plasma etching of a workpiece may be either physical or chemical or a combination of both. Physical etching consists of bombarding the surface with ions from the plasma so that material is sputtered away. The ions are accelerated across the potential difference set up between the plasma and the workpiece by the power supply connected to the support electrode. Chemical etching in contrast relies on radicals formed in the plasma, diffusing to the workpiece and reacting with the surface to produce volatile products which are then pumped away. Chemical etching per se is not reliant on workpiece biasing. Chemical etching, however, can be physically assisted by ion bombardment in cases where the energy thereby supplied to the surface of the workpiece is required to reach the activation energy of the chemical reaction.

The inventive step described here is the realisation that the application of high frequency radio frequency power to a support electrode can bring process advantages over the use of lower frequency power. Accordingly, this invention provides a method of treating a workpiece in a plasma, wherein the plasma is struck in a vacuum chamber by capacitative or inductive coupling of RF power and RF power is also applied to an electrode which provides support for the workpiece being treated at a high frequency well in excess of 13.56 MHz and equal to or higher than the ion plasma frequency, for all or part of the processing stage, in order to control the ion energy distribution. In this context, lower frequency power refers to 13.56

MHz or lower. 13.56 MHz is typically the frequency used to produce the inductively coupled plasma. High frequency radio frequency power, applied here to the support electrode and hence to the workpiece, refers to frequencies higher than 13.56 MHz, which may conveniently be 27 MHz or 40 MHz and higher order harmonics such as 54 MHz or 67 MHz, which are recognised frequencies for industrial usage, but without limit to these.

Specifically it can be advantageous to apply a high frequency bias to the support electrode when carrying out through-layer polymer etching with a mask that is sensitive to ion damage. Such application may be required for bi- layer, tri-layer or multi-layer resist systems as disclosed in the literature, including US 4980317. The mask may be for example photo-resist, other known polymer medium, silicon-containing resist (referred to as silated resist) or a hard mask (typically an oxide or a metal) previously opened by an etch step operated through a thin resist layer with high resolution critical dimension control .

High frequency biasing may also be advantageous in such applications with respect to etch profile control. In one case the profile may be controlled by a continuous process that provides directionality of the etch by means of biasing and hence acceleration of ions normally to the workpiece surface and/or by means of side-wall passivation via a carefully chosen plasma chemistry. In another case the profile may also be controlled by the use of a distinct passivation step in which a side-wall protection layer is deposited to inhibit lateral etch. The method is performed by cyclic etch and passivation.

The etch recipe in one case and the etch step in the other may make use of well known oxidising chemistry including gases comprising 0₂, C0₂, N₂0, S0₂. Additions such as N₂, Ar, He, are used to promote some aspects of the etching control .

The etch recipe in one case, and the passivation step in the other may utilise: a) A carbon/hydrogen polymer b) Polymeric precursor gas or gases which coat the side-wall with a "similar" material to the polymer being etched. e.g. to etch either polyimide or polymethylmethacrylate (PMMA) , carbon, hydrogen and possibly oxygen containing polymeric material may be used. In the case of a cyclic process, the passivation step may also utilise deposited material which is not etched chemically and hence requires the addition of a two-step etch process; the first step being to remove passivation from the base of the trench and the second to etch the polymer. The deposited material could be an insulating, semiconducting or metallic layer.

There are conflicting requirements on the ion energy when carrying out through-layer polymer etching. The ion energy should be reasonably high to minimise bowing of the trench profile (see Figure 2a) , while needing to be low to minimise damage, in particular faceting of the mask. It is this conflict in the ion energy requirements which can make the use of high frequency bias an advantage. As described above, for low frequency bias, the ion mobility allows them to respond to some degree to the RF waveform. This leads to an ion energy distribution with two peaks, the peak separation being determined by the frequency. For sufficiently high frequency bias (e.g. 67 MHz), the ions are unable to respond to the instantaneous RF waveform and have an energy distribution peaked around a single energy (see Figure 3) . Control of the mean ion energy is by control of the RF power applied to the support electrode, and control of the ion energy distribution function is in part by means of the frequency chosen. Adjustment of these two controls can allow both of the conflicting etch requirements described above to be satisfied, at least partially.

Severe faceting of the mask can be a particular problem for thin mask layers. This faceting may be affected by the frequency of the RF applied to the support electrode. The mask faceting makes trench profile control difficult, particularly as line widths decrease to sub one micron. Adjustment of the process chemistry such as the chemistry detailed above may improve the etch rate, but will have little effect on the faceting problem.

The faceting effect is due to the angular dependence of the physical etch rate of the mask on the incident angle of the bombarding ions. When a feature in the mask (e.g. the edge of the mask defining one side of where a trench is to be etched) , is exposed to ions with a low incident angle, the corners of the feature are eroded first, and small facets are produced. Once this has happened, the angle of the incident ions with respect to the facet normal will approach 45 degrees, and the etch rate is accelerated. The result is a significant reduction in the thickness of the resist adjacent to the side of the trench, so that as time passes, the resist is progressively removed adjacent to the trench and the trench side walls cease to be well defined. Figure 2a indicates how the resist faceting may appear, while Figure 2b shows the ideal resist shape. In addition local charging effects may well aggravate the faceting problem as seen for instance with most bi-layer resist etch processes.

Bowing of the sides of the trenches is due to a combination of either or both of a physical action (ions having lateral velocities which are not insignificant when compared with their velocities normal to the surface of the workpiece) and a chemical action. In general, and provided that charging effects that cause ion deflection are well controlled, increasing the RF power applied to the support electrode will increase the ratio of normal over lateral averaged ion velocities and hence will improve profile control . In cases where lateral etching occurs only above a threshold ion energy it may be found that lowering the power applied to the support electrode actually helps profile control, but does so usually at the cost of etch rate. In cases where the polymer etch is a physically driven chemical etch, such a threshold may exist and hence may be critical in devising the optimum process conditions and gas chemistry. The general and particular cases described above define two main potential process windows for improved profile control. In either case a narrowing of the ion energy distribution by the use of higher frequency biasing inherently benefits profile control. It effectively reduces the proportion of ions with a low velocity normal to the surface and hence reduces both the impact of the chemical part of the etch and the effect of surface charging on ion deflection, two major contributors to CD loss by lateral etching.

Claims

CLAIMS 1. A method of treating a workpiece in a plasma, wherein the plasma is struck in a vacuum chamber by capacitative or inductive coupling of RF power and RF power is also applied to an electrode which provides support for the workpiece being treated at a high frequency well in excess of 13.56 MHz and equal to or higher than the ion plasma frequency, for all or part of the processing stage, in order to control the ion energy distribution.

2. A method according to claim 1, wherein the RF power applied to the electrode is at a frequency of 27, 40, 54 or 67 MHz or other higher order harmonics.

3. A method according to claim 1 or claim 2, wherein the high frequency biasing of the electrode is performed during through-layer polymer etching with a mask that is sensitive to ion damage .

4. A method according to any one of claims 1 to 3 , wherein etch profile control is provided in a continuous etch process by the use of high frequency biasing of the electrode to provide directionality of the etch by means of the acceleration of the ions normally to the workpiece surface, with or without the use of side wall passivation.

5. A method according to any one of claims 1 to 3, wherein etch profile control is provided in a switched process by the use of high frequency biasing of the electrode to provide directionality of the etch step by means of the acceleration of the ions normally to the workpiece surface, with side wall passivation during the deposition step.

6. A method according to any one of claims 1 to 5, wherein the high frequency power applied to the electrode causes the ion energy distribution to peak around a single energy.

7. A method according to any one of claims 1 to 6, wherein the mean ion energy is controlled by the level of RF power applied to the support electrode.

8. A method according to claim 7, wherein both the frequency and power levels of the RF applied to the electrode are adjusted mutually to achieve optimal etch requirements .

9. Any novel combination of features of a method of treating a workpiece in a plasma whilst applying RF power to an electrode supporting the workpiece as described herein.