DE2736262A1 - METHOD AND DEVICE FOR END POINT CONTROL DURING PLASMA ETCHING - Google Patents

Description

2735262

The invention relates to a method and a device for end point control during plasma etching and in particular for Endpoint control by measuring a significant change in the intensity of an optical emission line from the plasma.

Plasma etching usually occurs from batch to batch

significant changes in the etching rate due to a variety of reasons such as changes in the initial temperature, changes in plasma conditions and changes in the area to be etched. It is therefore difficult ensure that the etching process is terminated at the correct point in time (i.e. end point) for optimal etching, i.e. when the layer to be etched has been removed but the pattern in this layer and the underlying layer not over-etched Layer are not noticeably etched.

The invention creates a method for precise endpoint control, which is based on the measurement or determination of a change in an emission line that occurs when the underlying layer has been reached, at which level a change in material occurs.

The invention is described below, for example, with reference to the drawing; in this shows:

1 shows a schematic representation of a plasma reaction chamber with the associated equipment,

2 shows a schematic representation of a reaction chamber and an emission detector,

Fig. 3 shows a typical emission signal for an etching gas for a special type of wafer,

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4 shows the use of blank disks for determining the end point of special materials,

Fig. 5 shows the emission signals for the detachment of photosensitive Paint.

A typical apparatus for plasma etching is shown in Fig. 1, which reproduces the reaction chamber in cross section perpendicular to its axis. A suitable etching gas, typically CF ^ + 5% 0pi or only CU for the plasma detachment of Photosensitive lacquer is introduced into the cylindrical quartz reaction chamber 10 through a needle valve 12 and on the pressure P held by means of a vacuum pump 13, which the Reaction chamber 10 evacuated through the throttle valve 14. The flow rate of the etching gas is adjusted by an adjustment of the needle valve 12 is set and precisely monitored with the aid of the flow meter 15, regardless of this the pressure is set with the help of the throttle valve 14 and with Using a precision manometer 16 monitored. Flow rates from 5 to 500 cnr / min and pressures from 0.25 to 1.5 torr is typical. A plasma is generated in the reactor by drawing energy from the RF power supply 17 to the capacitor plates 18 arranged around the reaction chamber 10; Power levels of 10 to 500 watts are typical. An impedance matching network in the RF power supply is used to couple the power into the plasma to optimize. The etch speeds are controlled through the use of a perforated RF shield 19 made of metal equalized and controlled, which serves to delimit the plasma from the ring-shaped area outside the plasma, thereby reducing the temperature and temperature non-uniformities on the workpieces 20, which in general are arranged on quartz holders.

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In a typical etching process, the workpieces are placed in the reaction chamber and the chamber is evacuated Etching gas is introduced and the etching begins when the RF power is applied. The reactor temperature rises during the etching process and because the etching rate is very temperature dependent, the thickness depends on the given time, is sensitive to the thermal properties of the reactor and the workpieces (i.e. from the initial temperature, the thermal mass or heat capacity and the heat time constant). For materials such as Silicon, polysilicon, and Si ^ N ^, and titanium are the etch speeds high and also depend on the area of the workpiece that is exposed to the etching gas, as well as the Distance between adjacent workpieces and the position of the workpieces and their number. The latter There is a dependency because the workpieces mutually deprive each other of the etching material available; accordingly, the etching rate decreases as the number of workpieces increases. Workpieces also become slower in the center of a batch etched than the end disks, which is due to depletion effects.

As a result of the dependence on temperature, plasma conditions and the load to be processed, the times that are required for the same etch, generally considerably from pass to pass, and it is therefore necessary to to monitor the progress of the etching so that the etching cycle can be ended in good time. The work pieces are irreparably damaged if they are excessively etched $ therefore, methods of monitoring or controlling the etch endpoint are extremely important and must be very accurate. For example, in the manufacture of semiconductor components, plasma etching is generally used for etching Patterns covered with photosensitive lacquer used in thin layers on slowly etchable substrates. That

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The substrate then serves as a partial etch stop and the effect of overetching is that undesirable etching take place under the mask edges (undercuts) and an excessive etching into the substrate layer. In some Cases the substrate layer is a thin layer over a rapidly etching bottom layer and going through the etching in this lower layer can lead to irreparable damage.

While it is possible to visually monitor the etching, it is often difficult to do so. You leave yourself in doing so, ensure that certain visible changes, e.g. a change in the color of the surface to be etched, occur. However, such a procedure is not always possible and depends to a very large extent on the level of perception of the operator away. Continuous observation by the operator is also required.

According to the invention, an optical emission from the plasma is monitored. It is possible to select emission lines that are sensitive to changes in the material being etched and in this way indicate when at the end point the etching is in an underlying layer begins. One form of apparatus for monitoring plasma emission is shown in FIG shown.

The optical emission from the plasma at 21 (in the reaction chamber 10) is detected with the aid of the photodetector 22 by the Filter 23 observed, the pass band is chosen so that an emission line is selected whose intensity in is particularly sensitive to changes in the material to be etched. The signal from the photodetector becomes continuous monitored with the aid of a line recorder 24 and can also be entered into a comparator circuit 25,

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which triggers the shutdown of the etching cycle with the aid of an output signal which is sent via connection 26 to the RF power source 17 is given. It can be a photomultiplier tube be used with an electrometer output amplifier as a detector, but solid-state detectors are preferred, since they are smaller and have a lower cost.

Tests using a monochromator showed that a line with a wavelength of approximately 0.70 yum changed considerably when the slice of SiO ^ in Si for an etch was changed to CF ^ + 0 ". The signal amplitude of this emission line was unchanged when the SiOp wafer has been removed or if a large number of SiOp or Si, N ^, wafers have been used. The addition of Si washers caused large decreases in the intensity of this emission line, with the proportional decrease greatest at large Disk loads, high RF power and low CF ^ + 5% 0p ~ Etching gas pressure was. The Si / SiO ^ signal ratio increased with increasing Temperature decreased somewhat and all signals decreased very strongly when No was added to the etching gas. The decrease in intensity the emission line with 0.70 yum with increasing etching rate or disk loading indicates that the intensity of this line depends on the number of available Si etching radicals in the plasma depends, which is depleted when the etching rate or etching speed or load increases. Alternatively there is also the possibility that this emission line is "poisoned" by the reaction products of the Si etching will.

An example of the endpoint monitor signal obtained in a typical polysilicon etch sequence is shown in FIG. The original curve had a signal-to-noise ratio that was better than 30: 1 and the signal change of approximately W% was obtained by etching a single photoresist covered wafer of polysilicon over SiOp at 50 watts in

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an etching gas of CF ^ + 5% C> 2 ^{at a} pressure of 0.25 Torr and a temperature of 50 ⁰ C achieved. The signal level at 30 is due to the backlight. The signal increases at when the plasma is ignited and at 33 reaches a level which is characteristic of the silicon etch. The signal then grows again at 3 ^ when parts of the SiOo substrate are exposed, and finally the end point of the etching is characterized in that the time dependence of the signal suddenly flattens out at 35. It has been shown that similar curves for successive etching runs can be reproduced extremely well in terms of shape and amplitude. Corresponding curves are also observed for larger batches of discs; the relative change in the signal at the end point is significantly greater and the abrupt change in the slope at the end point is somewhat smeared, which is due to the non-uniformity of the polysilicon thickness and the etching rate across the batch. The etching rate decreases with increasing disk loading and this effect can be compensated for by increasing the power, which results in a corresponding increase in the signal amplitudes supplied by the endpoint monitor. The peak 32 at the onset of the silicon etch generally only occurs if the etch is preceded by a brief Op plasma treatment which is used to improve etch uniformity by removing photoresist residues; the tip can be assigned to the etching of the thin SiOo layer that was formed during the plasma oxidation precleaning.

Points 35 and 36 in Fig. 3 correspond to the termination points for two wafers covered with patterns, which are etched using the inventive method for endpoint control became. The wafer with the etch stopped at point 35 was free of polysilicon at all exposed Make and showed negligible undercuts. the

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Disc, the etching of which was finished at 36, showed undercuts, which were small enough to allow the etching of good 2-jjm structures to enable. This shows that the endpoint can be easily recognized within a window during this time the etch should be stopped and that is far enough to accommodate the typical variations in polysilicon thickness or from non-uniformities in the etch rate within to enable batch loading.

The 0.70 emission line can also be used to monitor the plasma patterning of other structures, such as Si, N ^ on SiCU on Si, such as are typically used in the manufacture of silicon integrated circuits. This is done by using a blank disk, like the one in the Pig. 4a and 4b, FIG. 4a referring to Si, N ^ referring to SiO ₂ and FIG. 4b referring to SiOp referring to Si.

In Fig. 4a, a disk 40 comprises a silicon substrate 41 on which a layer of SiOp 42 on one surface and one Backside protective layer 43 attached on the other surface are. A layer 44 of Si, N ^, is on the SiO ^ layer 42 is formed and a masking layer 45 is on the Layer 44 is formed defining an area 46 to be etched. Together with the disk 40 is an empty disk 47 provided. The disk 47 has a silicon substrate 41 and a backside protective layer 43, but is the SiO ^ layer 42 omitted so that the Si ^ N ^ layer 44 directly on the substrate. The masking layer 45 is on top of the Layer 44 is formed and delimits the area 46. The Si, N ^ - Layers 44 on each disc are the same thickness. One or more discs 40 are put together in the reaction chamber introduced with an empty disk 47. The etching is ended immediately after the decrease in the intensity of the optical emission has occurred, which is associated with the etching through of the Si, N ^ -

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Layer 44 and the penetration of the etching into the silicon substrate 41 of the blank 40 is connected.

A similar arrangement is used for SiOo over Si. As shown in FIG. 4b, a disk 50 has a silicon substrate 51 ·> on one surface of which there is an SiO ^ layer 52 and on the other surface of which a protective backing layer 53 is applied. A masking layer ^ A is formed on the SiOP layer 52 and defines an area to be etched 55 · There is also provided an idle pulley 56, has the same configuration as the disk 50, a silicon substrate 51 ·> ie a SiOP layer 52, a Backside protective layer 53 and a masking layer 54 surrounding an area 55. The layers 52 have the same thickness, but in the present example the SiOp layer 52 of the blank was slightly pre-etched before use - for example about 500 to 1000 Å, like that is identified by the reference number 57. The disk or disks 50 are brought into the reaction chamber together with the empty disk 56. The etching is ended when the emission signal level corresponding to the Si etching is determined, which is assigned to the blank. A thin layer of SiOp then remains over the substrate 51 of the wafer or wafers 50 and the etching of the Si / SiOp interface can, if necessary, be completed by a chemical dip etch which does not degrade the pattern resolution. Silicon is etched away in the plasma so much faster than SiOp that the plasma etching at the Si / SiOp interface cannot be completed using CF ^ + 5% 0p etching gas without unacceptable damage to the areas of the underlying silicon due to irregularities the etching and the layer thickness occur.

About the non-uniformity of the etching speed and the layer thicknesses When using blank wafers, the Si, N ^, etching should be carried out for a short time beyond the endpoint signal

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to be continued and the SiO ₂ etching (etohback) of the SiO ₂ over-Si blank must be greater than the irregularities. In both examples, the blank panes must have the same layering and masking properties as the batch panes and the panes should have the same spacing in the reactor. The rear side protective layers 4-2 and 53 serve to avoid false Si signals from the rear sides of the wafers.

The endpoint monitoring device according to the invention for the Marking the onset (or completion) of the silicon layer etch has the following main advantages:

1. The endpoint display is clear and accurate and thus requires a minimum of judgment or skill on the part of the operator. In addition, the signal is reproducible and changes very quickly in the vicinity of the end point, so that it is suitable for use with a comparator circuit which is set, for example, to 80% of the SiO ₂ signal in order to trigger a time-delayed termination of the etching cycle. The set point can easily be calibrated with the aid of an empty loading system.

2. The process automatically compensates for changes in the number of slices, the distance between the disks, those exposed to etching Areas and the layer thickness from batch to batch.

3 · The procedure also automatically compensates from batch to Changes in the initial temperature and plasma conditions that occur during the batch. Thus it enables a Relaxation in the required accuracy of setting all plasma parameters.

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4. A consequence of point 3 is that the need for one Temperature control and preheating is eliminated and that short etch cycles with low temperature and high RF power and thus become possible with higher throughput without compromising the accuracy of the endpoint control or control compromises have to be made. The fact that the device according to the invention is a enables rapid etching at low temperature and high RF power, it results in maintaining good etch rate uniformity with minimal photoresist attack. This enables the application of patterns with finer lines over a more distinct or stepped one Layer than is usually the case.

The end point control with the aid of optical emission can also be applied to the stripping of the photoresist, as shown in FIG. In Figure 5, curve 60 corresponds to a 3 inch diameter disk while curve 61 corresponds to four 3 inch diameter disks. The coordinates of the curves are the intensity of the emission line or the detachment or development time. The release time is also related to the plasma temperature in the manner shown. The individual curves 60 and 61 relate to an emission line at a wavelength of Ö, ^ Ö ^L Jim for the detachment or development of the photoresist in an operating theater plasma with 0.5 Torr at an RF power of 300 watts within an etching tunnel. After an initial increase at 62, the signal shows a small peak at 63, initially decreases and then increases with increasing temperature and increasing development time or removal time to a maximum 64 when parts of the photoresist areas are removed. The signal then decreases until all of the photoresist is peeled off at 65 and then remains constant. The end point is even with a

Partial load clearly visible. The signal amplitude increases with increasing photoresist surface and reactor temperature

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and RF power, and decreases with increasing O pressure. the Emission line grows with increasing peeling, which indicates that it can be assigned to a reaction product.

The emission line at around 0.30 / im is particularly sensitive to photoresist detachment. However, there are groups of lines between about 0.250 and about 0.340 µm and between about 0.450 and about 0485 / µm and a single line at about 0.650 / µm that have similar sensitivity.

The emission comes from a component present in the plasma, which is associated with a change in the material to be etched. Thus, a signal caused by a change In the emission intensity generated, either the end point of the etching of a special material can be used or indicate the end point of etching another material over a particular material, i.e., the onset or the beginning of the etching of this particular material.

It has been shown that the emission line at a wavelength of approximately 0.70 Jim is particularly effective in plasma etching is from Si to SiOp or vice versa, but it will be understood that other lines may be effective with other materials.

It should also be noted that although CF ^ + 5% 0o than that Etching gas for the etching of Si has been described, the oxygen content can be changed starting from zero in a known manner can, and that CF ^ + 5% 0o only as a conventional etching gas has been described.

In Figures 1 and 2, there has been described an apparatus known as a volume loader. The inventive However, the device can also be used with a device of the surface loading form.

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The method is suitable for the photoresist used in the Integrated circuit manufacture is common. For example, typical photoresists made by Philip A. Hunt Co. Ltd. Waycoat HR series supplied and that of the Shipley Company Incorporated Shipley 1350 series.

An essential feature is the selection of the emission lines that are best used for the greatest possible sensitivity. A wide variety of emission lines can exist have useful properties and the method of selection and measurement can be varied.

The invention thus provides a method and a device by which the end point in the plasma etching is monitored an optical emission coming from the plasma is detected, a special optical emission line being selected and a substantial change in the emission intensity being detected. This indicates a change in the material to be etched or currently etched and thus shows the completion of the etching of one material and the beginning of the etching of the other material. The inventive method can also be applied to removal or use etching of photoresist material.

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Claims (12)

Patent claims 1.) A method for performing the endpoint control during plasma etching, characterized in that the change in the optical emission of a component present in the plasma is measured or observed, this component being associated with a change in the material to be etched, and that a signal is generated, which is indicative of the change in the optical transmission. 2. The method according to claim 1, characterized in that that the optical emission from the plasma is monitored, that an emission line assigned to the component it is selected that the emission intensity of this emission line is measured that a substantial change 809811/0672 DR. C. MANITZ ■ DIPL.-INC. M. FINSTERWALD β MÖNCHEN 22. ROBERT-KOCH-STR ASSE I TEL. IO89t 29 43 ti. TELEX OS - 99673 PATMF DIPL.-INC. W. C R Λ M K O W STUTTGART SO IBAD CANNSTATT) SEELBERCSTR. 93/95. TEL. (0711 156 79 61 CENTRAL TICKET OFFICE BAYER. VOLKSBANKEN MÖNCHEN. ACCOUNT NUMBER 7970 POST CHECK: MÖNCHEN 77062-805 2733262 detected in the emission intensity and generated a signal which is indicative of this change. 3. The method according to claim 2, characterized in that that the optical emission from the plasma is observed through a filter by means of a photodetector, wherein the filter passes the emission line, and that the photodetector generates an output signal indicative of for the intensity of the emission line. 4. The method according to claim 1 for the plasma etching of silicon on SiOp in a CF ^, - atmosphere, characterized in that an emission line from the plasma is monitored, that the emission line has a wavelength of approximately 0.70 pm and that a considerable increase is determined in the emission intensity of the emission line in order to mark the end point of the etching of the silicon. 5. The method according to claim 1 for the plasma etching of SiOp on silicon in a CF ^ plasma, characterized in that an emission line from the plasma it is monitored that the emission line has a wavelength of about 0.70 µm and that a substantial one Decrease in the emission intensity of the emission line is observed, which is indicative of the end point of the Etching of the SiOp is. 6. The method of claim 1 or 2 for removing a photosensitive Lacquer material in an oxygen plasma, characterized in that the change in the optical emission of a line is observed which is a line selected from the group between approximately 0.25 µm and 0.34yum is the emission intensity of the wavelength measured and a considerable decrease in intensity 809811/0672 2733262 is found to be indicative of the removal or removing the photoresist is. 7. The method according to claim 6, characterized in that that the change in optical emission of a line is detected which has a wavelength of approximately 0.30 µm that the emission intensity of the wavelength measured and a substantial decrease in this intensity is found, which is characteristic of the Removing the photoresist is. 8. The method of claim 1 or 2 for removing photosensitive Lacquer material in an oxygen plasma, characterized in that the change in the optical emission of a line is detected, the one line from a group between 0.4-5 / am and approximately 0.485 pn is the emission intensity of the wavelength measured and a considerable decrease in this intensity is noted, which is characteristic of the Removal of the photoresist is. 9 · Method according to claim 1 for the removal of photosensitive Lacquer material in an oxygen plasma, characterized in that the change in optical Emission of a line is found which has a wavelength of 0.65 / an that the emission intensity of this Wavelength is measured and a considerable decrease in intensity is found, which is characteristic of is the removal of the photoresist. 10. Device for performing the endpoint control at Plasma etching in a plasma reaction chamber, characterized in that a device (22) for monitoring the intensity of the emission of a given 809811/0672 2733262 Emission line from the plasma in the reaction chamber (21) and a device (24, 25) for generating a signal are provided, which is indicative of a change in intensity. 11. The device according to claim 10, characterized in that a device (23) for selection a predetermined emission line and a device (22) for measuring the emission and generating the Signal are provided. 12. The device according to claim 11, characterized net that a filter (23) for the selection of the given Emission line is provided and that the device for measuring the emission comprises a photodetector (22). 809811/0672