A method for reactive sputtering deposition

(54) (56)

INVENTOR

Tomas Nyberg, Uppsala SE, Sören Berg, Uppsala SE

OMBUD Aros Patent AB

NAME The procedure for reactive sputtering

CALLED PUBLICATIONS:

(57)

SUMMARY:

WO Al 92/02659 (C23C 14/34), US A 4,434,037 (u.204 / 192.12),

US A 5,685,959 (u.204 / 192.2)

The invention is a method of reactive sputtering with increased maximum tolerable current density and eliminated hysteresis. This is achieved by focusing the ionic current on a small area, a reduced erosion area (14), which constantly moves along the target (10) to avoid melting of target material. This means that the current density is very high at the reduced erosion area (14) while the average total current density is considerably lower. The reduced area moves at a rate high enough to avoid local melting of the target but at the same time low enough to ensure that most of the sputtered substances are metal atoms. In this way, a hysteresis-free process for reactive sputtering can be obtained. Furthermore, the problem of spark formation in reactive sputtering is solved and a significant increase in the proportion of ionized sputtered substances is obtained.

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The numbers in brackets indicate the international identification code, the INID chain. Letters in clamps indicate international document code.

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SUMMARY

The invention is a method of reactive sputtering with increased maximum tolerable current density and eliminated hysteresis. This is achieved by focusing the ionic current on a small area, a reduced erosion area (14), which constantly moves along the target (10) to avoid melting of target material. This means that the current density is very high at the reduced erosion area (14) while the average total current density is considerably lower. The reduced area moves at a rate high enough to avoid local melting of the target but at the same time low enough to ensure that most of the sputtered substances are metal atoms. In this way, a hysteresis-free process for reactive sputtering can be obtained. Furthermore, the problem of spark formation in reactive sputtering is solved and a significant increase in the proportion of ionized sputtered substances is obtained.

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TECHNICAL FIELD

The present invention relates to sputtering deposition processes.

BACKGROUND OF THE INVENTION

Sputtering is a preferred industrial process for thin film coating. In this process, a target material (target = cathode) is deposited over a substrate surface. By bombarding the target with gases accelerated with high voltage, target atoms are caused to be ejected, or sputtered, from the surface. Target particles then traverse the sputtering chamber and deposit on the substrate as a thin film.

In some sputtering applications, it is advantageous in many ways if a large proportion of the sputtered atoms are ionized. First, the sputtered ions can be attracted to the substrate by applying a bias to the latter. This adds energy to the growing film, which is advantageous for the growth of the film. Secondly, the attracted ions will have a favorable perpendicular direction when they arrive at the substrate surface, which allows deposition in grooves and pits. Third, if the ion fraction is large enough, it may be possible to operate the process in a self-sustaining state. This means that ions from the target material sputter themselves without the addition of extra (inert) gas. This will of course result in a much cleaner process where no inert gas substances contaminate the deposited film.

The proportion of ionized sputtered substances is correlated to the target ionic density. A higher current density means a greater proportion of ionized substances. In general, the maximum tolerable current density is limited by the efficiency of the target cooling system.

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By adding a reactive gas to the sputtering process, it is possible to reactively sputter thin films consisting of oxides, nitrides, carbides, etc. etc. Due to its highly complicated behavior, the reactive sputtering process is interconnected with a number of difficulties. The process usually exhibits a behavior that has a hysteresis effect, making it difficult to control. In addition, the deposition rate for compounds is usually much lower than the deposition rate for pure metal (sometimes as much as 10-20 times lower). Finally, deposition of insulating thin films of compounds from metal targets involves recharging and subsequent arcing at the target.

The only way to avoid hysteresis in the reactive sputtering process to date has been to increase the external pumping rate of reactive gas. This can sometimes be done for small systems but leads to unrealistically high pumping speeds in large industrial systems.

In an ideal controllable process, reactive sputtering would be performed from a target of pure metal and the metal atoms would react with the gas as they arrive at the substrate. Then it would be possible to change the content of various compounds in the deposited film through the flow of reactive gas. Unfortunately, it is not possible to achieve these ideal conditions because the reactive gas also reacts with the target, resulting in the formation of compounds at the target. Previously, experiments have been carried out to introduce a pressure gradient into the process chamber, thereby reducing the formation of compounds at the target. Such a gradient is not easy to create and the impact on the behavior of the process has so far been rather small.

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In order to solve the problem of charging and sparking when depositing insulating films, it is possible to use specially developed power supply which adds extra positive pulses in order to neutralize the target. Another way to partially solve this problem is to increase the total gas pressure such that a substantial portion of the sputtered metal from the target's high erosion parts sprays back onto the target's low erosion portions, thus making these parts more metallic, which suppresses recharging and subsequent sparking.

The concept of using a magnet moving relative to the target to induce a moving erosion area is well known and described in several patents, see, for example, US Patent US 6,183,614, WO 01/23634 and WO 92/02659. In US 6,183,614, asymmetric magnetic fields are introduced to achieve favorable high density plasma sputtering. WO 01/23634 uses a plurality of magnets to induce a magnetic field of a predetermined arbitrary shape and thereby improve material utilization. An alternative targe tip position is shown in WO 92/02659, which uses a cylindrical magnet for the purpose of reducing sparking.

Problems associated with sputtering processes according to the state of the art remain and there is a strong need for improved sputtering procedures.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a hysteresis-free reactive sputtering process.

This object is achieved in accordance with the appended claims.

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The invention is a method of reactive sputtering in which the hysteresis behavior is eliminated by reducing the area from which sputtering takes place. To compensate for the increased local heating, the reduced area from which sputtering takes place along the target. It is crucial that the reduced area is small enough to obtain sufficiently high current density. It is very important that the reduced area moves at a rate high enough to avoid local melting of the target. In addition, the velocity of the reduced area should be small enough to ensure that the vast majority of the sputtered substances are metal atoms.

The method of the invention has several advantages over conventional sputtering techniques. Due to the high current density, the proportion of ionized sputtered substances will increase dramatically, which is desirable for the above reasons. Another great advantage of a reactive sputtering process according to the invention is that the above-described problems associated with this process, in particular the problem of hysteresis, disappear. Furthermore, the problem of spark formation during reactive sputtering disappears, since the insulating compound material at the target is effectively removed by sputtering when the current density is high enough. Finally, the increased ionization rate allows sputtering at lower pressures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, and further objects and advantages thereof, are best understood by reference to the following description, together with the accompanying drawings, in which:

Fig. 1 illustrates the erosion groove on a typical prior art DC magnetic target;

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<td>Fig. 2</td><td>is an exemplary illustration of a reduced area in constant motion according to the invention;</td>

<td>Fig. 3</td><td>shows reactive gas pressure, sputtering rate, and target and substrate composition against reactive gas flow in a reactive sputtering system using a conventional standard size target;</td>

<td>Fig. 4</td><td>shows reactive gas pressure, sputtering rate and target and substrate composition against reactive gas flow in a reactive sputtering system with a small size erosion area in accordance with the invention;</td>

<td>Fig. 5</td><td>illustrates the situation at the target surface in a reactive sputtering process as a reduced area of ​​erosion moves along it at a velocity v, the thickness of the compound layer on the target surface being shown in a graph as a function of the position on the surface;</td>

<td>Fig. 6</td><td>illustrates the situation at the target surface in a reactive sputtering process as a reduced area of ​​erosion moves along it at a velocity 3v, the thickness of the compound layer on the target surface being shown in a graph as a function of the position on the surface; and</td>

<td>Figure 7</td><td>is a flow chart of a process for reactive sputtering in accordance with the invention.</td>

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DETAILED DESCRIPTION

Fig. 1 illustrates the erosion track on a typical prior art DC magnetic target. Sputtered material is removed from a target (cathode) 10, whereby an erosion groove 12 is formed at the surface of the target 10. Conventional DC magnet targets 10 generally have annular erosion areas 12 that extend over a substantial portion of the target.

The invention is based on a method of increasing the ionic current density of the target by reducing the area from which sputtering takes place. Fig. 2 is an exemplary illustration of a reduced erosion area in constant motion according to the invention. In order to avoid local heating of the target 10, a reduced erosion area 14 should be constantly in motion, as indicated in Fig. 2 with an arrow. Other embodiments of the invention may include other forms of reduced erosion area 14, which need not necessarily be circular. However, it is crucial that the reduced erosion area 14 is in constant motion to avoid local melting of the target 10. This ensures that the power is spread so that the average heating over the area of ​​the target 10 is substantially the same as for the large erosion area 12.

There are several ways to achieve the small erosion area 14 in motion according to the invention. One approach is to place a small magnet moving behind the target 10. Another approach is to arrange a screen with a small aperture near the surface of the target 10 and allow the screen to move. The screen effectively prevents plasma discharge except where the opening is located. Another way to obtain the desired small erosion area 14 in motion may be to divide the target 10 into several parts which are electrically insulated from each other. By distributing power to one part at a time, it is possible to control the location of the plasma discharge and thereby the area from

521 095 which sputtering takes place in a similar manner as in previous approaches. Other possible means of obtaining a small erosion area 14 in motion along the target 10 are also within the scope of the invention.

There are two crucial parameters to control in the process of the invention; the size of the reduced erosion area 14 and the rate at which the area 14 moves along the target 10.

In order to obtain a high ionization rate, ie a high proportion of ions among the sputtered substances, the current density should be as high as possible. Accordingly, the erosion area should be as small as possible and the requirements of the erosion area 14 speed are governed by the maximum local heating of the target 10. Thus, the speed of erosion area 14 must be higher than a certain minimum rate.

Since the main purpose is to obtain a hysteresis-free reactive sputtering process, other effects must also be considered. Fig. 3 shows results from simulations of a system of reactive sputtering with target and erosion area of ​​normal size, while Fig. 4 shows simulations of a system with a significantly smaller erosion area. The erosion area that results in Fig. 4 is approximately 30 times smaller than that leading to Fig. 3. Graphs of partial pressure of reactive gas, sputtering rate and proportion of compound are shown. It is clear that the hysteric behavior disappears as the erosion area is reduced substantially. In fact, it can be shown that hysteresis behavior can always be eliminated by a sufficiently small erosion area. It should be noted that the hysteresis behavior is not eliminated by increasing the current density by increasing the total current while maintaining a large area of ​​erosion. From this point of view, it is therefore advantageous to have as little erosion area as possible. However, because of cooling problems, it is not practically possible to have an arbitrarily small erosion area. However, under certain circumstances, it is equivalent to using the erosion area 14 i

521 095 movement as described above, instead of using the conventional rate 10 with a static erosion groove 12 of the same size. But for the above reasons, a moving erosion area can be made much smaller than a static area. This small area of ​​erosion has the advantage that a high current density is achieved, which as seen in Fig. 4 in turn gives a hysteresis-free process.

Fig. 5 illustrates the situation at the target surface in a process of reactive sputtering as the erosion area moves along it at a velocity v. It is seen that a certain amount of compound has formed in the area in front of the erosion area. An arbitrary point on the target surface will have this amount of compound before it is bombarded with ions for a certain time. This time equals the rate of erosion area multiplied by its length. As the point enters the erosion area 14, the compound is gradually sputtered away until a new composition balance with a significantly lower amount of compound is achieved. When it is no longer bombarded with ions, the target surface gradually becomes richer in compound again as the reactive gas forms a compound with the target metal.

When sputtering is performed with an erosion area 14 in motion, an important criterion is to meet that the compound-rich part of the target 10 entering the erosion area 14 is sputtered for a sufficient period of time to achieve the new, more metal-rich composition balance. In fact, it is crucial that the new composition balance be reached right early during the sputtering pulse. In other words, the rate of erosion area 14 must be sufficiently low to ensure that the target surface is sputtered for a relatively short time in compound mode, denoted C in Fig. 5, and most of the time in metal mode, denoted M in Fig. 5. This criterion introduces restrictions on erosion area 14 maximum speed. A diagram in Fig. 5 shows the composition at the target surface as a function of the position on the surface of the target 10 as the erosion area moves at the rate v.

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Fig. 6 illustrates the situation on the target surface in a reactive sputtering process when the erosion area scans it at a rate of 3v, whereby the thickness of the compound layer on the target surface is shown in a diagram as a function of the position on the surface. The same designations as in Fig. 5 are used. By comparing Figures 5 and 6, it is seen that when the erosion area 14 moves with the velocity v, the greater part of the erosion area is in metal mode, whereas it is most composed of compound when the velocity is 3v. In this case, the maximum velocity of the erosion arc 14 should be somewhere between v and 3v for a preferred reactive sputtering process.

If the rate of erosion area 14 exceeds the minimum rate required to avoid melting and falls below the maximum rate required to maintain most of the erosion area 14 in metal mode, the total reactive sputtering process will behave similarly to a target 10 process which has a static erosion track with the reduced size. Since simulations of small size static erosion traces clearly show that the hysteresis effect disappears, the result is that a sufficiently small erosion area 14 in motion will also yield a hysteresis-free process, provided that the velocity is between the given limits. A hysteresis-free process for reactive sputtering is highly desirable as it is easy to control. In addition, the problem of spark formation in reactive sputtering will disappear as the insulating compound material at the target is effectively removed by sputtering when the current density is high enough.

Fig. 7 is a flow chart of a method of reactive sputtering in accordance with the invention. The sputtering process begins in step SI. When a reactive sputtering process without hysteresis is sought, the target is arranged so that the ionic current is focused on the reduced area of ​​erosion or stain in a step S2. The area of ​​the erosion stain is small enough to provide the required current density for one

521 095 hysteresis-free reactive sputtering process. The position of the erosion leak must be possible to control. Movement of the stain is caused in a step S3. The rate at which the erosion stain is moved is higher than the minimum rate required to avoid melting of target material and lower than the maximum rate required for most of the erosion stain to remain in metal mode. The sputtering process is completed in step S4.

Those skilled in the art will recognize that various modifications and changes to the invention may be made without departing from the scope and spirit thereof, as defined by the appended claims.

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