The invention refers to a sputter deposition system according to claim 1 and to a sputter deposition process according to claim 10.

Technical Background

Control of stress uniformity during thin film deposition on wafers is an important issue with the semiconductor industry since long. This applies in particular to the deposition of piezoelectric thin films on substrates suitable for the realization of micromechanical systems (MEMS) or devices comprising such systems, where as an example Ali- _x Sc _x N films on conductive and/or semiconductive substrates are frequently used.

When a metallic target is used to produce an insulating film, a compound forming reaction due to the presence of reactive gas in the sputter atmosphere may take place in the plasma space, on the surface of the substrate to be coated and always on the target surface exposed to the sputter plasma, which thereby creates an insulating layer on the target side. In order to prevent the extinction of the plasma and to avoid electric arcing due to charge build-up phenomena with state of the art processes, a pulsed DC-supply could be connected to the target to dissipate accumulated electrical charges and allow further sputtering of the target surface. Such processes can be combined with a RF-bias supply to avoid charge build up on the substrate surface or parts of the chuck.

Alternatively a RF-supply could be used to sputter the target whereas the substrate to be coated is set on a floating potential, which means the substrate is electri cally isolated against any other potential but the plasma potential in front of the target.

Respective processes can be also used, when low conductive or isolating target bulk material has to be sputtered for deposition on a substrate.

So far good stress uniformities can be achieved in the production of Ali- _x Sc _x N thin films by a suitable selection of the gas flows, of the target-to-substrate distance, and of the magnetic field setup to shape the magnetic field at the target side. All these process parameters, however, do have a strong influence on the thickness distribution as well and combinations of the aforementioned parameters which result in improved stress uniformity usually impact negatively the thickness uniformity of deposited coatings.

Therefore, there is a need to provide a sputter system and a sputter process to diminish the mutual dependency of coating parameters and thereby tune as mentioned layer features separately without influencing the other(s). At the same time inventive systems and processes should have a potential to optimize further coating parameters like surface roughness, or uniformity of other e.g. electric parameters, like coating resistance and the like. Definitions: (Ali- _x Sc _x )N layers refer to any (Ali- _x Sc _x )N layers with 0 < x < 1, especially from 0 to 45 atomic/ of Scandium (0 < x < 0.45). Such layers can be deposited by use of alloyed targets of similar metal composition Ali- _x Sc _x whereat "x" or the relative Sc/Al ratio may vary slightly within some few percent from the target to the coating as deposited up to the actual process deposition.

Summary of the Invention

Surprisingly it has been found that when sputtering is performed with the help of a DC and an RF target supply, or a respective combined DC/RF target supply, and at the same time an RF bias supply is operated with the substrate pedestal, whereat both supplies produce and/or affect a cold plasma between the target and the substrate, the usage of a controlled phase shift between the RF or DC/RF target supply and the bias target supply can be used to control stress distribution on the substrate surface at very stable and reproducible processing conditions. At the same time such a phase shift control does not or at least nearly not influence other distribution sensible parameters like coating thickness, which can be optimized separately, e.g. by control of target to substrate surface distance, gas flow and/or target power separately.

Such processes can be performed with an inventive sputter system comprising: - a sputter source comprising a sputter cathode adapted for a sputter target to be mounted at, the cathode being electrically connected to a first RF-supply providing a first RF-signal of frequency fito the cathode which is further connected to a direct current(DC) supply via an RF-filter to provide a DC- signal to the cathode together with the RF-signal; the RF-filter protecting the DC-supply against RF voltage from the first RF-supply;

- a pedestal adapted to support a substrate to be processed connected to a second RF-supply, to provide a second RF-signal of frequency f2to the pedestal to form a bias electrode;

- a phase-shifter connected between the first and the second RF-supply.

Such frequencies can be operated in a range of 2 MHz < fi,2 < 100 MHz; e.g. from 5 to 25 MHz.

The frequency relation can be fi = n*f2, or fi = f2/n, where n is an integer > 0 and n can be 1.

In an embodiment of the invention the target or the magnet system can be mounted rotatably at the sputter cathode. Rotatable targets providing the additional benefit to allow the use of static asymmetric magnetic fields for special of axis sputtering configurations. In a further embodiment of the invention the pedestal can be provided with an electrostatic chuck (ESC) or a clamp chuck to ensure a save mounting of the substrate and may be provided with heating and/or cooling means to heat and/or cool the substrate.

The pedestal can be further provided with a back-gas supply to enhance thermal transfer from the pedestal to a wafer mounted to it or vice-versa. A back-gas supply may comprise a gas supply for at least one inert gas, e.g. He and/or Ar and at least one gas inlet leading to the surface of the pedestal, e.g. in the ESC-surface. Alternatively, there may be several inlets or gas distribution ducts, e.g. leading from a center towards further outside chuck or ESC surface areas and having a flow area to transport back-gas with a low flow resistance. The ducts may be in part or even completely open to the backside of the wafer and being connected to shallow but wide gas channels, e.g. from 10 m to 100 pm or 50 pm depth, having a considerable higher flow resistance than the ducts and covering an essential area of the pedestal/ESC surface to provide an effective thermal transfer between the wafer and the pedestal/ESC surface via the back-gas. Alternatively, the wafer may be positioned on spacers in a close distance above the pedestals or the ESCs surface, e.g. according to the channel depth as mentioned, thereby forming another kind of channel between the wafer and the pedestal/ESC. With both variations the wafer may be further positioned on a surrounding projection, e.g. a gasket to allow a higher back-gas pressure. In a further embodiment the projection may be provided with small outlet openings to the process atmosphere or outlet ducts may be provided to lead the back gas directly to the pump socket.

The invention further refers to a sputter process to deposit isolating coatings on a substrate. The process comprising the following steps:

- positioning the substrate on a pedestal of a sputter deposition system and applying vacuum and a first gas- flow to the system; - providing a first RF-signal of frequency fl and a DC- signal to a sputter cathode of the system, the cathode comprising a target to be sputtered and the DC-signal being provided via an RF-filter;

- providing a second RF-signal of frequency f2to the pedestal to form a bias electrode;

- providing a phase-shifter between the first and the second RF-supply to produce a phase shift Ps _h = Pi - P ₂ of at least ±10°, or at least ±40° between the first and the second radiofrequency. The "±" depends on the respective signal chosen as reference. It would minus for the first signal as reference signal, e.g. 360° - 10° = 350° if the first signal starts at zero, and it would be plus, e.g. 10° for the second signal starting from zero as reference signal. Therefore, the term "±" will be omitted with other information on phase-shift.

A good control of the stress distribution could be obtained in the following range of the phase shift: 40° < Ps _h £

320°. Frequencies fi,2 of the power supplies may be set as follows: 2 MHz < fi,2 £ 100 MHz, e.g. from 5 to 25 MHz. The relation of the frequencies should be fi = n*f2, or fi = f2/n, where n is an integer > 0, and n can be one which refers to fi = f2, e.g. fi or f2 or both supplies deliver a frequency of 13.56 MHz.

The first gas flow, is the process gas flow may comprise at least one of an inert gas, which can be Ar, Ne, Xe or the like and/or at least one of a reactive gas, which can be NH3, N2, N3, O2, O3, or the like. As an example, the first gas flow may contain or consists of a nitrogen containing gas, e.g. nitrogen, ammonia gas or hydrazine, when an AIN or an AlScN layer should be deposited. This gas flow can be channeled to the system via a gas line having gas-outlets circularly arranged round the target or round an anode encompassing the target.

Additionally to the first gas flow, a second gas-flow can be applied to the system via a back-gas system comprising gas channels at least in part open to the backside of the wafer. The second gas flow may comprise or consist of at least one inert gas, e.g. He, Ne, Ar, and/or Xe. Thereby a back-gas pressure can be produced in the gas channels open to the backside of the wafer, which pressure should not exceed 15 mbar, or even not exceed 8 mbar.

Appropriate target material may comprise at least one of a metal, a semiconducting or an isolating material, e.g. a ceramic material. For depositing AIN or aluminum alloy nitride coatings the target may consist of aluminum, doped aluminum, or an aluminum-alloy, e.g. an aluminum-scandium (All-xScx) alloy.

The invention further refers to a use of a process as described before to control a stress-level and a stress- level distribution of a coating. Thereby a stress-level distribution of ± 50 MPa, or even ± 30 MPa can be adjusted to a coating on a substrate of 200 mm diameter. Respective tight distributions could be met for different (Ali- _x Sc _x) N comprising also AIN layers. Layer thickness d can be in the following range: 100 nm < d £ 1200 nm.

In the following the invention is further presented at the hand of tables, examples and figures.

As for the coatings deposited to show the inventive effect, any of the examples with targets of different Ali- _x Sc _x composition has been performed within the range of the process parameters as shown in table 1. Most experiments were performed with fl = f2 = 13.56 MHz. However, some experiments with fl = 2f2 = 27.12 respectively fl = f2/2 = 6.78 MHz showed similar results. The coating equipment used was a Clusterline 200-11, target diameter 304 mm, if not otherwise indicated. An ESC was used to fix a wafer substrate on the pedestal.

Table 1: Figures

The invention shall now be further exemplified with the help of figures and examples. The figures show: Fig.l: A scheme of the electric components of an inventive sputter system;

Fig.2: A scheme of an inventive sputter system Fig.3: Stress and thickness distribution with target/substrate distance; Fig.4: Stress and thickness distribution with nitrogen flow;

Fig.5: Stress and thickness distribution with phase shift; Fig.6: Stress distribution with sputter power; Fig.7: Stress distribution of a state-of-the-art coating; Fig.8: Stress distribution of an inventive coating; Fig.9: AFM surface topography comparative example; Fig.10 AFM surface topography of an inventive coating;

In Fig.l a scheme of the electric components of an inventive sputter system is displayed comprising a sputter cathode fed by a DC-supply and an RF-Supply. The DC-supply being connected via a protective RF-filter, the RF-supply being connected via a sputter-matchbox to match the impedance of the sputter-supply to the system impedance. On the side of the substrate to be coated, the chuck is connected to the RF-bias-supply via a respective bias- matchbox to match respective supply impedance with the system. Additionally, a phase-shifter is provided between the RF sputter supply and the RF bias supply. The phase- shifter can be a master power oscillator (MOP), a device which generates several RF signals synchronized with a master signal internally generated, e.g. at 13.56 Mhz. Such a phase-shifter may cover a frequency range from 1 to 100 MHz. The output signals can have a phase offset selectable by the user. Each of the output signals can then be used to drive the RF generators to output signals of defined phase shift with respect to the master signal and therewith to each other. Such phase shifted signals can be applied to the target and the pedestal or chuck. A commercially available so called "master oscillator" from TRUMPF© (HtiTTINGER Elektronik GmbH + Co. KG) has been used as MOP together with respective RF-generators of the same company for all examples of the current invention as described below.

Fig.2 shows an outline of an inventive sputter system comprising a two-part vacuum chamber 10, split into a sputter compartment 18 and a pump compartment 17, both in terms of gas flow connected by a flow labyrinth 26. In this this embodiment the system has an essentially cylindrical setup, round a system axis A. Target diameter rt and inner diameter ra of the anode 2 which forms the sidewalls of the sputter compartment 18 can be chosen according to the substrate 4, for instance according to a wafer size. For a

200 mm wafer a target diameter rt can be chosen from 250 to

400 mm and an inner diameter ra from 300 to 450 mm. The sidewalls 11 are designed as a cathode 2 of the magnetron sputter source 22, on the top of the sputter compartment 18, which comprises the target 1 and the magnet system 23.

Further on a target backplate 24 may be provided in case of expensive or mechanically week target material. Between the target 1 and the cathode 2 an isolated mounted however conductive target ring 3 is positioned on a ceramic isolator 21 in form of a ring or ceramic supports arranged in an upper circumference of the anode 2 around the target

3. The target ring 3 is on floating potential. The isolator 43 sits hidden against any line of sight towards the sputter compartment 17 within a channel structure 34 which together with the inlet gap 35, gives the inlet 13 for the process gas which can be a reactive gas, a mixture of different reactive gases with or without addition of a diluent inert gas. In the embodiment as shown in Fig.2 the inlet gap 35 is formed between the anode and the target ring 3. An alternative or additional inlet for process gas can be provided in form of a gas ring 33 near a lower circumference of the anode. Thereby process gas and inert gas supply can be split, which can as an example help to prevent target poisoning when process gas is supplied only via the remote gas ring 33, which can be positioned in the pump compartment 17 as shown, and only inert sputter gas is provided to the upper inlet 13.

In the bottom of the sputter compartment 18 a vertically movable RF-pedestal, see reference number 5 and vertical double arrow, is mounted comprising an electrostatic chuck 6 to fix the wafer 4 and the pedestal base 5'. Together with the pedestal 5 a ring-shield 7 and a dark space shield 8 can be moved up and down. Both ring-shield 7 and dark space shield 8 are electrically isolated against the RF- potential of the pedestal and/or in dark space distance to respective RF-supporting parts of the pedestal 5 and the pedestals base 5'. However, the dark space shield 8 is on ground potential, whereas the ring-shield 7 is on floating potential or provided with a separate voltage source to form a third electrode in the sputter compartment 18. Such a third electrode 7 surrounding the wafer circumference can be used in addition to other known measurements, like e.g. target power and substrate bias, to optimize stress and stress distribution within the layers of a piezoelectric active coating. Via respective feedthroughs 32, the pedestal is connected to RF-lines 43 and to fluid lines 14 for heating and cooling of pedestal 5 and ESC 6. An optical temperature measurement device 9, e.g. a pyrometer can be used to control the temperature at the backside of the wafer 4, which either needs an additional optical feedthrough or may use a back-gas inlet 28, e.g. in a central position of the pedestal, connecting the back-gas supply 29 with the back-gas channel 27. At the bottom or a sidewall 11 of the pumping compartment 17 a pump socket is provided to connect to the high vacuum pump system 16.

The magnetron sputter source 22, respectively the backplate 24 of the target 1 is electrically connected to a first RF- supply 36 and to a DC-supply 38 via a common target supply line 41 and respective DC-line 39 and first RF-line 37. An RF-filter 40 is provided between DC-supply 38 and common target supply line 41 to protect the DC-source 38 against RF-signals from the first RF-source 36. The common target supply line 41 may comprise a cooling pipe for simultaneously cooling of the backplate and the target. Alternatively, such cooling may be provided separately.

At the substrate/wafer side the pedestal 5 is connected to a second RF-supply 42 via respective second RF-lines 43. Both the first RF-supply 36 and the second RF-supply 42 are connected to phase-shifter 25 via control lines 30 and 31 to control a defined phase shift between RF-signals one RF1 and two RF2. The phase shifter, here a MOP as mentioned above, comprises a quartz oscillator 44 as a master oscillator, to produce and provide the reference signals via control lines 30 and 31 which then replace the signal from the internal quartz oscillator (not shown) of the RF- supplies 36, 42. To produce curves in Fig.3 to Fig.6 process parameters were used as shown with table 2, to coat a wafer with an (Alo. ₉₀₅ Sco. ₀₉₅₎ N single layer by sputtering of a 9.5at% Sc alloy target (x = 0.095) in a nitrogen atmosphere. Process parameters like target to substrate distance, nitrogen flow, Phase Shift between target and chuck, as well as sputter power have been varied to demonstrate respective process dependencies. Average thicknesses of the exemplary coatings were from 250 nm to 1000 nm. Table 2:

Curves taken with a target to substrate distance from 66 to 68 and 70 mm (66 / 68 / 70 mm) are displayed in Fig.3 and show a strong influence of this parameter to the local stress of a coated wafer, see Fig.3a, as well to the layer thickness of the respective coatings, see Fig.3b. Similar figures showing a strong influence of the nitrogen flow to both coating features can be seen with respective N2-flow variations (66 / 100 / 133 seem) in Fig.4a and Fig.4b. A variation of the phase shift (0 / 45 / 135°) between the sputter frequency fl and bias frequency f2 however shows a completely different picture. Whereas the influence on the local stress distribution is still strong, as shown in Fig.5a, nearly no influence of different phase shift modes can be seen with Fig.5b. Therefore, inventive processes varying the phase shift between target and bias radio frequencies can be used to shape local stress distribution without influence to the thickness distribution on the wafer. The latter can be furthermore optimized by respective variation of one or usually a combination of other deposition parameters like, substrate distance to target, target power, bias power, gas flow, total pressure, substrate temperature and the like.

Exemplarily Fig.6 shows the influence of the sputter power to the stress distribution on one side of a wafer only, as such distributions are usually symmetrically with circular targets and respective circular symmetrically or rotating magnetic arrangements.

Due to the complex, non-linear behavior of most the parameters as can be seen by the figures discussed, plasma simulations performed up front can help to find the respective process parameters.

Fig. 7 shows the stress distribution of a state of the art coating on a 200 mm wafer. The coating has been deposited according to parameters in table 2, without variations and without a phase shift. Stress-level varies between - 60 and +80 MPa, which make a variation of ± 70 MPa.

The same process applying an inventive phase shift mode of 90° gives a very different result as can be seen in Fig.8. The Stress-level varies between zero and - 60 MPa, or ± 30 MPa only. At the same time thickness non-uniformity could be kept below 0.5% average standard deviation.

Similar progress could be found with surface quality. State of the art coatings of pure AIN deposited under constant conditions as shown in table 3, still show some bright spikes higher than 10 nm in a 5 pm x 5 pm AFM surface topography scan, as can be seen with Fig. 9. Such spikes are produced by abnormal crystallites during AFM scans. Table 3 Contrary to that a 10 mpix 10 mpiAFM surface scan of an Alo.8Sco.2N coating deposited by an inventive process as shown in Fig.10 has a perfect smooth surface without any spikes, which is particularly noteworthy as Sc-alloyed coatings usually have a higher tendency to form abnormal crystallites than pure AIN coatings as shown with Fig.9. Such a surface free from abnormal crystallites ensures superior dielectric properties and ease of process integration into bulk acoustic wave BAW manufacturing. The deposition process has been performed according to the parameters as shown in table 2, without variation and applying a phase shift of 90 degrees. The deposition processes have been performed on a Clusterline 200 system, using 6 mm targets of an Alo.sScom alloy having a diameter of 304 mm.

Reference numbers Fig . 2

1 target, sputtered electrode

2 anode

3 floating target ring

4 substrate/wafer

5 RF pedestal

5' base of the pedestal

6 electrostatic chuck (ESC)

7 ring-shield

8 darkspace shield

9 measurment device

10 sputtering system

11 vacuum chamber

12 side wall(s)

13 inlet process gas

14 heating and/or cooling line

15 substrate handling opening

16 vacuum pump system

17 pump compartment

18 sputter compartment

20 bottom of the sputter compartment

21 isolator

22 magnetron sputter source

23 magnet-system

24 backplate

25 phase-shifter, MOP

26 flow labyrinth

27 back-gas channel inert heating/cooling gas

28 back-gas inlet

29 back-gas supply

30 control line target RF

31 control line bias RF

32 feedthrough

33 gas ring, inlet process gas

34 channel structure

35 inlet gap

36 first RF supply (target)

37 first RF-line (target)

38 DC-supply (target)

39 DC-line (target)

40 RF-filter

41 common target supply line

42 second RF-supply (bias)

43 second RF-line (bias)

44 reference signal generator