DE19943741A1 - Plasma assisted chemical vapor deposition of silicon nitride, from silane, ammonia, and inert carrier gas, employs optimized gas flowrates - Google Patents

Abstract

An overall flowrate of 50-400 sccm of the reactive gases, is combined with a carrier gas flowrate of 25-200 sccm. An Independent claim is included for corresponding equipment.

Description

The invention relates to a method for ICP-CVD coating device according to the preamble of claim 1 and a Vorrich device for carrying out the method according to claim 20.

The application of plasma coating processes is one Key technology in surface modification, structure environment from micrometer to nanometer range and in the production of thin functional layers.  

There are z. B. silicon nitride applied to a wide variety of substrates. In the Microelectronics and microsystem technology are becoming such Coating process used for over 20 years. It so far, however, has not succeeded with simply controllable Technology good quality layers at process temperatures to produce significantly below 300 ° C.

The reduction of the thermal load by the Be Layering process is of great importance, however thermally sensitive components such as B. thin-film dis plays or solar cells coated on polymer substrates should be. Even with organic light transmitters, Mi microsystems or in nanotechnology, thermal Stress lead to problems. So especially in complex structures to be coated at high Temperatures undesirable gradients occur.

A PECVD technology (PECVD: Plasma enhanced chemical vapor deposition) is known, in which the RF energy for the plasma is capacitively coupled in in a parallel plate reactor. Much of the energy is converted in the plasma layers. Such plasmas are characterized by a relatively low clay density (approx. 10 ⁹ to 10 ¹⁰ ), a small ion-to-neutral particle ratio (approx. 10 ^-4 to 10 ^-3 ) and a relatively high plasma potential (approx. 50-1000 V) featured.

It is not possible to determine the ion density of the plasma Increase the RF power arbitrarily, since with awake transmitter power increasingly only the ion energy in the  Boundary layer of the plasma is increased. This can result in a Damage to the substrates to be coated. Is too this is accompanied by unwanted sputtering.

As a result, there are limits to the layer deposition Lich the pressure in the reactor room and the power of the HF source le set. So far z. B. good silicon nitride layer only achieved at temperatures above 200-240 ° C become. The deposition rate of approximately stoichiometric Silicon nitride remained below 30 nm / min. At indicated a deviation from these process conditions the layers have too much hydrogen and too many Voids.

You want usable layers at lower temperatures received, so the reactants had to be the actual ones Lead coating, very strongly with noble gases (e.g. helium or argon). This leads to an inefficiency procedure.

With a known inductive plasma source (ICP: inductively coupled plasma) higher ion densities can be achieved (more than 10 ¹¹ cm ^-3 ). The ion content is in the range of 10 ^-2 . The plasma potential can be less than 20 V. But even with previously known plasma sources of this type, it is not readily possible to efficiently produce high-quality coatings at low substrate temperatures; Corresponding combinations of operating parameters are hitherto unknown.

Basically, the efficiency and quality of a plasma gene layering process by a variety of parameters determines, in particular the volume flows of the carrier gases, the total gas flow, the ammonia content in the total gas flow, the process pressure, the RF power of the plasma source and the Substrate temperature.

With the lowering of the substrate temperature, so far known processes below 200 ° C considerable negative Changes in layer properties.

The present invention is based on the object To create procedures in which the operating parameters so are chosen that an efficient coating of high quality through a suitable ICP source at low temperatures is possible, so that the negative effects of a Low temperatures can be avoided. It is also one Object of the present invention, a corresponding Creating device.

This object is achieved by a method with solved the features of claim 1. By a current rate of the total gas flow between 50 sccm and 400 sccm and one Carrier gas flow rate between 25 sccm and 200 sccm can good silicon nitride layers at low temperatures ten are generated. In particular, a current rate of Total gas flow between 250 sccm and 350 sccm and also a flow rate of the carrier gas between 130 sccm to 150 sccm is advantageous. One is particularly advantageous Current gas flow rate of 298 sccm and one current  carrier gas rate of 139 sccm. At these flow rates is a particularly efficient, high quality Low temperature coating possible.

Nitrogen and. Are advantageously used as inert carrier gases Argon used.

In an advantageous embodiment of the method, a first reactant stream has 1 to 100 vol% SiH ₄ , the remaining portion being taken up by helium. The first reactant stream particularly advantageously has a proportion of 3 to 30 vol% SiH ₄ , in particular 5 vol% helium.

It is advantageous that the first reactant stream has a current rate between 100 sccm and 200 sccm. It is particularly advantageous if the first reactant current has a current rate of 146 sccm. At these operating conditions can also be found at low Temperatures a perfect coating with silicon ni ensure trid.

In a further advantageous embodiment of the method according to the invention, a second reactant stream has NH ₃ , the flow rate being between 5 sccm and 30 sccm. It is particularly advantageous if the current rate is 13 sccm to 15 sccm.

The process pressure is advantageously between 0.1 Pa and 60 Pa, particularly advantageous 15 Pa.  

The power range of an HF plasma is advantageously ma source between 350 W and 2500 W, particularly advantageous are 800 W.

In an advantageous embodiment of the invention The method is the temperature of the substrate holder between between 50 ° C and 200 ° C, particularly advantageously between 50 ° C and 100 ° C. To avoid negative it is particularly thermal effects on the substrate advantageous, a temperature of the substrate holder of 50 ° C. to use. Due to the low temperatures thermal stresses in the substrate avoided. Let also three-dimensional structures coat better because the Temperature gradients in the three-dimensional structure stay low.

An inventive device for performing the The method according to the invention has a conductivity agent with which the substrate is thermally effective on one temperature-adjustable substrate holder can be coupled. This is supposed to be avoided that between substrate and Substrate holders result in large thermal differences.

An advantageous embodiment of the invention The device has an inductively coupled plasma source with an antenna that has at least three arms.

An embodiment of the invention is particularly advantageous appropriate device, in the arms of the antenna planar are arranged, because it is a particularly compact design is possible.  

Furthermore, it is advantageous if the invention Device an HF generator for setting a pre has voltage of the substrate.

The invention is described below with reference to the Figures explained in more examples. Show it:

FIG. 1a is a representation of the influence of the total gas flow to the refractive index at a wavelength of 320 nm;

Figure 1b is a representation of the influence of the total gas flow to the deposition rate of the silicon nitride layer.

FIG. 1c is a representation of the influence of the total gas stream to the absorbance at a wavelength of 320 nm;

Figure 1d is a representation of the influence of the total gas flow to the refractive index at a wavelength of 633 nm.

FIG. 2a-d is an illustration of the influence of a variation of the NH ₃ to -Flusses layer parameters analogous to Figure 1a-d.

Fig. 3a-d is an illustration of the influence of temperature on Substrattem layer parameters analogous to Figure 1a-d.

Fig. 4a-d the influence of the RF power of the plasma source to layer parameters analogous to Fig. 1a-1d.

FIGS. 5a-d is a representation of the influence of a variation of the pressure on layer parameters analogous to Figure 1a-d.

Fig. 6 is schematic illustration of a device according to the invention;

Fig. 7 shows a schematic representation of an antenna for use in the device according to the invention.

In the following, examples are used to show which How the inventive method are used can.

The investigations were carried out with a SI 500D PECVD reactor carried out by SENTECH Instruments GmbH, Berlin. This Reactor has an ICP source. With inductive excitation the plasma only has a low DC bias of the substrate. This bias is for the energy of the ions impinging on the substrate, so that the layer properties are controlled via this pretension can be. To set the preload, a second RF generator with appropriate frequency and Phase difference used. So that the ion energy largely independent of the ion density become.  

The reactor has a vacuum pump system, a programmable computer control for the operating parameters and a data acquisition system.

A number of examples of the process are given below. The following gas flows are essentially used for a coating in the reactor:

• - First reactant stream (5 vol% SiH ₄ in helium)
• - Second reactant current (NH ₃ )
• - inert carrier gas flow (nitrogen or argon)

The supply of the first reactant stream in the direction of the Substrate is made via a gas ring that is 4 cm above the Substrate is arranged. The gas inlet for a carrier gas and the second reactant stream occurs in the ICP source. In an alternative embodiment, however, it is also possible the gas flows on the periphery of the reactor to let in.

The maximum total gas flow of the examples shown here was about 300 sccm. In the reactor there are minimal Pressures of 2 Pa-3 Pa reached.

The backflow of the first reactant stream from the gas Keep the ring in the ICP source as small as possible Flow rate of the gases fed into the source at least set equal to the flow rate of the first reactant stream. Basically, the current rates should be aimed at of the gas flows fed into the source are considerably larger to choose as the first reactant stream.  

Under the conditions described here, maxi results times a flow of 150 sccm for the first reactant flow and 150 sccm for the second reactant stream and Carrier gas flow.

For a layer consisting of approximately stoichiometric SiN, a reactant ratio of

NH ₃ : SiH ₄ = 3: 1. , , 2: 1

needed. Thus, at 5 vol% SiH ₄ in helium (first reactant stream) there is a requirement of approx.7.5 sccm SiH ₄ and 15-23 sccm NH ₃ .

The following current rates are therefore used as the starting operating point:
SiH ₄ in 5 vol% helium: 146 sccm
NH ₃ : 13 sccm
Carrier gas (Ar or N ₂ ): 139 sccm

The operating pressure of the starting working point is 15 Pa The power of the HF source is chosen to be 800 W. Unless otherwise stated, these values are shown below based measurements.

The influence of the carrier gas (here argon or nitrogen) and the variation of the total flow described the coating result.  

In Fig. 1a-1d, the influence of the flow rate of the Ge is samtgasstroms displayed on various parameters of the coating. The layer parameters were measured with an ellipsometer.

In each representation of FIG. 1, the test results for nitrogen as a carrier gas (circles) and argon as a carrier gas (filled circles) are shown.

The total gas flow was varied between 50 sccm and 300 sccm while maintaining the above-mentioned reactant ratios. Fig. 1a shows the influence on the refractive index (n ₃₂₀ ) for a wavelength of 320 nm. With increasing total gas flow, there is a slow increase in this refractive index, which could be related to the fact that with increasing flow rates reaction products are removed and hydrogen, which by the reaction with ammonia occurs, is displaced. In Fig. 1d is a very similar curve for the measurement of the Brechungsin dexes (n ₆₃₃₎ shows nm for a wavelength of 633. The refractive indices obtained using nitrogen as a carrier gas are significantly smaller (n = 1.54 to 1.75 ) than when using argon as the carrier gas (n = 1.9-2.05).

It is known that, in contrast to argon, nitrogen can form ammonia-containing radicals in the source, which leads to an increased incorporation of NH groups into the layer. The nitrogen may also be able to form and release NO groups with an existing moisture layer on the substrate. The low plasma potential also means that argon has no "sputtering effect". For this reason, argon is used as the carrier gas in the further investigations ( FIGS. 2 to 4).

Fig. 1b shows a steady increase in the separation rate (r _w ) with increasing total gas flow, this being true for both carrier gases. Obviously, the separation from the offered number of particles per unit of time is determined at the working point selected here. When using nitrogen as the carrier gas, the separation rate was higher than with argon.

Fig. 1c shows the influence on the absorbance value (k ₃₂₀₎ for a wavelength of 320 nm, which in any case is less than 10 ^-2. From this it can be concluded that hardly excess silicon has been installed in the layers. The extinction values are lower with nitrogen as the carrier gas than with the use of argon as the carrier gas.

In Fig. 2a-2d of the influence of the variation of the ammonia addition to the same layer parameters as in Fig. 1a-1d is shown. The ratio of the flow rates of SiH ₄ to NH _{3 was} varied. The stoichiometry of the nitride layer is controlled above all by varying the Ammnoni ak addition.

FIGS. 2a and 2c show again the refractive indices for the wavelengths 320 nm (Fig. 2a) and 633 nm (Fig. 2c), wherein the measuring ranges with respect to Figs. 1a and 1c have been enlarged. The silicon nitride is shown from a nitrogen-rich area to the installation of excess silicon.

FIGS. 2a and 2c show a change in the refractive indices of 1.9 to <2.3. The refractive index can be set from 1.9 to over 2 by varying the ammonia flow.

As can be seen from Fig. 2b, the growth rate (separation rate r _w ) falls significantly for a lower ammonia flow, ie with an increasing SiH ₄ : NH ₃ ratio. For larger ammonia flows, the separation rate changes to saturation.

From Fig. 2c, which shows the course of the absorbance value (k ₃₂₀ ) for 320 nm, and Fig. 2d, which shows the refractive index (n ₆₃₃ ) for 633 nm, an optimal ammonia flow rate of 14-15 sccm would result. The ratio of SiH ₄ : NH ₃ should be chosen a little less than 1: 2.

The extinction values of greater than 10 ^-2 and the refractive indices of greater than 2 show that excess silicon is being installed in these areas.

In Fig. 3a-3d, the influence of Subtrattemperatur on the layer parameters as in Figures 1 and 2. FIG. Varying the temperature is intended to provide information about the extent to which good silicon nitride coating results can be achieved. The temperature was varied from 50 ° to 320 ° C. In a parallel plate reactor with falling temperature, an extremely large amount of hydrogen is usually incorporated into the layer via NH groups. Due to the lower density, this leads to refractive indices that are less than 1.9. Also due to the high etching rates, these low temperatures are not practical for good coating results in conventional processes.

With the method according to the invention with the operating parameters used here, a good coating result can also be achieved at lower temperatures. Thus, Fig. 3b is a relatively constant deposition rate throughout the entire temperature range. The refractive indices (different Y-axis scaling than in FIGS. 1 and 2) and the extinction values are relatively constant over the temperature range. These are signs that good silicon nitride coating results can be achieved even at low temperatures in the range between 50 ° C and 100 ° C, in particular also at 50 ° C. With this low temperature nitride process, lacquer structures of approx. 5 µm could be coated.

Check-ups revealed that the RF power of the ICP source only increases the Substrate temperature by about 30 W.

In Fig. 4a-4d shows how change in FIGS. 1 to 3 already described layer parameters under the influence of the RF power. The RF power was varied in the range from 300 W to 1000 W.

As FIG. 4b shows, the deposition rate (r _w ) increases somewhat less than the square root of the RF power. The growth rate of the layer thickness is therefore approximately proportional to the current. It also shows that the coating process runs incompletely with decreasing output. Based on the courses of the refractive indices in FIGS. 4a and 4d, each for the wavelengths 320 nm and 633 nm, and the extinction value k _{320, it} can be seen that excess silicon is incorporated into the layers at lower powers. Throttling the river or lowering the operating pressure would solve this problem.

In FIGS. 5a-5d is illustrated as change in FIGS. 1 to 4 previously described layer parameters under the influence of pressure. The pressure was varied in the range from 2 to 40 Pa.

The refractive indices for the wavelengths of 320 nm and 633 nm show, in accordance with FIG. 5a and FIG. 5d, a respectively increasing course. The drop in the separator rate in FIG. 5b with increasing pressure is significant. According to FIG. 5c, there is an increasing course of the extinction value with increasing pressure.

In summary it can be said that at temperatures below 150 ° C a silicon layer can be achieved with the following properties:
Refractive index for wavelength 633 nm: 1.9-2.05
Absorbance value: <10 ^-2
Deposition rate: 30 nm / min-55 nm / min
Stress (tensile): 0.3.10 ⁹ dynes / cm ³ - 1.10 ⁹ dynes / cm ³
Maximum layer thickness: <10 µm

The following etching rates were available:
KOH: 30% KOH, 70% H ₂ O at a temperature of 85 ° C: etching rate <0.07 nm / min
BHF: 6.5% HF, 35% NH ₄ F, 58.5% H ₂ O at a temperature of 25 ° C: etching rate <4 nm / min

It is also possible to apply an isotropic coating to three-dimensional structures. The SiH ₄ conversion on the substrate is also very high at around 30%, so that only a relatively small amount of gas has to be disposed of.

The reactor chamber according to the invention has a reactor chamber known per se from plasma coating reactors and an inductively coupled plasma source. The most important components are shown in Fig. 6. The reference symbols have the following meaning: 10 reactor space
12 vacuum flange
20 substrate holders
22 substrate holder surface
24 substrate holder heating
25 electrical connections
26 cooling reservoir
27 Inlet / outlet coolant for substrate holder
30 inductive plasma source
32 antenna
33 counter electrode
34 dielectric coupling window
36 first gas connection
37 gas inlets
38 matching network
39 Source coverage
40 gas inlet module
42 gas ring shower
43 second gas connection
44 entry openings
50 RF generator of the source
60 Vacuum system with regulated pumping speed

An antenna with three planar arms (see FIG. 7) and a device for impedance matching 38 serve as antenna 32 .

The substrate holder 20 can be temperature-controlled and has good thermal coupling for the substrate to be coated, which is not shown here.

The gas supply system has at least one gas ring shower 42 , via which reactants and carrier gases can optionally be fed. The distance from the substrate is set so that optimal dissociation and ionization is ensured for the layer growth. Z. B. 4 cm distance between the gas ring shower 42 and the substrate.

The gases are sucked out of the reactor chamber 10 in a controlled manner. The device according to the invention also has a loading device, not shown here, for the substrate holder under vacuum.

FIG. 7 shows a top view of the antenna 32 , which is arranged in a planar manner above the reactor space 10 . Due to the arrangement and shape of the arms of the antenna 32 , a particularly effective radiation into the reactor space 10 can be achieved. The three arms extend symmetrically radially from the center at which the RF feed, not shown here, is located. From a certain length, the arms have a curvature so that the arms are shaped so that they lie on a circumferential line around the center. This results in an easy-to-manufacture antenna with an efficient radiation pattern.

In an alternative embodiment, the arms are arranged rectangularly around the center.

The invention is not limited in its execution the preferred embodiments given above le. Rather, a number of variants are conceivable, which of the inventive method and the inventive Device even with a fundamentally different design use.

Claims (23)

1. A method for chemical vapor deposition with an inductively coupled plasma source for the deposition of silicon nitride layers on a substrate, which is arranged on a substrate holder, with a total gas stream which as reactant streams contains at least one SiH ₄ stream and one NH ₃ stream and one inert stream Carrier gas portion, characterized by a flow rate of the total gas flow between 50 sccm and 400 sccm and a flow rate of the carrier gas between 25 sccm and 200 sccm. 2. The method according to claim 1, characterized by a flow rate of the total gas flow between 250 sccm and 350 sccm. 3. The method according to claim 1 or 2, characterized by a flow rate of the carrier gas between 130 sccm to 150 sccm.   4. Method according to at least one of the preceding Claims, characterized by a current rate of Total gas flow of 298 sccm and a flow rate of the carrier gas of 139 sccm. 5. Method according to at least one of the preceding Claims, characterized in that as a carrier gas argon or nitrogen is used. 6. The method according to at least one of the preceding claims, characterized in that a first reactant stream has a proportion of 1 to 100 vol% SiH ₄ , the remaining portion being taken up by helium. 7. The method according to at least one of the preceding claims, characterized in that the first reactant stream has a proportion of 3 to 30 vol% SiH ₄ in helium. 8. The method according to at least one of the preceding Claims, characterized in that the first Reactant flow a share of 5 vol% helium having.   9. The method according to at least one of the preceding Claims, characterized in that the first Re a current rate between 100 sccm and 200 sccm. 10. The method according to at least one of the preceding Claims, characterized in that the first Reactant current has a current rate of 146 sccm has. 11. The method according to at least one of the preceding claims, characterized in that a second reactant stream consists of NH ₃ , the second reactant stream having a flow rate between 5 sccm and 30 sccm. 12. The method according to at least one of the preceding Claims, characterized in that the second Reactant current a flow rate of 13 sccm to 15 sccm. 13. Procedure according to at least one previous one Claims, characterized in that the procedure pressure is between 0.1 Pa and 60 Pa.   14. The method according to at least one of the preceding Claims, characterized in that the procedure pressure is 15 Pa. 15. The method according to at least one of the preceding Claims, characterized in that the lei Power range of an RF plasma source between 350 W. and 2500 W. 16. The method according to at least one of the preceding Claims, characterized in that the performance the HF plasma source is 800 W. 17. The method according to at least one of the preceding Claims, characterized in that the tempera ture of the substrate holder between 20 ° C and 200 ° C lies. 18. The method according to at least one of the preceding Claims, characterized in that the tempera ture of the substrate holder between 20 ° C and 100 ° C lies. 19. Method according to at least one of the preceding Claims, characterized in that the tempera tur of the substrate holder is 50 ° C.   20. Device for carrying out a method according to at least one of the preceding claims, characterized by a conductivity means for thermal coupling of the substrate on a substrate holder ( 20 ). 21. The apparatus according to claim 20, characterized by using an inductive plasma source with an antenna at least three poor. 22. The apparatus according to claim 21, characterized net that the arms of the antenna essentially are arranged planar. 23. The device according to at least one of claims 20 to 22, characterized by an HF generator for Setting a bias of the substrate.