Method for plasma treatment of a carbon layer

Abstract

A method of treating a carbon layer depositited in or on a substrate, such as a silicon wafer is provided. At least one aliphatic organic compound chosen from the group of an aliphatic alkanes, aliphatic alkenes and aliphatic alkines is utilized as a precursor for plasma treatment of the carbon layer.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the plasma treatment of at least one carbon layer in or on a substrate for the production of semiconductor products. More particularly, the present invention relates to the treatment with a plasma generated with at least one precursor from the group of an aliphatic alkane, aliphatic alkene and aliphatic alkine.

2. Description of the Related Art

In the production of semiconductor products, especially dynamic random access memory (DRAM) devices, carbon layers are becoming more and more widely used as dielectric material.

Exemplary methods for the deposition of the carbon layers as such using chemical vapor deposition (CVD) methods are described in U.S. Pat. No. 6,423,384 B1 and WO 01/80309 A2.

Since the use of carbon layers in the fabrication of semiconductor products is relatively new, the methods for the treatment of such layers are not fully developed.

It is known that carbon layers can be etched using plasmas with Oxygen or Hydrogen as precursor components. As used herein, the term generally refers to a molecular compound (such as O₂, H₂) which is then used in atomic or ionized form in the plasma. Other precursor materials include, but are not limited to, CO, SO₂ or HBr/O₂.

SUMMARY OF THE INVENTION

The present invention generally provides a method of treating a carbon layer deposited in or on a substrate, such as a silicon wafer. In particular, at least one aliphatic organic compound chosen from the group of aliphatic alkanes, aliphatic alkenes and aliphatic alkines is utilized as a precursor for the plasma treatment of the carbon layer, not the deposition.

Those aliphatic compounds are a convenient source of Hydrogen atoms which can be used for the etching of the carbon layer. A passivation reaction would occur through the —CH₂— radicals.

In one aspect of the invention, the precursor for the plasma comprises one of the group of CH₄, C₂H₄ and C₂H₂. These compounds are the simplest aliphatic compounds, CH₄ having the highest Hydrogen to Carbon ratio.

In another aspect of the invention, the precursors according to the invention are used together with one compound from the group of Oxygen and Nitrogen. In the plasma, molecular Oxygen and Nitrogen form radicals.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages and objects of the present invention are attained can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by the reference to the embodiments thereof which are illustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIGS. 1A, B depict sectional views of a structure in a carbon layer generated by a method known from prior art.

FIG. 2 depicts a sectional view from a similar structure as in FIG. 1 generated with a first embodiment of the present invention.

FIG. 3 depicts a sectional view from a similar structure as in FIG. 1 generated with a second embodiment of the present invention.

FIGS. 4A, B depict schematically the principal effects of an excess concentration of CH4 in the method according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, the reaction mechanism is described by way of example for a process according to the present invention. One skilled in the art will appreciate that this example is not to be understood as limiting the scope of the claimed invention.

A carbon layer 1 is deposited on a silicon wafer 10 and is to be further processed. In this example, the carbon layer is to be etched, more particular a groove 2 needs to be etched into the carbon layer 1.

For one embodiment, CH₄/N₂/O₂ are used as precursors to form a plasma which would achieve the desired etching. Suitable plasma etching apparatus are generally known to the person skilled in the art (e.g. from Campbell, The Science and Engineering of Microelectronic Fabrication, Second Edition, Oxford University Press) so that no detailed description of the apparatus is necessary here.

These precursor materials may be introduced into the plasma chamber with the following flows:

• 40 sccm CH₄
• 40 sccm O₂
• 120 sccm N₂

The ratio of CH₄/O₂ may be approximately 1:1. The plasma chamber may be operated at a pressure of 30 mTorr and may use 500 W high frequency power, 150 W low frequency power.

In the plasma chamber the substrate 10 with carbon layer 1 is exposed to the described plasma.

In a simplified form the following reactions may take place: CH₄+e⁻→CH₃+H+e⁻  (1) CH₃+e⁻→CH₂+H+e⁻  (2) C_(solid)+O→CO   (3) C_(solid)+CH₃→C_xH_{y (solid)}   (4) C_(solid)+CH₂→C_xH_{y (solid)}   (5) C+N+H→C_xH_yN_{z (solid)}   (6)

Reaction (1) describes the formation of radicals through collisions in the plasma. Reaction (2) describes the further reaction from the product of reaction (1). The reaction product CH₂ might react with C to form C_xH_y in a passivation reaction in the gaseous state or on the surface of the substrate.

Reaction (3) describes the etching of the carbon with oxygen radicals.

Since this is anisotropic etching, the downwards (Y) direction of the etching reaction C+O═CO is dominant. The passivation CH₂+C reaction may be responsible for deposition of material on the sidewalls (X-direction). The Oxygen may etch the C_xH_y to some minor extent.

Reactions (4, 5) describe the passivation of the carbon layer by the formation of a solid C_xH_y film. Reaction (6) describes the passivation of the carbon layer by the formation of a solid C_xH_yN_z film.

The profile tuning may be influenced by pitch and the aspect ratio of the etched recess. High aspect ratio structures can lead to different surface areas of exposed films on the wafer. This may likely lead to the need to alter pressure and/or gas ratio in order to maintain a suitable profile.

In FIGS. 1A, B sectional views taken from a SEM (scanning electron microscope) image are given. In both figures a structured carbon layer 1 on an oxide substrate 10 is shown. The structures are a series of long trenches 2 of which sectional views for one of the trenches are shown here in detail. A capping layer 3 is positioned on top of the carbon layer 2. Typically the capping layer 3 is made from dielectric material.

The sectional dimensions at the center of the trench 2 in FIG. 1A is approximately 330 nm deep and 75 nm wide (at the bottom). Thus, the aspect ratio (depth/width) is approximately 4.4.

The sectional dimensions at the edge of the trench 2 in FIG. 1B is approximately 270 nm deep and 84.46 nm wide (at the bottom). The aspect ratio is approximately 3.2. The trench 2 becomes shallower at the edges compared to the center.

The trench 2 of which sectional views are given in FIGS. 1A, B is produced by etching according to a prior art process with a plasma in which o₂/N₂ were used as precursors. As can be seen from FIGS. 1A, B the sectional profile of the trench is not optimal since the side walls of the trench are not straight as would be desirable. In effect, the etching produced a bottle shaped trench 2 which is wider at the bottom than on the top.

This contrasts with a similar structure etched with a first embodiment of the present invention as depicted in FIG. 2. The trench 2 structure includes basically the same layers and materials as in FIG. 1A, B. However, the walls of the trench 2 shown in FIG. 2 are almost vertical and no sizable line edge roughness is detectable. This may be attributed to using CH₄ as precursor for the etching plasma. For the illustrated example, the etch parameters were as follows. The other precursors used were O₂ and N₂. The ratio between O₂ and CH₄ was approximately 1:1 (40 sccm O₂ and 40 sccm CH₄). The N₂ flow was 120 sccm. The etching was performed at a pressure of 30 mTorr. 500 W high frequency Power and 150 W low frequency power were used. The temperature at the top was 60° C., at the bottom 20° C. The magnetic flux density was 300 Gauss.

In FIG. 3 the result of using a second embodiment of the present invention is depicted. The same structure was produced in essentially the same method as described in connection with FIG. 2. Only the ratio of O₂ and CH₄ was changed to 1:0.5, i.e. the CH₄ content was halved.

Even though the sectional profile in FIG. 3 is considerably better than the profile obtainable through the prior art method described in connection with FIG. 1, the side wall of the trench is not as straight as with the application of an O₂/CH₄ ratio of 1:1 (shown in FIG. 2). This shows the sensitivity to the CH₄ concentration.

In FIGS. 4A, B two applications of embodiment of the present invention are shown, that serve to illustrate the ability to taper walls or create straight sidewalls. In FIG. 4A a sectional view is presented having straight sidewalls. This is achieved by a lean process, i.e. a process using a low CH₄ flow. The shape of the sectional profile is tuned by changing the CH₄ flow to the plasma, as described in connection with FIGS. 2 and 3.

If the CH₄ flow is in excess of O₂ (greater flow rate means more CH₂), the rich process moves into a deposition mode, i.e. carbon is deposited on the side walls as shown in FIG. 4B. This can be used to protect carbon sidewalls while etching with a known O₂ plasma. Deposition on the etch front direction (Y) is zero since the high energy incident ions will remove deposition under the action of the reactive ion etching (RIE) process. This is understood for RIE applications. The low frequency RF enables this to occur.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

1. A method for the plasma treatment of at least one carbon layer in or on a substrate for the production of a semiconductor device, comprising: generating a plasma with at least one precursor selected from a group comprising an aliphatic alkane, aliphatic alkene and aliphatic alkine; and exposing the at least one carbon layer to the plasma during an etching process.

2. The method of claim 1, wherein the precursor comprises at least one of: CH4, C2H4 and C2H2.

3. The method of claim 1, wherein the plasma comprises at least one of: Oxygen radicals and Nitrogen radicals.

4. The method of claim 1, wherein: the plasma comprises O2; and the ratio between the O2 and the at least one precursor is approximately 1:1.

5. The method of claim 4, wherein: the at least one precursor comprises CH4; and the ratio between the O2 and CH4 is approximately 1:1.

6. The method of claim 1, wherein generating the plasma comprises: flowing methane at a rate of approximately 40 sccm; flowing oxygen at a rate of approximately 40 sccm; and flowing nitrogen at a rate of approximately 120 sccm.

7. The method according to claim 1, wherein the plasma is at a pressure of approximately 10 to 100 mTorr at some time during the etching process.

8. The method according to claim 7, wherein the plasma is at a pressure of approximately 40 mTorr at some time during the etching process.

9. The method of claim 1, wherein: high frequency power of the plasma is approximately 500 Watts; and low frequency of the power of the plasma is approximately 150 Watts.

10. The method of claim 1, wherein an etch rate and an etch profile is tunable by adjusting the ratio of oxygen in the plasma.

11. A method of fabricating a semiconductor device, comprising: depositing a layer of carbon on a substrate; generating a plasma with at least one precursor selected from a group comprising an aliphatic alkane, aliphatic alkene and aliphatic alkine; and forming one or more structures in the layer of carbon by exposing the layer of carbon to the plasma.

12. The method of claim 11, wherein the layer of carbon comprises a layer of amorphous carbon.

13. The method of claim 11, wherein the semiconductor device is a memory device.

14. The method of claim 13, wherein the memory device is a dynamic random access memory (DRAM) device.

15. The method of claim 11, wherein the precursor comprises at least one of: CH4, C2H4 and C2H2.

16. The method of claim 11, wherein: the plasma comprises O2; and the ratio between the O2 and the at least one precursor is approximately 1:1.

17. The method of claim 6, wherein: the at least one precursor comprises CH4; and the ratio between the O2 and CH4 is approximately 1:1.

18. A method of forming a structure in a carbon layer of a semiconductor device, comprising: generating a plasma with at least one precursor selected from a group comprising an aliphatic alkane, aliphatic alkene and aliphatic alkine; and forming the structure in the layer of carbon by exposing the layer of carbon to the plasma.

19. The method of claim 18, wherein: the plasma comprises O2; and the ratio between the O2 and the at least one precursor is approximately 1:1 to form the structure with substantially straight sidewalls.

20. The method of claim 18, wherein: the plasma comprises O2; and the at least one precursor comprises CH4, wherein CH4 flow rate is in excess of the O2 flow rate causing carbon deposition on the sidewalls to form the structure with tapered sidewalls.