WO2002037549A2

WO2002037549A2 - Method for structuring a silicon oxide layer

Info

Publication number: WO2002037549A2
Application number: PCT/EP2001/012538
Authority: WO
Inventors: Matthias Goldbach; Bastian HAUSSDÖRFER; Ortrun Grahl
Original assignee: Infineon Technologies Ag; Applied Materials, Inc.
Priority date: 2000-10-30
Filing date: 2001-10-30
Publication date: 2002-05-10
Also published as: DE10053780A1; WO2002037549A3

Abstract

The invention relates to a method for structuring a silicon oxide layer. According to said method, a substrate comprising a silicon oxide layer with a mask is provided in a plasma reactor. The silicon oxide layer is exposed to a plasma which is produced from an etching gas containing at least one fluorocarbon compound that is selected from the group consisting of compounds of the empirical formula CxHyFz, wherein x = 1 to 5, y = 0 to 4 and z = 2 to 10. The process is optimised by direct switching between the etching and deposition modes, which is achieved by varying the potential difference between the substrate and the plasma.

Description

Be honored

Process for structuring a silicon oxide layer.

The invention relates to a method for structuring a silicon oxide layer.

In the structuring processes for the construction of integrated circuits, processes for etching silicon oxide layers play an important role. In the integrated

Semiconductor circuits serve silicon oxide layers, among other things, as insulating, passivating layers or even as hard masks. Silicon oxide layers find e.g. Application in trench isolation technology, where they are used to isolate neighboring transistors. In addition, they serve as insulation layers in multi-layer wiring or when building e.g. MOS transistors. The construction of integrated semiconductor circuits thus requires the provision of suitable etching methods by means of which a silicon oxide layer which has been deposited or is generated by thermal oxidation can be structured. In particular, plasma etching processes are used as the etching process.

Due to the ever increasing integration density of the circuits, the etching process has to be able to realize ever smaller critical dimensions and ever higher aspect ratios. Around structures such as contact holes, vias or hard masks with ever larger ones

To be able to etch aspect ratios, the etching must firstly be as anisotropic as possible. In addition, the etching should be as selective as possible to that of the Etching materials used as a mask, such as silicon or polymer lacquer masks. If the etching has only a low selectivity, the mask is attacked in the etching process, in particular on its profile flanks, to an extent that leads to an undesired processing of the structure specified by the mask. As a result, both the aspect ratio to be achieved and the critical dimension to be achieved are adversely affected. In addition, the etching depth to be achieved is reduced by a low selectivity of the etching process, since the mask itself is removed very quickly. The improvement of the selectivity of etching processes for silicon oxide layers is therefore the subject of constant research in semiconductor technology.

Processes which use fluorocarbons or fluorocarbons in the etching gases have become established as the plasma etching process for silicon oxide layers. A condition arises in the plasma gas in which, in addition to the etching, a deposition of polymers occurs at the same time

Materials expires. The deposition and the etching rates are in an interplay and are different from different materials. Through a suitable choice of the process parameters, they can be set so that an effective etching can be achieved selectively for one material, whereas at the same time the deposition predominates on another material. For example, when using CF ₄ in the etching gas, it is possible to set the etching parameters so that a fluorine-containing polymer is deposited on silicon and silicon nitride, whereas silicon oxide is etched by the plasma. The etching parameters can therefore be set such that the material of the layer to be structured is etched and at the same time a polymer is deposited on the material of the mask.

Attempts to increase the selectivity of etching processes to date have essentially concentrated on increasing the selectivity by using new etching gases, varying the system parameters in the etching mode or changing the chamber design. The etching parameters used in these processes always represent a compromise between the protection of the mask by the polymer deposition on the one hand and the etching of the silicon oxide layer in the exposed areas on the other hand optimal etching rate for the material to be structured, still the optimal deposition rate of the polymer on the mask reached.

US Pat. No. 6,074,959 describes an etching process in which an early termination of the

Etching process by excessive polymer deposition in structures with high aspect ratio is avoided by performing the etching process in a two-step process. In a first step, an etching recipe is selected with which the oxide layer is quickly but non-selectively etched. In a second step, by changing the etching chemistry, i.e. the fluorine-containing compounds and the power coupled into the plasma reactor created more selective etching conditions.

In addition to the general disadvantages described above, this method also has the disadvantage that the increase in the selectivity of the etching process is a change in the Chemical etching. The plasma is not maintained between the changes in the etching chemistry. This change therefore entails waiting times during which a steady state in the plasma has to be established again.

The present invention is therefore based on the object of providing a method for structuring a silicon oxide layer which reduces or completely avoids the disadvantages described above. In particular, the object of the present invention is to provide a method with which structures with a high aspect ratio can be etched into a silicon oxide layer with a high selectivity in relation to the mask used.

This object is achieved by the method for structuring a silicon oxide layer according to claim 1. Further advantageous embodiments, configurations and aspects of the present invention result from the dependent patent claims, the description and the figures.

According to the invention, a method for structuring a silicon oxide layer is provided, which comprises the following steps: A substrate, which comprises a silicon oxide layer and a mask partially covering the silicon oxide layer, is provided in a plasma reactor. A plasma is generated from an etching gas which contains at least one fluorocarbon or fluorocarbon compound selected from the group consisting of compounds of

_Molecular formula C _x H _y F _Ä7 where x = 1 to 5, y = 0 to 4 and z = 2 to 10. For at least a first period of time, a first potential difference between the substrate and the plasma generated from the above-mentioned etching gas, which is selected such that at least the silicon oxide layer undergoes an etching removal (process step c). For at least a second period of time, a second, different from the first potential difference between the substrate and the plasma generated from the above-mentioned etching gas is set, which is chosen such that a layer of a fluorine-containing polymeric material is deposited on the mask, the layer thickness of which during the second period grows. (Process step d).

All plasma reactors that can be used for the usual chemical-physical dry etching processes can be considered as plasma reactors. Such dry etching processes can e.g. reactive ion etching, anodically coupled plasma etching in the parallel plate reactor, magnetic field-supported reactive ion etching, triodes reactive ion etching, inductively coupled plasma etching or etching with an inductively coupled plasma source. In the context of the present invention, masks are to be understood as layers of materials which are suitable for protecting a silicon oxide layer from etching removal during plasma etching. In particular, this includes materials such as Silicon, silicon nitride or polymeric materials that are used as photolithography masks or lacquer masks. The only exception is silicon oxide itself.

In the context of the present invention, the term “fluorocarbon compound” is to be understood as meaning both compounds which are composed only of fluorine and carbon or compounds which also contain hydrogen in addition to fluorine and carbon. Such connections can be, for example. CF ₄ , CH ₃ F, C ₂ F ₄ , C ₂ F ₆ , C ₃ F ₆ , C ₃ F ₈ , C ₄ F ₆ , C ₄ F ₈ , C ₄ F ₁₀ , CH ₂ F ₂ , C ₂ HF ₃ , C ₂ HF ₅ , C ₃ HF ₅ , C ₃ H ₂ F ₆ , C ₄ F ₈ or C ₅ F ₈ . In a preferred embodiment of the present invention, C ₄ F _{8 is} used as the fluorocarbon compound, the constitutional isomer of this compound not being important.

In an advantageous variant of the method according to the invention, the gas flow rate of the C ₄ F _{8 is} between 10 and 50 sccm, in particular between 20 and 50 sccm. The higher gas flow rates are preferred since higher etching or deposition rates can be achieved when they are used.

In the context of the present invention, the potential difference between the plasma and the substrate is understood to mean the potential difference that forms between the plasma envelope and the adjacent substrate. This potential difference determines the energy with which the ions are accelerated from the plasma onto the substrate. This potential difference can be influenced by various process parameters that can be set on the plasma reactor. As a rule, the potential difference is varied by changing the high-frequency power coupled into the plasma reactor. This power is also referred to as "bias power". Depending on the type of plasma reactor, the coupling can take place inductively and / or capacitively. In a preferred embodiment of the method according to the invention, the first (in method step c) and the second potential difference (in method step d) between the substrate and the plasma generated from the etching gas used is set by coupling a power into the plasma reactor. The variant in which this power is capacitively coupled into the reactor is particularly preferred. This can be done, for example, by adding power to the plasma in addition to inductively coupled power is thereby coupled in that the substrate or the electrode on which the substrate is attached is capacitively connected to a high-frequency source.

The sequence of process steps c) and d) is not specified in the process according to the invention. The method can be designed in such a way that the SiO ₂ layer is etched in a first etching step and the polymer is deposited on the mask in a subsequent deposition step. Conversely, the polymer can first be deposited on the mask and then the silicon oxide layer can be etched. In the first case, the mask is reinforced by the deposition after the etching, in the second case the mask is reinforced before the etching.

For the first period, the first potential difference is selected in the method according to the invention in such a way that the silicon oxide layer experiences an etching removal. Only the silicon layer alone or also the mask protecting it can be etched. If both the silicon oxide layer and the mask are etched, the first period must be dimensioned so that the mask is not completely removed in any area.

In a preferred embodiment of the method according to the invention, the power which is coupled into the plasma reactor and with which the first potential difference is set during the first period is at least 400 W, preferably at least 600 W and particularly preferably at least 800 W.

It is particularly advantageous if the parameters that can be set on the plasma generator are set in this way be that the etching of the Si0 ₂ layer already has a high selectivity towards the mask during the first period.

In a preferred variant of the invention

The first potential difference is set in such a way that a layer of fluorine-containing polymer forms on the mask during the first period, the layer thickness of which remains essentially constant during the first period. That the potential difference is chosen here such that a balance is established between the etching of the mask and the deposition of the polymeric material on the mask during the first period. The layer which is deposited is generally only a few nm thick.

For the second period, the second potential difference between the substrate and the plasma generated from the etching gas is selected in the method according to the invention in such a way that a layer of a fluorine-containing polymeric material is deposited on the mask, the layer thickness of which during the second

Period grows.

The preferred power for setting the second potential difference during the second period is less than 400 W, preferably at most 200 W. In a particularly advantageous variant of the method according to the invention, the deposition rate of the fluorine-containing, polymeric material on the mask is at least 50 nmmin ⁻¹ , preferably is at least 240 nmmin ^"1 , in particular at least 350 nmmin ^{" 1} .

By separating out over the second period. The polymer layer becomes the mask against further etching protected. This protective effect significantly improves the selectivity of the etching process between silicon oxide and the mask material during the second period. In addition, the mask is renewed by the polymer deposition so that it can withstand a subsequent renewed etching longer. This increases the etch depth that can be achieved.

The change of the first potential difference to the second potential difference can e.g. by simple

Change in the power coupled into the plasma reactor. The changeover of the coupled power, and thus the changeover from etching mode to deposition mode, takes place instantaneously, i.e. without delaying the process.

The method according to the invention thus offers the advantage that rapid, selective etching of the silicon oxide layer can be achieved without having to make time-consuming changes to the plasma.

The etching chemistry of the plasma generated in the method according to the invention is crucially based on the use of a fluorocarbon compound which is suitable for depositing a fluorine-containing, polymeric material on the mask from the plasma. This component is retained in the etching gas over the entire period of the process. The term “the etching gas generated in b) mentioned” is also to be understood in this sense in the context of the present invention. The other etching gas components can be varied throughout the process. To increase the selectivity of the process, additional gas components can either be used during the entire process or also only be mixed in during individual steps. In particular, additional components which increase the etching rate can be added to the etching gas during method step c). In this case, the use of molecular oxygen as an additional component in the etching gas is particularly preferred.

In addition to increasing the selectivity of the etching removal, the polymer deposition offers a further advantage during the second period. In addition, the mask is reinforced by the deposition that takes place selectively on the mask material, so that the mask can then be exposed again to a relatively unselective etching step over a longer period of time. This is done in a particularly preferred variant of the invention

Process used in that process steps c) and d) are repeated several times.

With these repetitions, the duration of the first and second periods can differ from that of the first or previous cycle. Furthermore, the respective first and second potential differences can be changed in the repetition steps. This also applies to all other selectable process parameters, insofar as they can be changed without lengthy time delays. By changing the process parameters in the repetition steps, the etching and deposition conditions in each step can be optimally adapted to the aspect ratios of the structure to be etched during the respective cycle , This results in a higher selectivity and a better profile control during the etching, whereby time-consuming changes in the process conditions can be dispensed with. In a further preferred variant of the method according to the invention, the etching gas additionally contains an inert gas, preferably argon. Its gas flow rate is preferably between 100 and 1000 sccm, particularly preferably between 200 and 700 sccm, in particular between 200 and 500 sccm.

It is further preferred that the etching gas additionally contains molecular oxygen 0 ₂ . It is particularly preferred here that the gas flow rate of the molecular oxygen is between 5 and 25 sccm, in particular 10 sccm. By adding molecular oxygen, the potential difference, or the power coupled into the plasma generator, at which the deposition of the fluorine-containing polymeric material converts into an etching of the polymeric fluorine-containing material, can be shifted to lower potential differences or coupled-in powers.

In addition, the etching removal in method step c) can be increased further when using molecular oxygen in the etching gas. In a further particularly advantageous variant of the method according to the invention, the etching gas additionally only contains molecular oxygen in method step c).

In the following the method according to the invention will be explained in more detail using the exemplary embodiments and in connection with the drawings. It shows

Fig. 1: A graph showing the dependence of the deposition rate of the fluorine-containing, polymeric material Silicon from the capacitively coupled power (bias power) for various etching gases is shown.

Embodiment 1

In this example, the polymer deposition from the plasma onto a silicon mask was determined as a function of the capacitively coupled power.

For this purpose, silicon wafers were placed in a plasma generator of the type IPS Dielectric Etcher from Applied Materials and exposed to a plasma. The polymer deposition on the silicon wafers was then determined using scanning electron microscopy for various capacitively coupled powers.

The following basic recipe was used in this series of experiments:

Argon gas flow rate: 200 sccm

Gas flow rate C ₄ F ₈ : 20 sccm

Power (inner source): 600 W.

Power (outer source): 200 W chamber pressure: 13 mTorr

He pressure: 5 torr

Chilier (heat exchanger): 20 ° C

Roof: 200 ° C

Ring: 375 ° C bias power: 0 - 400 W.

Two series of measurements were carried out. The basic recipe was used in the first series of measurements and in the second In addition, molecular oxygen with a gas flow rate of 10 sccm was added to the basic recipe. The measured values of the first series of measurements are represented in the graph shown in FIG. 1 by the full symbols. The full circles correspond to the measured values at the edge of the silicon wafers, the full squares represent the measured values in the center of the silicon wafers.

The measured values of the second series of measurements are represented by the half-open symbols, the circles again representing the measured values on the edge and the squares representing the measured values in the center of the silicon wafers.

It can be seen from the measurement data that the deposition rate decreases with increasing capacitively coupled power and converts into an etching rate from a certain power.

The addition rate of molecular oxygen reduces the deposition rate. In addition, the deposition rate shows only slight local deviations. For the first series of measurements (basic recipe), the deposition rate for an arbitrarily selected power results according to the formula: r _D = 417, 3 nm / min - 1.06714 nm / min W * P _B (r _D = deposition rate; P _B = capacitive coupled power). For the second series of measurements (oxygen-containing recipe), the respective deposition rate can be calculated using the formula r _D = 389.3 nm / min - 1.583 nm / min W * P _B (r _D = deposition rate; P _B = capacitively coupled power).

From these deposition rates, an optimized time can be determined, which is necessary for the deposition of a defined layer thickness. Embodiment 2

In this example, an Si0 ₂ layer was structured with a silicon mask. The Si0 ₂ layer already had contact holes with an approximate depth of 500 nm and an aspect ratio of about 5.

The following two-step process was used: In a first step (DI), the etching conditions were chosen such that a deposition took place on the silicon mask. In a subsequent etching step (E1), the etching conditions were chosen so that the Si0 ₂ layer was etched.

The process parameters for both steps are listed in Table 1.

Table 1: Process parameters for Dl and El:

Under these conditions, an approximately 80 nm thick polymer layer was deposited on the mask in deposition step D1 within 12 seconds. Only negligible deposition took place on the profile edges. Deposition within the contact hole was not observed.

In the subsequent etching step E1, about 150 nm of SiO _{2 were} etched within the contact hole within 20 seconds. The polymer layer on the mask had almost completely disappeared after the etching step E1. It was therefore possible to etch 150 nm Si0 ₂ silicon oxide without loss of mask material.

Example 3:

In this exemplary embodiment, the procedure was analogous to exemplary embodiment 2, except that the process conditions for the deposition step (D2) were changed. The process parameters are given in Table 2.

Table 2: Process parameters for D2 and E2

This process control shows results similar to those in embodiment 2. However, with these process parameters, a slight edge coverage of the mask can be determined. In addition, the Si0 ₂ layer in the contact holes is already etched in the deposition step D2 in this process.

Claims

claims

1. A method for structuring a silicon oxide layer, comprising the steps:

a) a substrate comprising a silicon oxide layer and a mask partially covering the silicon oxide layer is provided in a plasma reactor,

b) a plasma is generated from an etching gas which is at least one fluorocarbon or

Fluorocarbon compound selected from the group consisting of compounds of the empirical formula C _x H _y F _z , where x = 1 to 5, y = 0 to 4 and z = 2 to 10,

c) for at least a first period of time, a first potential difference is set between the substrate and the plasma generated from the etching gas mentioned in b), which is selected such that the silicon oxide layer experiences an etching removal, and

d) for at least a second period of time, a second potential difference, different from the first, between the substrate and the plasma generated from the etching gas mentioned in b) is set, which is selected such that a layer of a fluorine-containing polymeric material is deposited on the mask whose layer thickness increases during the second period.

2. The method according to claim 1, d a d u r c h g e k e n n z e i c h n e t that steps c) and d) are repeated several times.

3. The method according to claim 1 or 2, so that the first and the second potential difference between the substrate and the plasma generated from the etching gas mentioned in b) takes place by coupling a power into the plasma reactor.

4. The method according to claim 3, d a d u r c h g e k e n n e e i c h n e t that the power is capacitively coupled into the plasma reactor.

5. The method according to any one of the preceding claims, characterized in that the etching gas as a fluorocarbon or fluorocarbon compound is a compound selected from the group consisting of CF ₄ , CH ₃ F, CH ₂ F ₂ , C ₂ F ₄ , C ₂ F ₆ , C ₂ HF ₃ , C ₂ HF ₅ ,

C ₃ F ₆ , C ₃ HF ₅ , C ₃ F ₈ , C ₃ H ₂ F ₆ , C ₄ F ₆ , C ₄ F ₈ , C ₄ F ₁₀ , or C ₅ F ₈ .

6. The method according to any one of the preceding claims, characterized in that the etching gas contains C ₄ F ₈ as the fluorocarbon compound.

7. The method according to claim 6, dadurchgekennzeic hn et that the gas flow rate of C ₄ F _{8 is} between 10 and 50 sccm, preferably 20 sccm.

8. The method according to any one of the preceding claims, characterized in that the Etching gas contains an inert gas, preferably argon.

9. The method according to claim 8, so that the gas flow rate of argon is between 100 and 1000 sccm, preferably between 200 and 700 sscm, in particular between 200 and 500 sccm.

10.The method according to any one of the preceding claims, characterized in that the etching gas additionally contains molecular oxygen 0 ₂ .

11. The method according to claim 10, so that the gas flow rate of the molecular oxygen is between 5 and 25 sccm, preferably 10 sccm.

12. The method according to any one of claims 10 or 11, that the etching gas additionally contains molecular oxygen only in step c).

13. The method according to any one of the preceding claims, characterized in that the power for setting the potential difference between the plasma and the substrate in step c) is at least 400 W, preferably at least 600 W, in particular at least 800 W and less than 400 W in step d) , preferably at most 200 W.

14. The method according to any one of the preceding claims, characterized in that the first potential difference between the substrate and the the etching gas generated in b) is adjusted so that a layer of fluorine-containing polymeric material forms on the mask during the first period, the layer thickness of which remains essentially constant during the first period.

15. The method according to any one of the preceding claims, characterized in that the deposition rate of the fluorine-containing polymeric material on the mask during the second period is at least 50 nmmin ^"1 , preferably at least 240 nmmin ^{" * 1} , in particular at least 350 nmmin ^-1 .

16. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that the

Mask is a silicon mask.

17. Method according to one of the preceding claims, that the mask is a lacquer mask.