WO1995025186A1

WO1995025186A1 - Surface treatment process for flat silicon bodies

Info

Publication number: WO1995025186A1
Application number: PCT/DE1995/000391
Authority: WO
Inventors: Jörg Rappich; Helmut Jungblut; Mohammed Aggour; Hans-Joachim Lewerenz
Original assignee: Hahn-Meitner-Institut Berlin Gmbh
Priority date: 1994-03-17
Filing date: 1995-03-17
Publication date: 1995-09-21
Also published as: DE19510852A1; AU2067295A; DE19510852C2

Abstract

Semiconductor wafers for the production of electronic components must have a surface as perfect as possible for subsequent processes. For that purpose a flat n-Si body is electrolytically polished in a fluoride-containing electrolyte solution to which is applied an anodic potential while the surface to be treated is irradiated with light whose energy is at least equal to that of the band gap, or a flat p-Si body is similarly treated in a dark environment. At the same time, an electroanalytic monitoring process is carried out to detect the occurrence of a periodic current oscillation. The surface is polished during a few such current oscillations. The treatment comprises in particular the anodic oxidation and electrochemical etching of the surface, if required also an electric polishing process, and allows a high electronic quality characterised by the interface state density to be obtained by immediate hydrogen termination during the transient dark current that sets in.

Description

description

Process for treating surfaces of flat silicon bodies

The invention relates to a method for treating surfaces of flat silicon bodies, which includes electro-analytically monitored, electrolytic smoothing of the surface to be treated.

The semiconductor wafers used for the production of electronic components must have a surface that is as perfect as possible on which subsequent processes, e.g. epitaxial layer growth to be carried out. Usual methods for polishing etching are based on removing the material peaks when reducing the thickness of the generally disk-shaped semiconductor bodies (wafers) which appear as roughness on the desired flat surface formation and have low binding energy in the crystal structure.

DE 40 33 355 A1 specifies a method for the electrolytic etching of silicon carbide, as a result of which flat semiconductor bodies have flat surfaces which are suitable for further processing into electronic components. The peculiarity of the polishing etching of silicon carbide is that this material is not attacked by acids and bases; therefore an alkaline solution should be provided as an electrolyte. If the electrolytic solution contains fewer protons than negative hydroxide ions, its pH value is greater than 7 and it is present in a high concentration which is kept constant at least in the vicinity of the surface to be polished, completely smooth and flat surfaces can be achieved without any structuring, which in itself do not reveal any unevenness in the scanning electron microscope with high resolution. This prior publication does not contain any information about the duration and the necessary measures for monitoring and / or controlling such a polishing etching process as well as about units of measurement and values for the surface quality achieved.

The prior art, which is assumed in the invention, is known from DE 40 31 676 AI. The treatment of the surface of a silicon body to be carried out subsequently takes place essentially by etching in a fluoride-containing solution in a dark environment. Of crucial importance, however, is the fact that there is a clearly correlated relationship between electroanalytical methods and, for example, optical investigation methods for checking the nature of semiconductor surfaces. Thus, an electroanalytical measurement that can be carried out in situ allows monitoring of the treatment taking place.

The excellent results that can be achieved in the above-mentioned manner with regard to the smoothing and also an electronic passivation of the surfaces of Si wafers must, however, be compared with the fact that the technology on which they are based may not be convincing on its own. In this context can also refer to: "J. Electroanal. Chem." 351 (1993) 159-168 ("On the origin of photocurrent oscillation at Si electrodes" by HJ. Lewerenz and M. Aggour). The influence of the molarity and the pH of the electrolytic fluoride solution on the oscillation of the photocurrent, which occurs during the electrochemical surface treatment of n- and p-Si (100) surfaces, is dealt with there. For reasons of comparability, both the n- and the p-type crystals were connected to a potential of +6 VgCE and the n-Si was irradiated with an intensity of 70 mWcrn " ² .

The technical problem with which the invention is concerned is to show further possibilities for a Si surface treatment with electrochemical processes to be used in successive stages, which offer at least similar qualitative results and further simplify the treatment method and its controllability.

In a method of the type mentioned at the outset, the solution according to the invention is that a) a pretreatment of the silicon body is carried out and then bl) the electrolytic smoothing of the surface to be treated in a fluoride-containing electrolyte solution essentially by anodizing and a subsequent electrochemical etching and b2) the electro-analytical monitoring of the electrolytic smoothing according to a current measurement - between the silicon body, which is at the anodic potential as the working electrode and whose surface to be treated is in contact with the electrolyte solution, and a counter electrode immersed in the electrolyte solution - with regard to the occurrence of a periodic oscillation of the measured current b3) takes place during a period in which the periodicity of the oscillation can be clearly registered.

With regard to current oscillation, experimental studies have shown that there is a connection between such an oscillation curve and changes in the surface morphology. These investigations were e.g. with combined in-situ ellipsometry, in-situ infrared spectroscopy, scanning tunneling microscopy (STM) and ex-situ electrochemical surface analysis. As well as other surprising effects that can be observed in electrochemical studies on single-crystal silicon / electrolyte contact, e.g. Photocurrent multiplication, the occurrence of a dark current in fluoride-containing electrolyte solutions after and during the removal of oxide, also includes current or photocurrent oscillation. Accordingly, an essential feature of the invention in the scope of the priority-based application is that the electrolytic smoothing of n-Si is brought about by irradiation of the surface to be treated with light, the energy of which is at least equal to the band gap, and the electroanalytical monitoring with the aid of the measured photocurrent occurs.

Although there are hypotheses for the causes of the occurrence of photostromoscillation, the elucidation of their physico-chemical foundations still requires further research. The inventors have described the state of this research work, in particular on changing the surface morphology in connection with photocurrent oscillations and the measurement methods used to investigate surfaces treated in this way and by other methods. in the publication "Journal of the Electrochemical Society" 141, (1994) L 99 to L 102, which appeared after the priority date.

Thereafter, when treating not yet redesigned Si (III) -H (III) surfaces, both chemical and electrochemical processes, in particular with regard to the hydrogen termination of the surface, lead to comparable results. However, an electrochemical process not only offers the possibility of using current or voltage as parameters for monitoring the process, these parameters can also be used to easily detect the changes which are brought about in the far submonolayer region. Thus comes the electrochemical Conditioning certainly a privilege for creating excellent surface textures of silicon too.

The interface state density Djt is an important criterion for the surface quality, in particular for the electronic properties of the surface of a Si wafer. The values of Dft are usually lxlO ¹² eV ^ cnr ² . More recently - cf. eg "J. Electrochem. Soc." 141_ (1994) 3595 to 3599 ("Surface Electronic Properties of Electrolytically Hydrogen Terminated Si (lll)" by H. Dittrich et al) - values of D _it idgap = \ _x ιol l eV ' ^{2 have been} reached. Also electropolishing of n-Si (l 1 l) surfaces - cf. J. Rappich et al: "Electrochemical and Electronic Passivation by Hydrogenation of n-Si (l ll)" on the occasion of the "7th International Symposium on Passivity: Passivation of Metals and Semiconductors" TU Clausthal, DE 21.-26.08.1994 - led to interesting results. Further work in these areas planned for publication in the near future is also concerned with improving the interface state density of n-Si with (111) and (100) -oriented surfaces, so that the now more comprehensive technical problem is to be seen in it to be able to carry out the treatment method according to the invention independently of the orientation of the surfaces and also independently of the conductivity type (n- or p-type Si).

Accordingly, the invention now also provides a variant with its own inventive significance as its essential feature of bringing about the electrolytic smoothing of the surface of p-Si to be treated in a dark environment, the measured currents flowing due to the applied potentials being used for the electroanalytical monitoring. Here, too, the oscillating current - like the oscillating photocurrent with n-Si while illuminating the treated surface - leads to surface smoothing.

With n-Si, the required charge carriers (holes) are generated by the exposure; with p-Si, these charge carriers are present anyway due to the doping of the material. It is surprising, however, that a dark current transient can also be observed with p-Si and thus the hydrogen termination that takes place during the duration of the dark current transient can be monitored electroanalytically. With regard to the type of conduction, there are therefore no fundamental differences when carrying out the surface treatment according to the invention. With regard to the achievable interface state density, the orientation of the crystal surfaces is also without significant meaning. The (III) orientation is indeed the thermodynamically more stable compared to other orientations, for example the (100) orientation. Compared to other known surface treatments, the D ^ values achievable according to the technical teaching according to the invention are significantly better, so that any differences attributable to the orientation of the treated crystal surfaces are less.

Preferred embodiments of the invention relate to the areas of the process parameters that are essentially to be observed or their values that can be defined more precisely within these areas. Furthermore, both serial and parallel treatment of several wafers of a batch can advantageously be carried out, with in each case only one reference specimen being measured in-situ or beforehand, e.g. needs to be carried out by means of optical methods. Finally, further process stages can follow the surface smoothing, in particular those for further conditioning of the wafer surfaces, before e.g. epitaxial layer growth, lithographic processes or the like take place, for which optimal conditions are created with the invention and its embodiments. It is of particular importance here that the smoothing treatment according to the teaching according to the invention leads to a particularly favorable distribution of the interface states which characterizes the electronic quality. In a particularly advantageous manner, the electrolyte liquid used for the electrolytic smoothing is freed of oxygen by flushing with inert gas. For example, nitrogen can keep the treatment liquid in motion both in a storage container and in the treatment room and in the process carry oxygen bubbles to the surface. In this way the Djf values are additionally improved.

For this purpose, reference is made both to the information in the respective subclaims and to the details in the description of the schematic representations in the drawing. They show schematically:

Fig. 1: the structure of a treatment device for electrolytic smoothing

Silicon surfaces; Fig. 2: the structure of a container in which a larger number of Si wafers can be treated at the same time

3: a concept for a system in which several treatment devices according to FIG. 1 or 2 work as a follower of a guide device: 4 and 5: diagrams for the time profile of the oscillating current which occurs during the electrolytic surface smoothing and the subsequent dark current transient;

Fig. 6: a graph of the time course of a roughness parameter and Fig. 7: a graph of the atomic structure of a Si (l 1 l) surface with one step.

The construction of a treatment device shown in FIG. 1 is essentially based on an arrangement as shown in FIG. 5 of DE 40 31 676 AI.

In embodiments of the invention, such a device has a container 2, the bottom of which can be sealed with a plate, the flat silicon body 1. The surface of the silicon body 1 to be treated lies in the interior of the container 2 and is in direct contact with the electrolyte 7. The back of the silicon body 1 is electrically conductively connected to a contact tab 3. A ring-shaped counter electrode 4 is attached to the wall of the container 2 in the vicinity of the lid surface. At least this lid surface of the container 2 is transparent, e.g. designed as a window 5. The surface to be treated of the silicon body 1 made of n-conducting material can be irradiated from a light source 6 located above it.

The electrolyte 7 reaches the desired composition from a reservoir 11 via an inlet pipe 8 into the container 2 and can be removed from there via an outlet pipe 9. A water reservoir 10 is provided for subsequent rinsing cycles. The liquids from the reservoirs 10/11 enter the container 2 via the inlet pipe 8 via valves 12.

The silicon body 1, the counter electrode 4 and a reference electrode 15 immersed in the electrolyte 7 are connected to a potentiostat / galvanostat, of which a voltage measuring device 14 and a current measuring device 13 are shown symbolically.

A container 27 shown in FIG. 2 - in cross section and in detail - can simultaneously accommodate a larger number of silicon bodies 1 (wafers) for surface treatment. The treatment and rinsing liquids pass through an inlet pipe 18 into a trough 28 and flow out of there again through an outlet pipe 19. At the bottom of the trough 28, the individual silicon bodies 1 are to be placed in openings, the edge regions of which have an electrically insulating double ring seal, that is, an outer sealing ring 20 and an inner sealing ring 21 are provided. An annular space 22 between these two sealing rings 20, 21 is connected to negative pressure, so that any liquids that are not retained by the outer sealing ring 20 are suctioned off.

The silicon bodies 1 cover a contact spring space 26 which may also have to be placed under reduced pressure. The electrical potential required for the electrolytic treatment is applied to each silicon body 1 via a contact spring 25 which is connected to a terminal 24 which is accessible from the outside. The silicon bodies 1 therefore do not need to be provided with a fixed contact tab or the like.

The negative pressure in the contact spring chamber 26 and the spring force of the contact spring 25 are to be coordinated with one another such that, on the one hand, the electrical contact is ensured and, on the other hand, the silicon bodies 1 are not raised.

Further details not shown in Fig. 2, e.g. a counter electrode, a reference electrode, a transparent cover for the trough 28, and a light source located above it, as well as devices of a potentiostat / galvanostat are to be provided if necessary in accordance with the device shown in FIG. 1. For the smoothing of p-type silicon bodies 1, the light source is not necessary and, if necessary, a transparent cover is to be shaded from ambient light that penetrates from the outside.

3 shows an arrangement in which a device for the electrolytic, electroanalytically monitored or controlled surface treatment of Si bodies 1 and measuring devices 16 and a control device 17 is provided, with the aid of which correlating process parameters are determined, from which the electrochemical setting values are used Control of the process sequence in subsequent devices - containers 2Sj, 2S2, ..., Sk - serve. There, several or all copies of a batch of wafers are treated in accordance with measures which are specified by a guide device - container 2 _m .

From the graphs shown in FIGS. 4 and 5 it can be seen for the course of the surface smoothing carried out according to the technical teaching according to the invention that - according to FIG. 4 - a photocurrent flows at the beginning of the irradiation of n-Si, which oscillates. After 15 oscillations, the light source 6 was switched off. On 5 shows an analogous behavior for the current flowing in the surface smoothing of p-Si when an anodic potential is applied. After a few oscillations, here five peaks, the potential is lowered. Then a dark current transient known from DE 40 31 676 AI, already mentioned, was observed, also with p-Si, which was followed by a further current drop.

During this treatment process in-situ or afterwards measurements of the changes in the surface morphology using optical methods showed that material peaks and steps were removed from the Si surface as long as the current oscillated. The surface smoothed in this way was sealed with hydrogen when the low dark current flowed. The smoothing was carried out in acidic fluoride solutions and was examined on n-Si using FTIR (fourier transform infrared) spectroscopy with ATR (attenuated total reflection) in-situ and by STM (scanning tunneling misroscopy) in air. The roughness parameter R _a - cf. DIN 4762 - from the STM examination shows a reduction from 40 Ä ± 8 Ä (4 nm ± 0.8 nm) to 7 Ä ± 1Ä (0.7 nm ± 0.1 nm) on a 7 cm ² area ( see Fig. 6 and associated description below).

The electrolyte solutions 7 were made from p.a. -Prepared reagents and triple distilled water and had the following compositions / pH values / temperatures

- 0.1 M NH4F; pH = 4.0, adjusted with H ₂ SO ₄ ; 22 ° C ± 1 ° C for STM examination

- 0.1 M NaF; pH = 4.0, adjusted with HF; 22 ° C ± 1 ° C for the FTIR experiment.

For the electrochemical measurements were used: ^•

- platinum counter electrode 4;

- saturated calomel electrode or 0.1 mol KCl / AgCl / Ag reference electrode 15.

A potentiostat-galvanostat (from Heka, model 128) was used to set the potential.

The surface to be smoothed with n-Si was irradiated with white light at 70 mW / cm ² in the STM examination and 20 mW / cm ² in the FTIR experiment.

At a potential of + 2.8 Vjjjg (normal hydrogen electrode) no photostromoscillation occurred, but at + 6 VNJJ due to ellipsometric Investigations showed that an optimal surface smoothing is achieved in the area of this potential setting.

A final etching step in dark surroundings at a potential of + 0.78 VNHE leads to hydrogen termination of the smoothed surface (approx. 80% to 90%, meanwhile 95% of a SiH monolayer with e.g. n-Si (l 11).

During the individual current or photocurrent oscillations, the surface gradually becomes smoother until, after the desired smoothing effect has been reached, the applied potential is lowered or the light source 6 is switched off and the smoothing process can be ended by anodic oxidation.

The duration during which the current or photocurrent oscillation can be shortened with a changed electrolyte composition. However, such optimizations are to be set in relation to the time required for the entire conditioning measures of wafers, including set-up times and the like for the treatments to be carried out, and may then make only minor contributions to the economy.

6 shows from the course of the roughness parameter R _a (dim: Ä; 1 Ä = 0.1 nm) over time (dim: minutes) that a considerably smoothed surface is created in less than an hour. In view of the fact that, when recording this course, greater emphasis was placed on the determination of the surface quality achieved than on the time required for the duration of the treatment, the periods to be taken indicate an upper limit which is easily undercut in practice.

It can be seen from the atomic structure shown in Fig. 7 that the roughness of surfaces to be smoothed is due in particular to steps of the (1 reoriented silicon which, depending on the orientation of the steps, have one or two open bonds. There, material can be removed more easily than in places with closed bonds, that is, according to the teaching of the invention, the smoothing of Si surfaces takes place predominantly by leveling steps.

The technical teaching of the invention and its embodiments thus makes it possible to provide the surfaces of Si wafers with excellent electronic quality in a manner which is simple to carry out and controllable. A prerequisite for this is smoothed surface to the lowest possible roughness. For the interface state density, which is characteristic or achievable as a quality standard, the values Dj _t (dim: eV '^ cπr ² ) show, at least in the tendency, improvements of one to two orders of magnitude compared to the values achievable with previously customary surface treatment methods.

Pretreatments of the Si wafers, which are required depending on the delivery state, are limited, for example chemical etching or must include previous mechanical pre-cleaning processes or the like. All the process steps to be carried out subsequently are electrochemical, take place at room temperature, do not require any transport of the wafers between these process steps and are completed in a few minutes. The diluted fluoride solutions - NH4F - used can, for example, for electrochemical etching, electropolishing and mainly for electrolytic smoothing and the immediately following hydrogen termination with regard to molarity and / or the pH value and are technically easy to handle.

The electrical contact for the potential to be applied to the Si wafers for electrolytic treatment can be brought about via contact springs which are pressed against the rear sides of the wafers. This avoids both the effort and the risk of damage associated with the attachment and removal of a contact firmly adhering to the wafer.

In general, a single pass through the treatment steps "electrolytic smoothing / hydrogen termination" is sufficient for the surface treatment of Si bodies according to the invention. In any case, this step is the essential one for a surface treatment and must therefore be completed at the end of the treatment, which can then be followed by general processes and / or processes used to prepare treatments for other purposes. Electropolishing instead of electrolytic smoothing by current oscillation is not recommended, but can favor this process step, which is essential for the invention and its embodiments, for example if the Si wafer is particularly roughened in the delivery state or after mechanical pre-cleaning. Electrolytically smoothing the surfaces of p-Si in a dark environment does not require absolute darkness. Weak ambient light can be allowed, otherwise it is sufficient to shade the treatment container against the incidence of light.

For the duration of the electrochemical treatment stages that are essential for the invention and its embodiments, it has hitherto been possible to determine the minimum duration for electrolytic smoothing by current oscillation and 1 minute for the subsequent hydrogen termination, for about 2 to 3 minutes. Longer treatment times showed no significant changes in the results. This means that a relatively broad time window between minimum and maximum ^'duration of treatment is available and optimal, adjustable over a wide range parameters are selectable.

Claims

claims

1. A method for treating surfaces of flat silicon bodies, which includes an electroanalytically monitored, electrolytic smoothing of the surface to be treated, characterized in that a) a pretreatment of the silicon body is carried out and then bl) the electrolytic smoothing of the surface to be treated in a fluoride-containing electrolyte solution essentially by anodic oxidation and subsequent electrochemical etching, and b2) the electroanalytical monitoring of the electrolytic smoothing in accordance with a current measurement - between the silicon body, which is at the anodic potential as the working electrode and whose surface to be treated is in contact with the electrolytic solution , and a counter electrode immersed in the electrolytic solution - with regard to the occurrence of a periodic oscillation of the measured current b3) during a period in which the periodicity of the Osz illation must be clearly registered.

2. The method according to claim 1, characterized in that the electrolytic smoothing of n-Si is brought about by irradiating the surface to be treated with light, the energy of which is at least equal to the energy of the band gap, and the electroanalytical monitoring is carried out with the aid of the measured photocurrent .

3. The method according to claim 2, characterized in that - the working electrode is set to a fixed anodic potential greater than 1 V against normal hydrogen during the anodizing,

- An acidic fluoride solution is used as the electrolyte;

- The surface power density of the light is at least 10 mW / cm ² and there is - for the clear periodicity of the photocurrent oscillation, a duration of between 10 minutes and 60 minutes, depending on the initial roughness of the surface to be treated.

4. The method according to claim 1, characterized in that the electrolytic smoothing of the surface to be treated by p-Si is brought about in a dark environment, and the measured currents flowing due to the applied potentials are used for the electroanalytical monitoring.

5. The method according to claim 4, characterized in that - the working electrode is set to a fixed anodic potential greater than 1 V against normal hydrogen during the anodizing,

- An acidic fluoride solution is used as the electrolyte solution; and yourself

- For the clear periodicity of the current oscillation, a minimum duration of 1 minute, depending on the initial roughness of the surface to be treated, results.

6. The method according to claim 3 or 5, characterized in that

- The potential of the working electrode is set to 6V ± 10% against normal hydrogen and

- The electrolyte is a 0.1 M fluoride solution with a pH of 4.0.

7. The method according to any one of claims 1 to 6, characterized in that in the treatment stage of the electrochemical etching after the anodic oxidation, the hydrogen termination of the smoothed surface takes place during a period in which a dark current transient can be registered.

8. The method according to any one of claims 1 to 7, characterized in that the parameter values for the electrolytic smoothing and the electroanalytical monitoring on a reference specimen of a batch of wafers by means of a correlated, in-situ optical measurement of the surface quality determined and the rest Copies of the batch are treated solely on the basis of the correlating parameter values determined on the reference copy.

9. The method according to claim 8, characterized in that the treatment of the reference copy is carried out as a guide treatment for the simultaneous treatment of further copies of the same batch.

10. The method according to any one of claims 1 to 9, characterized in that on the electrolytic smoothing of surfaces flat silicon bodies as further process steps

- Separate the silicon body from the electrolyte solution and

- Connect the aftertreatment of the silicon body with liquid and / or gaseous substances.

11. The method according to any one of claims 1 to 10, characterized in that the electrolyte liquid used for the electrolytic smoothing by rinsing with

Inert gas is freed from oxygen.