DE19510852A1 - Treating surfaces of flat silicon bodies - Google Patents

Abstract

Treating surfaces of flat Si bodies comprises: (a) pretreating the Si bodies; (b1) electrolytically polishing the surfaces in an F-contg. electrolyte soln. by anodic oxidn. and electrochemically etching; (b2) electro-analytically monitoring the polishing using a current measurer-between the Si body, present as working electrode of anodic potential, and in contact with the electrolyte soln. and a counter electrode immersed in the electrolyte soln., w.r.t. occurrence of periodic oscillation of the measured current; and (b3) registering the periodicity of oscillation over a period of time.

Description

The invention relates to a method for treating surfaces flatter Silicon body, which is an electroanalytically monitored, electrolytic smoothing of the treating surface.

The semiconductor wafers used to manufacture electronic components must have as perfect a surface as possible on which subsequent processes, e.g. B. epitaxial layer growth to be performed. Usual procedures for polishing etching are based on reducing the thickness in general disk-shaped semiconductor bodies (wafers) to remove such material tips, which as Roughness appear on the desired flat surface formation and have low binding energy in the crystal structure.

DE 40 33 355 A1 describes a method for the electrolytic etching of Silicon carbide specified, as a result of which flat semiconductor bodies are flat Have surfaces that are used for further processing to electronic components are suitable. The peculiarity of the polishing etching of silicon carbide is that this material is not attacked by acids and alkalis; therefore is one provide alkaline solution as electrolyte. If the electrolytic solution is less Contains protons as negative hydroxide ions, their pH is greater than 7 and in higher, kept constant at least in the vicinity of the surface to be polished Concentration is completely smooth and flat surfaces without any Structuring achievable even with high scanning electron microscopy Resolution does not reveal any unevenness. About the duration and the required Measures for monitoring and / or controlling such a polishing etching process as well as units of measurement and values for the surface quality achieved Pre-release no information.

The prior art, which is assumed in the invention, is from the DE 40 31 676 A1 known. The subsequent treatment of the surface A silicon body is essentially found by etching in a fluoride Solution takes place in a dark environment. However, that is of crucial importance The fact that there is a clearly correlated connection between  electroanalytical methods and z. B. optical examination methods for the Examination of the nature of semiconductor surfaces exists. Thus, a electroanalytical measurement that can be carried out in situ, monitoring 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 the sake of comparability, both the n- and p-type crystals were placed at a potential of +6 V _SCE and the n-Si was irradiated with an intensity of 70 mWcm ^-2 .

The technical problem with which the invention is concerned is that of others Possibilities for a Si surface treatment with in successive stages to show applicable electrochemical processes that are at least similar Provide qualitative results and the mode of treatment and its controllability further simplify.

In a method of the type mentioned at the outset, the solution according to the invention exists in that

• a) a pretreatment of the silicon body is carried out and then
• b1) the electrolytic smoothing of the surface to be treated in a fluoride-containing Electrolytic solution essentially by anodizing and adhering to it subsequent electrochemical etching takes place,

and

• b2) the electro-analytical monitoring of the electrolytic smoothing accordingly a current measurement - between the silicon body, which acts as the working electrode anodic potential and its surface to be treated with the Electrolyte solution is in contact, and one in the electrolyte solution immersed counter electrode - 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 clearly increases is register.

With regard to current oscillation, experimental studies have shown that a Relationship between such an oscillation course and changes in Surface morphology exists. These studies have been carried out e.g. B. with combined in-situ ellipsometry, in-situ infrared spectroscopy, scanning tunneling microscopy (scanning tunneling microscopy-STM) and ex-situ electrochemical Surface analysis carried out. Like others, with electrochemical Investigations on single-crystalline silicon / electrolyte contact, surprising effects, e.g. B. photocurrent multiplication, occurrence of a dark current in fluoride-containing electrolyte solutions after and during the removal of oxide also the current or the photocurrent oscillation for this. Accordingly, there is one essential feature of the invention in the scope of the priority Registration - in that the electrolytic smoothing of n-Si while irradiating the treating surface with light, whose energy is at least equal to the band gap is brought about, and the electroanalytical monitoring with the aid of the measured photocurrent occurs.

There are hypotheses for the causes of the occurrence of photostromoscillation before, the elucidation of their physico-chemical basics is still required further research. About the status of this research, in particular related to changing the surface morphology Photocurrent oscillations and the study of in this way and after other methods treated surfaces was used by the inventors z. B. in the publication published after the priority date "Journal of the Electrochemical Society" 141, (1994) L 99 to L 102 in detail reported.

Thereafter, when treating Si (111) -H (1x1) that has not yet been redesigned, Surfaces both in terms of chemical and electrochemical processes the hydrogen termination of the surface for comparable results. A However, electrochemical process not only offers the possibility of electricity or Use voltage as a parameter for monitoring the process with these Parameters can also be easily set in the far submonolayer area Capture changes brought about. Thus comes the electrochemical  Conditioning certainly a privilege for creating excellent ones Surface textures of silicon too.

The interface state density D _{it is} an important criterion for the surface quality, in particular for the electronic properties of the surface of a Si wafer. The values of D _it are usually 1 × 10 12 eV ^-1 cm ^-2 . More recently - cf. e.g. B. "J. Electrochem. Soc." 141 (1994) 3595 to 3599 ("Surface Electronic Properties of Electrolytically Hydrogen Terminated Si (111)" by H. Dittrich et al) - values of D _it ^midgap = 1.7 × 10 11 eV ^-1 cm ^{-2 have been} achieved. Also electropolishing n-Si (111) surfaces - cf. J. Rappich et al: "Electrochemical and Electronic Passivation by Hydrogenation of n-Si (111)" 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 ones 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 can be seen therein: 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 sees a variant with its own inventive Significance as their essential feature, the electrolytic smoothing of the bring about treating surface of p-Si in a dark environment, whereby for the electroanalytical monitoring of the measured, based on the applied potentials serve flowing currents. Here, too, the oscillating current carries - like that oscillating photocurrent at n-Si with exposure of the treated surface - for 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 density of interface states that can be achieved, the orientation of the crystal surfaces is also of no significant importance. The (111) orientation is the thermodynamically more stable compared to other orientations, e.g. B. the (100) orientation. Compared to other known surface treatments, the D _it 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 a plurality of wafers in a batch can advantageously be carried out, with in each case only one reference specimen being measured in-situ or beforehand, for example, to measure the surface quality. B. 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 z. B. an epitaxial layer growth, lithographic processes or the like. 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 doing so carry oxygen bubbles to the surface. In this way, the D _it values are additionally improved.

For this purpose, reference is made both to the information in the respective subclaims as also below in the description of the schematic representations in the Drawing went into more detail. They show schematically:

Fig. 1 shows the structure of a treatment apparatus for the electrolytic smoothing of silicon surfaces;

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

Fig. 3 shows a design for a plant in which a plurality of treatment devices according to Fig 1 or 2 operate as a result of guide means.:

. Figs. 4 and 5 shows diagrams of the time course of the oscillating current which occurs during the electrolytic surface smoothing, and the subsequent Dunkelstromtransienten;

Fig. 6 in a perspective view the time course of a roughness parameter and

Fig. 7 is a graph of the atomic structure of a Si (111) surface with a 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 A1.

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. B. 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 sections 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 tub 28 , the individual silicon bodies 1 are to be inserted into openings, the edge regions of which are provided with an electrically insulating double ring seal, that is to say an outer sealing ring 20 and an inner sealing ring 21 . 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. B. a counter electrode, a reference electrode, a transparent cover for the tub 28 , and a light source located above and equipment of a potentiostat / galvanostat are to be provided if necessary according to 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.

Fig. 3 shows an arrangement in which a device for the electrolytic, electro analytically monitored or controlled surface treatment of Si- bodies 1 and Meßgerätschaften 16 and control means 17 is provided, are determined with the aid of correlating process parameters, of which the electrochemical setting values for Control of the process sequence in subsequent devices - container 2 S₁, 2S₂,. . . , 2S _k - 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. An analogous behavior is shown in FIG. 5 for the current flowing in the smoothing surface of p-Si on application of an anodic potential stream. After a few oscillations, here five peaks, the potential is lowered. Then a dark current transient, known from DE 40 31 676 A1 already mentioned, was also 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 Å ± 8 Å (4 nm ± 0.8 nm) to 7 Å ± 1Å (0.7 nm ± 0.1 nm) on a 7 cm² area (see also Fig. 6 and associated description below).

The electrolyte solutions 7 were prepared from pure reagents and triple distilled water and had the compositions / pH values / temperatures given below

• - 0.1 M NH₄F; pH = 4.0, adjusted with H₂SO₄; 22 ° C ± 1 ° C for the STM investigation
• - 0.1 M NaF; pH = 4.0, adjusted with HF; 22 ° C ± 1 ° C for the FTIR experiment.

The following were used for the electrochemical measurements:

• - platinum counter electrode 4 ;
• - Saturated calomel electrode or 0.1 mol KCl / AgCl / Ag reference electrode 15 .

A potentiostat-galvanostat (from Heka, model 128).

The surface to be smoothed with n-Si was irradiated with white light 70 mW / cm² in the STM study and 20 mW / cm² in the FTIR experiment.

At a potential of + 2.8 V _NHE (normal hydrogen electrode) there was no photocurrent oscillation but at + 6 V _NHE . Based on ellipsometric investigations, it was found that optimal surface smoothing is achieved in the area of this potential setting.

A final etching step in a dark environment at a potential of + 0.78 V _NHE leads to hydrogen termination of the smoothed surface (approx. 80% to 90%, now 95% of a SiH monolayer with e.g. n-Si (111 ).

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 thus be ended by anodic oxidation.

The duration during which the current or photocurrent oscillation occurs can be determined with shorten a changed electrolyte composition. Such optimizations but are in proportion to the time spent on the whole Conditioning measures of wafers, including set-up times and the like for the treatments to be carried out and then possibly only perform minor contributions to profitability.

Figs. 6 omits the course of the roughness parameter R _a (dim: Å 1 Å = 0.1 nm) versus time (dim: minutes) recognize that a substantially smooth surface is obtained in less than an hour. In view of the fact that, when recording this course, greater emphasis was placed on determining the surface quality achieved than on the time required for the duration of the treatment, the time periods to be taken indicate an upper limit which in practice is easily undercut.

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 (111) -oriented silicon which, depending on the orientation of the steps, have one or two open bonds. There, material can be removed more easily than at locations with closed bonds, ie, 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 a surface smoothed to the lowest possible roughness. For the interface state density, which is characteristic or achievable as a quality standard, the values D _it (dim: eV ^-1 cm ^-2 ) show, at least in the tendency, improvements of one to two orders of magnitude compared to the values which have been achievable with conventional surface treatment methods.

Pretreatments of the Si wafers, which are required depending on the delivery condition, limit z. B. on chemical etching or must be previous include mechanical pre-cleaning processes or the like. All then Process steps to be carried out are electrochemical in nature, see Room temperature, do not require transportation of the wafers between them Process steps and are completed after a few minutes. The one there used, dilute fluoride solutions - NH₄F - can, for. B. for electrochemical etching, electropolishing and mainly for the electrolytic Smoothing and the immediately following hydrogen termination regarding Molarity and / or the pH value can be adjusted and are technically good manageable.

The electrical contact for the electrolytic treatment on the Si wafer Potential to be applied can be brought about via contact springs on the back the wafer are pressed. This eliminates both the effort and the risk of Damage avoided with attaching and removing one on the wafer firmly adhering contact are connected.

Generally sufficient for the surface treatment of Si Bodies a single pass through the "electrolytic treatment stages Smoothing / Hydrogen Termination ". In any case, this level is at one Surface treatment the essential and therefore at the end of the treatment too to which general and / or for the preparation of Treatments can follow processes that serve other purposes. A Electropolishing instead of electrolytic smoothing by current oscillation is not recommendable, but can this for the invention and its embodiments essential process step z. B. with particularly high roughness of a Si wafer in Favor delivery condition or after mechanical pre-cleaning.  

The electrolytic smoothing of the surfaces of p-Si in a dark environment requires no 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 essential for the invention and its embodiments Electrochemical treatment stages have so far been the minimum duration for the electrolytic smoothing by current oscillation 1 minute and for the subsequent Hydrogen termination can be determined for about 2 to 3 minutes. Longer Treatment times showed no significant changes in the results. The means that a relatively wide time window between minimum and maximum Treatment duration is available and optimal, adjustable in wide ranges Parameters are selectable.

Claims (14)

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
• b1) the surface to be treated is electrolytically smoothed in a fluoride-containing electrolyte solution essentially by anodic oxidation and 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.

2. The method according to claim 1, characterized in that the electrolytic smoothing of n-Si while irradiating the surface to be treated with light, the energy of which is at least equal to the energy of the band gap, is brought about, and the electroanalytical monitoring with the aid of the measured photocurrent occurs. 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 area power density of the light is at least 10 mW / cm² and thereby
• - For the clear periodicity of the photostromoscillation, a duration of between 10 minutes and 60 minutes, depending on the initial roughness of the surface to be treated, results.

4. The method according to claim 1, characterized in that the electrolytic smoothing of the surface to be treated by p-Si in darker Environment is brought about, and for the electroanalytical monitoring the measured currents flowing due to the applied potentials. 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 6 V ± 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 step of electrochemical etching after the anodic oxidation the hydrogen termination of the smoothed surface over a period of time takes place 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 electrolytic smoothing and electroanalytical Monitor on a reference copy of a batch of wafers using a correlating optical measurement of the Surface quality determined and the remaining copies of the batch alone on the basis of the correlating parameter values determined on the reference copy be treated.   9. The method according to claim 8, characterized in that the treatment of the reference copy as management treatment for the simultaneous Treatment of further copies of the same batch is carried out. 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.