WO2016151001A1

WO2016151001A1 - Method of reduction of defects and method of fabrication of soi structures comprising such method

Info

Publication number: WO2016151001A1
Application number: PCT/EP2016/056346
Authority: WO
Inventors: Oleg Kononchuk; Aurélien MAK
Original assignee: Soitec
Priority date: 2015-03-24
Filing date: 2016-03-23
Publication date: 2016-09-29
Also published as: FR3034108A1

Abstract

The present invention relates to a method of reduction of defects for substrates comprising silicon, in particular formed by Czochralski process, comprising a first thermal treatment in a substantially non-oxidizing atmosphere and a second thermal treatment in an oxidizing atmosphere., and to a method of fabrication of a silicon-on-insulator substrate obtained by layer transfer technique further comprising said method of reduction of defects.

Description

Method of reduction of defects and method of fabrication of SOI structures comprising such method

Field of the invention

The present invention relates to a method of reduction of defects in substrates comprising crystalline silicon, in particular monocrystalline silicon as obtained by crystal growth methods such as for example Czochralski process.

Background of the invention

The objective of the invention is to propose a method of reduction of defects in silicon which are inherent due to its production method such as silicon ingot production using the Czochralski process.

The basics of the Czochralski process are explained as follows. A precisely oriented seed crystal is dipped into molten silicon and the rod on which is mounted the seed crystal is slowly pulled upwards and rotated simultaneously. The precise control of the temperature gradient, the rate of pulling and speed of rotation allows the extraction of large single crystal cylindrical ingots from the melt.

It has been found that the control of above mentioned parameters is essential in order to control the amount of defects and the type of defects in such ingots, especially for applications in microelectronics for which even the slightest amount of defects would render subsequently formed devices unusable. In order to obtain so-called near perfect crystals of silicon one has to precisely control the ratio of the rate of pulling v_p to the temperature gradient dT such that v_p/dT is as close as possible to the value 2x10^"5 cm²/Ks. Only this value allows to grow near perfect crystals suitable for microelectronic applications. Already slight deviations from above value leads to the incorporation of defects in the solidified silicon ingot, the types of defects can be mainly qualified as either being of vacancy type or interstitial type, for values above or below above threshold, respectively. Such defects may have a spatial extent of several nanometers, in particular 1 nm to 5nm or even 10nm. This might be the case whenever one chooses growth conditions rather far away from the critical value v_p/dT mentioned above whereby vacancy or interstitial type defects as well as so-called oxygen precipitation defects in the nanometer range or even larger can occur. Such rather large defects have detrimental effects on microelectronic applications. Closer to the critical value the spatial extent of created vacancy or interstitial type defects might be in the range of tenth of A. These small defects might not be directly detrimental to the substrate quality, but for instance small vacancy type defects play an essential role as nucleation sites for oxygen precipitation defects of larger extent, such precipitation might occur during temperature treatments necessary in device processing of such substrates. As already mentioned such larger defects are particularly detrimental to device applications having the same range of magnitude, as it is the case for instance for so-called fully depleted silicon on insulator (FD-SOI) substrates obtained by layer transfer of a thin silicon layer with thickness in the range of 5nm to 10nm. For more details regarding defects in silicon we refer either to "Grown-in defects in silicon produced by agglomeration of vacancies and self-interstitials" by Voronkov et al, Journal of crystal growth 04/2008, 310, pages 1307-1314, or to "The engineering of intrinsic point defects in silicon wafers and crystals" by Falster et al., MRS Bulletin June 2000, pages 28-32. Techniques such as layer transfer, as for instance obtained by the well- known SmartCut™ technique, are rather cost extensive, and in order to increase the throughput for above mentioned types of substrates it is necessary to seek for methods of reduction of defects in the base material which is silicon obtained for instance by the Czrochalski process.

It is well-known (see for instance "Comparison of the impact of thermal treatments on the second and on the millisecond scales on the precipitation of interstitial oxygen" by Kissinger et al., ECS J. Solid State Sci. Technol. 2012, Volume 1 , Issue 6, pages 269-275) that defects of the vacancy type can be healed by a thermal treatment in an oxidizing atmosphere at temperature in the range of 1000°C to 1 150°C and, thus, leading to final defect densities well below 10⁵ cm-3 (with respect to the volume) which is acceptable for microelectronic applications. Concerning interstitial type defects one might refer to thermal treatments requiring temperatures as high as 1200°C to 1250°C as proposed in US6635587. However such high temperature treatment alone leads to the creation of vacancy type defects which play the detrimental role of nucleation sites for oxygen precipitation. One is therefore in need of a method of reduction of defects able to reduce the overall density of defects independent of the kind of defect (vacancy and/or interstitial) which might be present even in mixed form.

The present invention is obviating the above mentioned problems which allows to reduce the defect density, in a substrate comprising silicon formed by growth methods, such as Czochralski process, and giving thus more flexibility in the choice of the starting material as the defect density of the as- grown silicon substrate can be healed. Description of the invention

In particular, the present invention relates to a method of reduction of defects for substrates comprising silicon, in particular formed by Czochralski process, comprising a first thermal treatment in a substantially non-oxidizing atmosphere and a second thermal treatment in an oxidizing atmosphere.

Further advantageous embodiments relate to a method of reduction of defects according to claim 1 wherein the first thermal treatment is performed at a temperature in the range of 1 150°C to 1300°C, preferentially in the range of 1200°C to 1250°C. Further advantageous embodiments relate to a method of reduction of defects according to one of the preceding claims wherein the substantially non-oxidizing atmosphere of the first thermal treatment is an argon atmosphere containing oxygen of at most l OOOppm, more preferentially 500ppm, even more preferentially 200ppm, or even more preferentially containing no oxygen.

Further advantageous embodiments relate to a method of reduction of defects according to one of the preceding claims wherein the second thermal treatment is performed at a temperature in the range of 900°C to 1 150°C, preferentially in the range of 950°C to 1 100°C. Further advantageous embodiments relate to a method of reduction of defects according to one of the preceding claims wherein the oxidizing atmosphere of the second thermal treatment is an argon atmosphere containing oxygen in the range of 100ppm to l OOOOppm, preferentially 800ppm to l OOOppm. Further advantageous embodiments relate to a method of reduction of defects according to one of the preceding claims wherein the first thermal treatment is performed with a duration of at most 60s, preferentially 20s, or even more preferentially 10s. Further advantageous embodiments relate to a method of reduction of defects according to one of the preceding claims wherein the second thermal treatment is performed with a duration of at most 60s, preferentially 20s, or even more preferentially 10s.

Further advantageous embodiments relate to a method of reduction of defects according to one of the preceding claims wherein the heating and/or the cooling rate of the first thermal treatment is higher than 5 °C/s, more preferentially higher than 50°C/s.

Further advantageous embodiments relate to a method of reduction of defects according to one of the preceding claims wherein the heating and/or the cooling rate of the second thermal treatment is higher than 5 °C/s, more preferentially higher than 50°C/s.

Further advantageous embodiments relate to a method of reduction of defects according to one of the preceding claims wherein the first thermal treatment is followed by a cooling step in order to obtain the temperature of the subsequent second thermal treatment.

The present invention also relates to a method of fabrication of a silicon-on- insulator substrate obtained by layer transfer technique further comprising a step of reduction of defects according to one of the preceding claims. The present invention relates also to a method of reduction of defects for substrates comprising silicon as well as a method of fabrication of a silicon- on-insulator substrate comprising a step according to the method of reduction of defects. The present invention will now be described with reference to specific embodiments. It will be apparent to the skilled person that features and alternatives from any of the embodiments can be combined, independently of each other, with features and alternatives of any other embodiment in accordance with the scope of the claims. As already mentioned two types of intrinsic defects, so-called vacancy and interstitial type, are formed depending on the local conditions under which the silicon is solidified, in particular on the above mentioned ratio _vp/dT where v_p is the rate of pulling and dT is the local axial temperature gradient in the crystal near the melt-solid interface. It is well-known that both types of defects can recombine which results in supersaturation of one species and an undersaturation of the other. Most of silicon produced is grown under conditions resulting in of almost only vacancy type defects and one approach to improve and tailor the reactions leading to such vacancy defects in order to reduce their density. Such engineering might be obtained by a thermal treatment in an oxidizing atmosphere. This kind of treatment has a sort of stretching effect on the lattice parameter which, once a local vacancy type defect is encountered, allows to recombine with the latter and relax to the normal equilibrium lattice parameter of defect free silicon. However, such a treatment is well-known not to have any effect on the remaining type of interstitial defects which might still be present in such silicon substrates. The present invention addresses a combination of thermal treatments leading to nearly defect free silicon containing substrates. Such control and reduction of defects can be obtained near the surface of the respective substrate or even deep into the bulk of said substrate, depending on the conditions of the thermal treatments. Such control is particularly interesting for application like fully depleted silicon on insulator substrates (FD-SOI) for which very thin layers of silicon (thickness range from 1 nm to 5 nm, or even up to 10 nm) are involved.

A thermal treatment generally involves a step of increasing the temperature following a predetermined heating rate up to a temperature level. This temperature level is situated in the temperature range suitable for the thermal treatment, for the duration of the thermal treatment. The temperature level might be varied during the duration of the thermal treatment within the range applicable for the thermal treatment. The thermal treatment at the temperature level is followed by a step of decreasing the temperature following a predetermined cooling rate down to a second temperature level, in particular ambient temperature conditions. As will be pointed out later, in some embodiments it might be interesting to choose the second temperature level in the range of a subsequent thermal treatment. The combination of thermal treatments in our present invention comprises a first thermal treatment applied to the silicon containing substrate in a substantially non-oxidizing atmosphere. Such non-oxidizing atmosphere is preferentially a pure argon atmosphere, but other types are also envisageable as for instance H₂ or N₂. Further, in case of argon atmosphere (or any other inert gas), one might add slight amount of oxygen. The introduction of oxygen influences the ability of the first thermal treatment to treat the interstitial type of defects. However, even with oxygen contents up to l OOOppm a considerable effect in reduction of such defect type can be achieved. The lower the amount of oxygen the better is the reduction of interstitial type defects. The introduction of oxygen therefore deviates from the ideal non-oxidizing pure argon atmosphere, and oxidizing effects and reactions occur at the surface to the treated substrate. Preferentially an oxygen content below l OOOppm, or even more preferential below 500ppm, or even more preferentially below 200ppm, or even more preferentially no oxygen, is used. However, a process window in between 1 ppm and 100ppm should be avoided as such conditions lead to etching of the surface due to the chemical reaction producing volatile SiO. Oxygen contents higher than 100ppm however lead to the stable S1O2 oxide which does not detrimentally affect the surface roughness properties. Therefore, and particularly interesting for applications as the SOI process for which thin layers are transferred (and bonding properties and parameters such as roughness have to optimized), either a high purity argon atmosphere should be used which, in turn, might be technically difficult to control, or an argon atmosphere containing slight amount of oxygen above 100ppm should be used in order to avoid surface degrading process window mentioned above. This first treatment is held in the range of 1 150°C to 1300°C, preferentially in the range of 1200°C to 1250°C. The duration for the first thermal treatment is thereby at most 60s, preferentially 20s, or even more preferentially 10s.

The first thermal treatment is particularly important for defect of interstitial type and leads to a significant reduction in the density of this type of defects. However the reduction of the interstitial type of defect is simultaneously accompanied by a raise of the density of vacancy type defects. The first thermal treatment leads to a somewhat conversion of defect type. The extent of this reduction and conversion of defect density on the substrate containing silicon depends on above mentioned parameters of temperature, duration, but also the rate used to achieve and control temperature. Further the thickness of the substrate also plays a role as thermal equilibrium conditions invoke the boundary conditions to a certain extent. For thicker substrates longer annealing conditions and/or higher temperatures might necessary in order to have an effect deeper located in the substrate with respect to its surface. The heating rate as well as the cooling rate for the first thermal treatment should be rather steep, in particular higher than 5°C/s, or preferentially higher than 50°C/s, in order to avoid the effect of other temperature ranges on the silicon containing substrate. For instance as it is well-known that a thermal treatment in between 800°C to 900°C would lead to oxygen precipitation using small vacancy type defects as nucleation sites, it is important not to maintain such temperatures for long duration, as for instance several seconds. It would be even advantageous to avoid such temperature exposition to silicon containing substrates comprising such type of vacancy defects. The combination of thermal treatments further comprises a second thermal treatment in an oxidizing atmosphere, as already indicated above important for the vacancy defects remaining in the substrate containing silicon. Such oxidizing atmosphere is preferentially an argon atmosphere containing small amounts of oxygen in the range of 100ppm to l OOOOppm, preferentially 800ppm to l OOOppm. This second treatment is held in the range of 900°C to 1 150°C, preferentially in the range of 950°C to 1 100°C. The duration for the first thermal treatment is thereby at most 60s, preferentially 20s, or even more preferentially 10s. Such second thermal treatment leads to a reduction of the density of vacancy defects as already mentioned above, and the combination with the first thermal treatment allows an overall reduction of both types of defects, leading thus to nearly defect free silicon containing substrates. Similar reasoning applies regarding the influence of the parameters on the effect of the method of reduction of defects of the second thermal treatment regarding the thickness of the substrate.

Similar reasoning applies also for the cooling and heating rate for the second thermal treatment. For instance after the second thermal treatment it might be possible that still very small amounts of vacancy type defects exist in the silicon containing substrate for which one should avoid oxygen precipitation as mentioned above at temperatures around 800°C to 900°C.

One advantageous embodiment would be the immediate subsequent performance of both thermal treatments in order to avoid an intermediate cooling step passing through the lower temperature region of 800°C to 900°C.

Such method of reduction of defects is particularly interesting for so-called silicon on insulator substrates (SOI), in particular above mentioned FD-SOI substrates.

SOI substrates are generally formed by so-called well-known layer transfer techniques such as SmartCut™. In that case implantation of hydrogen and/or helium leads to the creation of a weakened zone in the silicon substrate. After formation of a precisely controlled oxide on the surface of said silicon substrate molecular bonding is performed in order to assemble the implanted silicon substrate with its oxide finished surface onto another receiver substrate, in particular another silicon substrate. Subsequent splitting at the weakened zone due to thermal and/or mechanical stress allows a thin layer of silicon material remaining, separated by its oxide from the underlying received substrate. Other possibilities than implantation based SmartCut™ might be bonding of a silicon substrate with its respective oxide to another silicon substrate and mechanical polishing/grinding in order to reduce the thickness of the remaining silicon layer. The method of reduction of defects of our present invention can be directly applied and integrated in such a fabrication method of silicon on insulator substrates, and that even at all stages of above mentioned steps of the involved layer transfer technique. It can be applied once the SOI structure obtained, but preferentially before the bonding.

Claims

1 . Method of reduction of defects for substrates comprising silicon, in particular formed by Czochralski process, comprising a first thermal treatment in a substantially non-oxidizing atmosphere and a second thermal treatment in an oxidizing atmosphere.

2. Method of reduction of defects according to claim 1 wherein the first thermal treatment is performed at a temperature in the range of 1 150°C to 1300°C, preferentially in the range of 1200°C to 1250°C.

3. Method of reduction of defects according to one of the preceding claims wherein the substantially non-oxidizing atmosphere of the first thermal treatment is an argon atmosphere containing oxygen of at most l OOOppm, more preferentially 500ppm, even more preferentially 200ppm, or even more preferentially containing no oxygen.

4. Method of reduction of defects according to one of the preceding claims wherein the second thermal treatment is performed at a temperature in the range of 900°C to 1 150°C, preferentially in the range of 950°C to 1 100°C.

5. Method of reduction of defects according to one of the preceding claims wherein the oxidizing atmosphere of the second thermal treatment is an argon atmosphere containing oxygen in the range of 100ppm to l OOOOppm, preferentially 800ppm to 10OOppm.

6. Method of reduction of defects according to one of the preceding claims wherein the first thermal treatment is performed with a duration of at most 60s, preferentially 20s, or even more preferentially 10s.

7. Method of reduction of defects according to one of the preceding claims wherein the second thermal treatment is performed with a duration of at most 60s, preferentially 20s, or even more preferentially 10s.

8. Method of reduction of defects according to one of the preceding claims wherein the heating and/or the cooling rate of the first thermal treatment is higher than 5 °C/s, more preferentially higher than 50°C/s.

9. Method of reduction of defects according to one of the preceding claims wherein the heating and/or the cooling rate of the second thermal treatment is higher than 5 °C/s, more preferentially higher than 50°C/s.

10. Method of reduction of defects according to one of the preceding claims wherein the first thermal treatment is followed by a cooling step in order to obtain the temperature of the subsequent second thermal treatment.

1 1 . Method of fabrication of a silicon-on-insulator substrate obtained by layer transfer technique further comprising a step of reduction of defects according to one of the preceding claims.