CLEANING STEP OF A METHOD FOR MANUFACTURING A SUBSTRATE, THE METHOD COMPRISING THE DEPOSITION OF A THIN MONOCRYSTALLINE LAYER ON A SUPPORT

Abstract

The invention relates to an RCA-type cleaning step for a method of manufacturing a substrate by transferring a thin monocrystalline layer onto a support. The SC1 solution and the SC2 solution which make up this RCA cleaning are agitated by means of an acoustic field. The rinsing solution, in which the substrate is immersed directly before its immersion in the SC2 solution, is not agitated by means of an acoustic field. Figure 1

Description

Title of the invention: CLEANING STEP OF A METHOD FOR MANUFACTURING A SUBSTRATE, THE METHOD COMPRISING THE TRANSFER OF A THIN MONOCRYSTALLINE LAYER ON A SUPPORT FIELD OF THE INVENTION

[0001] The present invention relates to the field of composite substrates, that is to say substrates formed by transferring a thin monocrystalline layer onto a support. These may in particular be silicon-on-insulator (SOI) substrates. These substrates find particular applications in the fields of microelectronics, optoelectronics, and electromechanical microsystems. More particularly, the present invention relates to a cleaning step of a method for manufacturing such substrates. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Smart Cut™ technology, well known to those skilled in the art, very generally proposes manufacturing a composite substrate by transferring a thin layer, usually monocrystalline, onto a support.

[0003] In the methods based on this technology, the thin layer is taken from a donor substrate, which may for example be made of a solid monocrystalline silicon substrate. The thin layer is defined in the donor substrate during a weakening step by introducing light species, usually by implantation of hydrogen and / or helium ions, these species tending to form a fragile layer in the depth of the donor substrate, delimiting the thin layer. The donor substrate is then assembled to the support. A dielectric layer, typically a silicon oxide, is previously formed on the donor substrate and / or on the support. It is interposed, after assembly, between the transferred thin layer and the support.

[0004] The detachment of the thin layer from the rest of the donor substrate, and its transfer to the support, is obtained by mechanically or thermally stressing the assembly formed from the donor substrate and the support, which causes the fracture of the donor substrate at the level of the fragile layer.

[0005] This detachment step is followed by a sequence of finishing steps aimed at improving the crystalline quality of the transferred thin layer, its surface condition, its adhesion to the support, and at giving it, by thinning, a determined and very uniform thickness. This sequence is conventionally composed of a plurality of steps including annealing in a reducing or neutral atmosphere, sacrificial oxidations, stabilization, polishing, etc.

[0006] Regardless of the manufacturing process used, the composite substrate must meet very precise specifications. These relate in particular to the thickness and thickness uniformity of the thin layer and the dielectric layer.

[0007] The exposed face of the thin layer must also have a flawless surface condition. For this purpose, it is usual to inspect the substrate by dark-field microscopy using an incident light beam projected onto an inspection point scanning the exposed surface of the substrate. A light-collecting device and a detector make it possible to measure the scattered light at the inspection point. Such an inspection can in particular be carried out using the Surfscan™ SP1 inspection equipment from KLA.

[0008] When the inspected surface is perfectly flat and free of any particles, the light beam is reflected there, without undergoing any deviation, and no light radiation is collected and detected by the equipment. Conversely, any surface irregularity or particle present at the inspection point leads to spatial diffusion of the light of the beam according to preferred azimuthal and / or elevation angles which are collected and directed towards the detector. The measurement of this radiation constitutes a signature carrying information on the presence, position and / or nature of a surface defect or a particle at the inspection point.

[0009] The expression "Light Point Defect" or LPD (which could be translated into French as "light point defect") is usually used to refer to any surface defect leading to the production of diffuse radiation at the inspection point, measured by the detector. In this way, a map of the defects present on the exposed surface of the thin layer can be formed, these defects can be grouped into classes according to the signature of the collected radiation, or these defects can be counted to obtain an indicator of the quality or defect of the surface of the thin layer.

[0010] The particles present on the exposed surfaces of the donor substrate, the support and the thin layer, before and after each of the stages of the manufacturing process, are the cause of numerous LPD-type defects on the final substrate. This is why these surfaces are very meticulously and regularly cleaned during the manufacture of a composite substrate.

[0011] To implement some of these cleaning steps, it is known in particular to apply RCA cleaning (“RCA clean” according to the terminology of the field, acronym for Radio Corporation of America.) Such cleaning comprises the successive immersion of the substrate in a solution of SCI (“Standard Clean 1” for “standard cleaning 1”) containing ammonium hydroxide (NH40H), hydrogen peroxide (H2O2) and deionized water, then in a solution of SC2 (“Standard Clean 2” for “standard cleaning 2”) containing hydrochloric acid (HCl), hydrogen peroxide (H2O2) and deionized water.

[0012] RCA cleaning also includes immersing the substrate in rinsing solutions, typically deionized water, inserted before and / or after each of the immersions in the SCI, SC2 solutions.

[0013] The SCI solution is intended mainly to remove isolated particles on the surface of the substrate and particles buried in the vicinity of this surface, and to prevent them from redepositing. The SC2 solution is mainly intended to remove metallic contaminants which may have been deposited on the surface of the thin layer, in particular by forming chlorides.

[0014] A detailed description of this RCA cleaning and its evolution can be found in the brochure “RCA Critical Cleaning Process” dated 06 / 08 / 2007 and published by the company Microtech System, inc.

[0015] It is also customary, during RCA cleaning, to agitate the SCI solution by means of an acoustic field, for example by producing a megasonic or ultrasonic field. In this regard, reference may be made to the document “Megasonic Particle Removal from Solid-State Wafers” by Shwartzman, S., Mayer, A., and Kern., RCA Review, 46:81 (1985).

[0016] In view of their effects on the defectivity of the final substrate and their repetitions in the manufacturing process of this substrate, the cleaning steps therefore form important steps in this process. They have significant consequences both on the quality of the substrate and on the rate of the process, on its cost and its environmental impact, due to the consumption of the resources used, in particular chemical solutions and water. SUBJECT OF THE INVENTION

[0017] An aim of the invention is to propose a cleaning step of a method for manufacturing a substrate, this method comprising the transfer of a monocrystalline thin layer onto a support. This cleaning step is distinguished from the cleaning methods of the prior art. When used in the method for manufacturing the substrate, this cleaning step makes it possible to reduce the surface defect of the thin layer in comparison with the performance of other cleaning steps and / or to maintain this defect at a comparable level, but by reducing the immersion time and, consequently, the quantity of chemical resources required. BRIEF DESCRIPTION OF THE INVENTION

[0018] With a view to achieving one of these aims, the subject of the invention proposes a step of cleaning a substrate during a method of manufacturing a substrate composite. The manufacturing process comprises transferring a thin monocrystalline layer taken from a donor substrate onto a support substrate, the cleaning step comprising: a. immersing the substrate in a first container comprising a SCI solution, then; b. immersing the substrate in a rinsing container comprising a rinsing solution; c. then, directly after immersing the substrate in the rinsing container, immersing the substrate in a second container comprising a solution of SC2.

[0019] The SCI solution is agitated by means of an acoustic field during immersion of the substrate in the first. According to the invention, the rinsing solution is not agitated by means of an acoustic field during immersion of the substrate in the rinsing container and in the second container.

[0020] Surprisingly, by avoiding agitation of this rinsing solution, the cleaning step can be simplified without compromising the quality of the substrate.

[0021] According to other advantageous and non-limiting characteristics of the invention, taken alone or in any technically feasible combination: - the cleaning step comprises, between the immersion of the substrate in the first and the second container, its immersion in at least one other rinsing container comprising the rinsing solution, the rinsing solution present in the at least one other rinsing container being agitated by means of an acoustic field; - the cleaning step comprises, between the immersion of the substrate in the first and second container, its successive immersion in two rinsing containers comprising the rinsing solution; - the rinse solution includes deionized water; - the cleaning step comprises, prior to immersing the substrate in the first container, immersing the substrate in a rinsing container filled with ozonated deionized water.

[0022] According to another aspect, the invention proposes a method for manufacturing a composite substrate, this manufacturing method comprising the transfer of a thin monocrystalline layer taken from a donor substrate onto a support substrate by means of: a. a step of weakening the donor substrate; b. a step of assembling the donor substrate to the support substrate; c. a step of detaching the thin layer from the donor substrate; d. the application of a sequence of finishing steps of the thin layer transferred to the support substrate;

[0023] This method is remarkable in that it comprises, before or after one of these steps, the application of a cleaning step as defined previously.

[0024] According to other advantageous and non-limiting characteristics of this aspect of the invention, taken alone or in any technically feasible combination: - the cleaning step is applied directly after the detachment step; - the cleaning step is applied after a step of the finishing sequence; - the cleaning step is applied directly after the embrittlement step; - the thin monocrystalline layer is made of silicon. Brief description of the drawings

[0025] Other characteristics and advantages of the invention will emerge from the detailed description of the invention which follows with reference to the single appended figure in which:

[0026] [Fig.l]

[0027] [Fig.l] represents a measurement of the defectivity of a thin layer transferred onto a support, the method implementing a variety of cleaning steps. DETAILED DESCRIPTION OF THE INVENTION

[0028] This detailed description of the invention presents the experiments which were carried out by the Applicant and which led to the establishment of a cleaning step in accordance with the invention.

[0029] As a preamble, it is specified that these experiments were carried out by integrating the cleaning steps evaluated within a manufacturing process of a silicon-on-insulator substrate conforming to Smart Cut™ technology, the main steps of which were recalled in the introduction. This substrate consists of a stack formed of a thin layer of monocrystalline silicon, this thin layer forming a surface layer of the substrate whose thickness, at the end of its manufacturing, is of the order of 100 nm, of a dielectric layer of silicon oxide and of a support formed of a solid wafer of monocrystalline silicon.

[0030] More specifically, the cleaning steps evaluated were applied to this substrate directly after the step of detaching the thin layer from the donor substrate. It is noted that this detachment step leads to the generation of a high density of particles on the surface of the thin layer, these particles resulting from the “tearing off” of this layer from the donor substrate and / or from the rupture of fragile portions of this layer, particularly at its periphery which, at this stage of the process, is not in total adhesion with the support. The substrate obtained at the end of this detachment step therefore forms, due to the high density of particles present on the surface, a particularly interesting test vehicle to use to evaluate the effectiveness of a cleaning step.

[0031] The substrates cleaned according to the different cleaning steps evaluated were then subjected to the same finishing sequence, consisting here of a long smoothing anneal ("batch anneal" according to the Anglo-Saxon terminology used in the field) under a neutral atmosphere at a temperature between 1000°C and 1200°C.

[0032] The final substrates obtained at the end of this finishing sequence were then inspected using dark field microscopy inspection equipment such as that presented in the introduction to this application.

[0033] Such an inspection of a substrate makes it possible to measure the defect of the exposed surface of the thin layer by counting the detected LPD defects. These can be previously grouped by classes, for example on the basis of their size. The count can be carried out on each of the defect classes. The defect measurement can combine, in any suitable way, the counts carried out. In all cases, and whatever the way in which the data provided by the inspection equipment is used, a measurement of the defect of the exposed surface of the thin layer is available and, by comparison, it is possible to determine the relative effectiveness of two separate cleaning steps.

[0034] To return to the cleaning steps evaluated, these were carried out in cleaning equipment composed of containers filled with solutions in which a substrate is successively immersed. In such equipment, the containers are configured to maintain the temperature of the solutions they contain at a target temperature. They are also equipped with a transducer making it possible to develop an acoustic wave field whose power is controllable, in order to agitate the solution. In the experiments reported below, and when the transducer associated with a container was actually activated, the wave field had a frequency of 750 kHz and a power of 600 W.

[0035] A robotic mechanism of the cleaning equipment makes it possible to successively immerse a substrate in the containers, in a determined order and for a determined duration, in order to apply a perfectly controlled cleaning sequence to it. Generally, the robotic mechanism manipulates a nacelle in which a plurality of substrates are positioned in order to collectively apply the cleaning step to them.

[0036] The cleaning equipment is also provided with means allowing, at the end of immersion in the successive containers, to subject the substrate to its rapid rinsing. ("Quick Dump Rinse" as the expression generally used in the field) and its drying. These two steps are well known to those skilled in the art and will therefore not be detailed in this description, for the sake of brevity.

[0037] In the context of the present description and the experiments carried out by the Applicant, the equipment was composed of a first container comprising a SCI solution, a second container comprising a SC2 solution and a plurality of rinsing containers filled with a rinsing solution.

[0038] The SCI solution is maintained in the first container at a temperature within the conventional range of 70°C to 75°C. The SC2 solution was maintained in the second container at a temperature between 28°C and 32°C.

[0039] The rinsing solution which fills the rinsing containers consists of deionized water maintained at room temperature, 21°C.

[0040] In certain cases which will be detailed below, to promote the elimination of organic pollutants, and in particular hydrocarbons, the rinsing solution which fills the rinsing container in which the substrate is immersed before its immersion in the SCI solution consists of deionized water comprising gaseous ozone, the solution being maintained at ambient temperature, 21°C.

[0041] The following tables define seven different cleaning steps (Cleaning 1 to Cleaning 7) which were evaluated with a view to finding those leading to an acceptable compromise between the defectiveness of the final substrate, the duration of the treatment and the consumption of chemical resources.

[0042] Each cleaning step is defined by the immersion conditions of the substrate in each of the containers (numbered 1 to 7 in the lines of the table) of the cleaning equipment used. More precisely, for each immersion (each line of the table), the duration of this immersion, the agitation or the absence of agitation caused by means of the acoustic field have been specified.

[0043] It is first noted that all the cleaning steps evaluated had the same final immersions (lines 6 and 7 of the table) in the second container containing the SC2 solution (for 180s) and in the rinsing solution consisting of deionized water (for 180s). Since the experiments carried out sought to improve the defect of particulate origin, the main objective was to optimize the immersion conditions in the SCI solution and in the rinsing solutions which follow this immersion.

[0044] [Fig.l] represents the defect D of each of these cleaning steps (numbered 1 to 7 in this figure), measured by dark field microscopy, to detect LPD defects (expressed in arbitrary units in the figure, a relatively higher measurement denoting a relatively greater defect).

[0045] Cleaning numbered 3 forms a kind of reference cleaning: the substrate is immersed in the solutions contained in all the containers of the cleaning equipment. The rinsing and SCI solutions are all agitated by applying an acoustic field.

[0046] Cleaning numbered 7 forms a second reference cleaning, which differs from cleaning 3 by the longer duration (600s vs 346s) of immersion in the SCI solution. This duration of immersion and treatment of the substrate in this solution being longer, it is natural to observe that the measured defect (of particulate origin, we recall) is lower than that measured in the case of cleaning 3. However, this results in a longer duration of the cleaning step as a whole, which impacts the rate of the production line. In addition, a longer duration of immersion in the SCI solution requires renewing this solution more regularly, and the quantity of solutions used relative to the number of substrates treated is therefore higher, which, in an industrial context, is unfavorable. Order Container Cleaning 1 Cleaning 2 Cleaning 3 Cleaning 4 1 - Rinse Without 03 346s Agitation With 03 346s Agitation With 03 346s Agitation With 03 346s Agitation 2-SCI 180s Agitation 346s Agitation 346s Agitation 180s Agitation 3 - Rinse 180s Agitation 180s Agitation 180s Agitation 180s Agitation 4 - Rinse 180s Without agitation 180s Agitation 180s Agitation 180s Without agitation 5 - Rinse - - 180s Agitation - 6-SC2 180s Without agitation 180s Without agitation 180s Without agitation 180s Without agitation 7 - Rinse 180s Agitation 180s Agitation 180s Agitation 180s Agitation Order Container Cleaning 5 Cleaning 6 Cleaning 7 1 - Rinse With 03 346s Agitation With 03 346s Agitation With 03 346s Agitation 2-SCI 346s Agitation 346s Agitation 600s Agitation 3 - Rinse 180s Agitation 180s Agitation 180s Agitation 4 - Rinse 180s without agitation 180s Agitation 180s Agitation 5 - Rinse - 180s Without agitation 180s Agitation 6-SC2 180s Without agitation 180s Without agitation 180s Without agitation 7 - Rinse 180s Agitation 180s Agitation 180s Agitation

[0047] As can be seen in [Fig.l], cleanings 4, 5 and 6 have a defect measurement D intermediate between those of cleaning 3 and cleaning 7, taken as a reference.

[0048] In interpreting these results, the inventors of the present application observed that in these cleanings 4,5,6 the immersion in the rinsing solution which directly precedes the immersion in the SC2 solution was carried out without agitation. The performances are similar whether the immersion in a rinsing solution is repeated three times (as in cleaning numbered 6) or only twice (cleanings 4 and 5) omitting the immersion in line 5 of the table. In cleaning 4, the duration of the immersion in the SCI solution is even reduced to 180s.

[0049] Cleaning steps 4 and 5 are therefore particularly advantageous, for the reasons of throughput and resource saving already mentioned. This is also the case for cleaning step 6 which achieves a defect close to that of cleaning 7, but with a shorter immersion time in the SCI solution.

[0050] By comparing cleanings 4 and 5, it is observed that it is notably possible to omit an immersion in a rinsing solution, insofar as the immersion in the rinsing solution which directly precedes the immersion in the SC2 solution is carried out without agitation, without notably affecting the defect measurement.

[0051] It is noted that this advantageous configuration is also present in cleaning 1 of the table, without leading to the same level of performance as that of cleanings 4, 5 and 7. In this cleaning 1 however, the duration of the immersion in the first container containing the SCI solution is relatively reduced (180s) and the ozone of the rinsing solution which precedes the immersion in the SCI solution has been omitted. This result tends to show the benefit of this particular rinsing.

[0052] In cleaning 2, immersion in a rinsing solution was simply omitted, and it is observed that this omission tends to degrade the quality of the cleaning.

[0053] According to a possible interpretation of these results, the agitation of the solutions in which the substrates are immersed tends to break fragile portions of the transferred thin layer. These fragile portions are in particular those arranged at the periphery of the thin layer, which are not in strong adhesion with the support. The particles which result from these breakages can be removed during a subsequent rinsing immersion. However, the SC2 solution which tends to make the treated surface hydrophilic also tends to retain on this surface these particles which cannot be effectively removed. To avoid this phenomenon, it is understood that it is preferable not to agitate the rinsing solution by means of an acoustic field when it directly precedes immersion in the SC2 solution, in order not to cause the generation of particles which could not be removed.

[0054] Of course, the invention is not limited to the embodiments described and variant embodiments can be made without departing from the scope of the invention as defined by the claims.

[0055] Although the experiments presented were carried out on the surface of the thin layer directly after the fracture step, this in no way limits the possible use of a cleaning step in accordance with the invention. This cleaning step can be applied at the end of any step of a method of manufacturing a substrate by transferring a layer onto a support.

[0056] A cleaning step in accordance with the invention is of particular interest at the end of manufacturing steps likely to generate particles. This is the case, for example, during the steps of thinning the transferred thin layer, for example by sacrificial oxidation. During such a treatment, the etching solution of the surface oxide is likely to penetrate peripherally under the thin layer, which tends to suspend it and make it fragile. This is also the case for cleaning the donor substrate directly after it has undergone the embrittlement step, by implantation.

[0057] Furthermore, the effectiveness of a cleaning step according to the invention, in particular when it is applied after the step of detaching the thin layer, is in no way linked to a particular finishing sequence of the substrate. Additional tests carried out by the applicant have shown that the same benefits could be obtained for a wide variety of finishing sequences, for example sequences implementing rapid annealing rather than a single long annealing used in the experiments presented in the body of this description, or implementing a plurality of long annealings.

[0058] In some cases, it may be beneficial to repeat, in a loop, a plurality of times the cleaning step on the same substrate. For example, this may involve repeatedly applying this step two or three times.

Claims

Claims

1. A method of manufacturing a composite substrate comprising transferring a thin monocrystalline layer taken from a donor substrate onto a support substrate, the manufacturing method comprising a step of cleaning a substrate comprising: a. immersing the substrate in a first container comprising an SCI solution, then; b. immersing the substrate in a rinsing container comprising a rinsing solution; c. then, directly after immersing the substrate in the rinsing container, immersing the substrate in a second container comprising an SC2 solution; the SCI solution being stirred by means of an acoustic field during immersion of the substrate in the first container, the cleaning step being characterized in that the solution is not stirred by means of an acoustic field during immersion of the substrate in the rinsing container and in the second container.

2. A method of manufacturing a composite substrate according to the preceding claim comprising, between the immersion of the substrate in the first and second containers, its immersion in at least one other rinsing container comprising the rinsing solution, the rinsing solution present in the at least one other rinsing container being agitated by means of an acoustic field.

3. Method of manufacturing a composite substrate according to the preceding claim comprising between the immersion of the substrate in the first and the second container, its successive immersion in two rinsing containers comprising the rinsing solution.

4. A method of manufacturing a composite substrate according to one of the preceding claims wherein the rinsing solution comprises deionized water.

5. A method of manufacturing a composite substrate according to one of the preceding claims comprising, prior to immersing the substrate in the first container, immersing the substrate in a rinsing container filled with ozonated deionized water.

6. A method of manufacturing a composite substrate according to one of the preceding claims further comprising: a. a step of weakening the donor substrate; b. a step of assembling the donor substrate to the support substrate; c. a step of detaching the thin layer from the donor substrate; d. the application of a sequence of finishing steps of the thin layer transferred to the support substrate;

7. Manufacturing method according to the preceding claim in which the cleaning step is applied directly after the detaching step.

8. Manufacturing method according to the preceding claim in which the cleaning step is applied after a step of the finishing sequence.

9. Manufacturing method according to one of claims 6 to 8 in which the cleaning step is applied directly after the embrittlement step.

10. Manufacturing method according to one of claims 6 to 8 in which the monocrystalline thin layer is made of silicon.

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