JP2006303088A - Cleaning method of silicon substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cleaning method in which surface roughness can be recovered easily when it is worsened by cleaning in the fabrication process of an SOI substrate. <P>SOLUTION: Ozone cleaning (O<SB>3</SB>cleaning) is performed before substrates are stuck. The O<SB>3</SB>cleaning may be performed following to SC-1 cleaning or HF cleaning. Ozone concentration of ozone water is, preferably, set at 1 ppm or above. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a bonded silicon substrate, and more particularly to a cleaning method before bonding substrates as materials.

The so-called SOI (Silicon On Insulator) substrate formed by bonding hydrogen ion implanted substrates exhibits numerous excellent performances such as high speed, low power consumption, high temperature operation, and radiation resistance when an integrated circuit is formed. Therefore, it has attracted attention as a semiconductor substrate for next-generation VLSI.
Conventionally, as a method for manufacturing an SOI substrate, after forming an oxide film on the surface of the first silicon substrate by a thermal oxidation method or the like, a microbubble layer (injection layer) is formed inside the substrate by implanting hydrogen ions from the surface. A hydrogen ion implantation step, a bonding step in which the hydrogen ion implanted substrate is washed and then brought into close contact with the surface of the second silicon substrate, and a peeling step in which the two bonded substrates are separated from each other with the microbubble layer as a boundary And an annealing process for improving the adhesive strength of the peeled second silicon substrate, a planarization process for flattening the surface, and a thinning process for obtaining a predetermined thickness are known. (For example, refer to Patent Document 1).

This ion implantation delamination method is an extremely excellent method for manufacturing bonded SOI substrates. However, in order to produce these SOI substrates at a mass production level with a high yield, bonding defects called voids generated at the bonding interface are known. Need to be reduced.
The method described in Patent Document 1 proposes a method in which substrate cleaning and hydrogen ion implantation are divided into several times to remove particles. As a cleaning method, cleaning with ammonia, hydrogen peroxide, pure water, electrolytic ion water, a mixed solution of NH ₄ OH / H ₂ O ₂ / H ₂ O (SC-1) or the like is proposed.

In addition, as another method for obtaining a bonded substrate free from blister defects by removing particles, cleaning the _two substrates with a mixed solution of NH ₄ OH / H ₂ O ₂ / H ₂ O (SC-1 cleaning). A temperature of the cleaning liquid in the SC-1 cleaning step, including a step of drying the cleaned substrate, a step of bonding the dried substrate, and a step of thinning one of the bonded substrates. A method of manufacturing a bonded substrate having a temperature of 25 ° C. or more and 60 ° C. or less has also been proposed (see, for example, Patent Document 2). According to this method, it is possible to prevent the occurrence of blister defects and void defects even for a bonded substrate having a large diameter exceeding 300 mm, and the manufacturing yield of the bonded substrate can be greatly improved. It is said that.
International Publication No. 01-093334 Pamphlet JP 2003-309101 A

When a silicon substrate is washed with an aqueous solution containing hydrogen peroxide or hydrofluoric acid, the surface roughness of the substrate surface deteriorates, and when bonding is performed and then separated, the region where the ion implantation layer does not separate, that is, bonding is performed. A region (void) that peels off at the interface occurs.
Conventionally, since there is no recovery means once the surface roughness is deteriorated, it has been necessary to strictly control the temperature control and the cleaning time so as not to roughen the surface.
This invention is made | formed in view of the above situations, Comprising: It aims at providing the cleaning method which can recover surface roughness easily, even when surface roughness deteriorates by washing | cleaning.

In order to solve the above problems, a method for cleaning a silicon substrate according to the present invention is a method for cleaning a substrate, characterized in that in the method for manufacturing a bonded SOI substrate, ozone water cleaning (O3 cleaning) is performed before bonding the substrates. Adopted.
In the silicon substrate cleaning method of the present invention, ozone water cleaning may be performed subsequent to SC-1 cleaning or HF cleaning.
Moreover, it is preferable that the ozone concentration of ozone water shall be 1 ppm or more.

  According to the present invention, even if the surface roughness deteriorates in the step of completely removing particles by SC-1 cleaning or HF cleaning, the surface roughness can be recovered again, so that the void peels off at the bonding interface. Thus, the manufacturing yield can be improved and an SOI substrate can be provided at a low cost.

  FIG. 1 shows an outline of a manufacturing process of an SOI substrate using the silicon substrate cleaning method of the present invention. This SOI substrate manufacturing process includes an oxide film forming step (a) in which an oxide film is formed on the surface of the first silicon substrate by a thermal oxidation method or the like, and a microbubble layer (injection) is injected into the substrate by injecting hydrogen ions from the surface. Layer)), a cleaning step (c) for cleaning the hydrogen ion-implanted substrate, a bonding step (d) for closely contacting the surface of the second silicon substrate, and two bonded sheets An exfoliation process step (e) for exfoliating the substrate with the microbubble layer as a boundary, a flattening process step (f) for flattening the microbubble layer surface of the exfoliated second silicon substrate surface, and a predetermined thickness. It manufactures through a thinning process step (g).

The outline of processing in each step will be described with reference to FIGS.
First, as shown in FIG. 2A, an oxide film 11 is formed on the surface of a first silicon substrate (top wafer) 1 by thermal oxidation, CVD, or the like. The thickness of the oxide film is preferably about 100 to 300 nm.
If a single crystal silicon substrate is used as the first silicon substrate 1, an SOI substrate having an SOI layer with a very uniform film thickness can be manufactured at low cost, so that it can be widely applied to various devices. A single crystal silicon substrate is also suitable for increasing the diameter.

Next, as shown in FIG. 2B, hydrogen ions of 2 × 10 ¹⁶ / cm ² to 1 × 10 ¹⁷ / cm ² are implanted from the surface of the oxide film 11. Then, a hydrogen ion implanted layer 12 is formed at a desired depth under the oxide film 11 via the active silicon layer 13. The depth of the hydrogen ion implantation layer determined by the energy at the time of ion implantation determines the thickness of the active silicon layer. Since the hydrogen ion implantation layer 12 includes many lattice defects and is made of a brittle material, it becomes the separation surface S in the subsequent separation step.

  Next, prior to the bonding step, the second silicon substrate to be bonded to the first silicon substrate that has been implanted with hydrogen ions is cleaned. Since the cleaning method is the most characteristic part of the present invention, it will be described in detail later.

Next, the first silicon substrate 1 and the second silicon substrate that have been cleaned are overlapped and bonded together. As the second silicon substrate (base wafer) 2, a single crystal silicon substrate that is similarly mirror-polished can be used.
If a single crystal silicon substrate is used as the second silicon substrate, a substrate having excellent flatness can be obtained, and a large-diameter substrate having a diameter of 200 mm, 300 mm or more can also be obtained.
As shown in FIG. 2 (d), the first silicon substrate 1 is turned upside down on the mirror-polished surface of the second silicon substrate 2 at room temperature and overlapped with the oxide film 11 face down. Match.

  Next, in the peeling step shown in FIG. 3E, the peeling surface S can be used as a boundary to peel off.

On the peeled surface of the second silicon substrate (base wafer) 2 having the thin hydrogen ion implantation layer 12 on the surface thus obtained, irregularities of about several tens of nanometers are formed. Therefore, the surface is smoothed by using a mechanical polishing method or a chemical polishing method as necessary (see FIG. 3F).
Finally, as shown in FIG. 3G, the remaining hydrogen ion implantation layer 12 and part of the active silicon layer 13 are removed to obtain an SOI substrate having a desired thickness.

Here, the description returns to the cleaning step of FIG. As is usually done in the present invention, the first silicon substrate into which hydrogen ions have been implanted is cleaned before being bonded to the second silicon substrate. At this time, the greatest feature is to use ozone cleaning. Cleaning is performed to remove particles adhering to the surface of the silicon substrate to be bonded.
Even if it is for removing particles, if the ion-implanted substrate on which the oxide film is formed is washed more than necessary with, for example, a hydrofluoric acid aqueous solution, the surface roughness deteriorates, and then bonding and further peeling are performed. A region that cannot be separated from the ion-implanted layer as a boundary, that is, a region (void) that is separated from the bonded surface is generated.

Even when the surface roughness deteriorates as described above, the surface roughness is recovered by performing ozone water cleaning (O3 cleaning), and then voids are prevented in the case of further bonding and peeling. It can be peeled cleanly with the hydrogen ion implanted layer as a boundary.
Moreover, since O3 cleaning also has an effect of removing organic substances on the surface, voids and blister defects can be prevented (reference: Japanese Patent Laid-Open No. 2000-30992)
An aqueous solution in which ozone is dissolved is used for O3 cleaning, but an ozone (O ₃ ) concentration that generates ozone of 1 ppm or more is necessary. If the O ₃ concentration is less than 1 ppm, the surface roughness is reduced even after cleaning for a long time. It cannot be recovered. At an O ₃ concentration of _{1 to 10} ppm, the surface roughness is recovered, but cleaning requires a long time. Accordingly, the concentration is preferably 10 ppm or more.
Further, the O3 cleaning time is not particularly limited and depends on the roughness of the substrate surface. That is, it takes a long time to recover when the surface roughness is rough, and when the surface roughness is not so rough, the O3 cleaning time may be short.

  Regarding the surface roughness, the surface roughness is measured with an AFM (atomic force microscope), and the cleaning time for O3 cleaning is determined based on the surface roughness. Specifically, it is necessary to determine the O3 cleaning time so that the finished surface roughness after O3 cleaning is 0.1 nm (measurement region: 10 × 10 μm) or less. For example, when the SC-1 or HF cleaning results in 0.1 to 0.2 nm (measurement region: 10 × 10 μm) or more, the cleaning time is set to 20 minutes or more, and further 0.2 to 0.3 nm ( When the measurement area is 10 × 10 μm) or more, the cleaning time is 40 minutes or more. Thus, as the value of the surface roughness increases, it is necessary to lengthen the O3 cleaning time.

The relationship between surface roughness and appropriate conditions for O3 cleaning was examined. Table 1 shows that a silicon substrate having a surface roughness of 0.1 to 0.2 nm was cleaned by changing the O ₃ concentration of O ₃ cleaning and the cleaning time, and the improvement of the surface roughness was investigated. In Table 2, the same investigation was conducted on a silicon substrate having a surface roughness of 0.2 to 0.3 nm. The results are shown in Tables 1 and 2.

In the case where the substrates cleaned under the conditions described in Tables 1 and 2 as “improvement to <0.1” were used for bonding, no peeling area was generated at the bonding interface. In addition, when bonding was performed using a substrate cleaned under the condition described as “no improvement effect”, a region peeled off at the bonding interface occurred.
From the results of Tables 1 and 2, it can be seen that the O ₃ concentration of the O ₃ cleaning liquid needs to be 1% or more. It can also be seen that the higher the O ₃ concentration of the O ₃ cleaning solution, the shorter the required cleaning time, and the longer the surface roughness before cleaning, the longer the required cleaning time.

In the present invention, O3 cleaning is SC-1 (H ₂ O / H ₂ O ₂ / NH ₄ OH-based cleaning solution, 70 ° C., 3 minutes immersion) cleaning or HF (oxygen saturation ultrapure with addition of 5 to 50 ppm of HF). Water, room temperature) can be used in combination with acid cleaning such as cleaning or alkaline cleaning.
If an example of a washing | cleaning process is shown, it will wash | clean in order of an alkaline type washing tank-> pure water rinse tank-> acid type washing tank-> O3 washing tank-> pure water rinse tank. It is effective to perform O3 cleaning after the surface becomes rough by acid cleaning.

(Example)
A silicon substrate having a diameter of 200 mm on which an oxide film having a thickness of 150 nm is formed is implanted with hydrogen ions at a dose of 1 × 10 ¹⁷ / cm ² , an acceleration voltage of 50 keV, and a beam current of 20 A.
The following cleanings (a) to (d) were performed as substrate cleaning before substrate bonding.
(A) SC-1 cleaning; NH ₄ OH: 1% / H ₂ O ₂ : 2.5% / H ₂ O cleaning solution, 60 ° C., immersion for 5 minutes,
(B) Rinse: Ultrapure water, immersed for 5 minutes,
(C) HF cleaning; HF 0.1%, immersion for 5 minutes,
(D) O3 cleaning; _O 3 0.1%, since the silicon surface roughness of the substrate after cleaning SC-1 cleaning and HF cleaning for 20 minutes was 0.2nm (10 × 10μm), O3 cleaning time 20 Minutes.
Thereafter, the substrate was bonded to a silicon substrate having a diameter of 200 mm, an SOI substrate was formed by a peeling process, and the occurrence of a void region peeled off at the bonding interface was inspected by visual appearance inspection. As a result, no void area was observed.

(Comparative example)
For the silicon substrate implanted with hydrogen ions similar to the example, O3 cleaning was omitted as cleaning before bonding, and the following cleanings (a) to (d ′) were performed.
(A) SC-1 cleaning; NH ₄ OH: 1% / H ₂ O ₂ : 2.5% / H ₂ O cleaning solution, 60 ° C., immersion for 5 minutes,
(B) Rinse: Ultrapure water, immersed for 5 minutes,
(C) HF cleaning; HF 0.1%, immersion for 5 minutes,
(D ′) rinse; ultrapure water, immersed for 5 minutes,
Thereafter, the substrate was bonded to a silicon substrate having a diameter of 200 mm, an SOI substrate was formed by a peeling process, and the occurrence of a void region peeled off at the bonding interface was inspected by visual appearance inspection. As a result, several void areas were observed.

It is a figure which shows the outline of the manufacturing process of an SOI substrate. It is a figure explaining the outline | summary of the process in each process of FIG. It is a figure explaining the outline | summary of the process in each process following FIG.

Explanation of symbols

1... 1st silicon substrate 2... 2nd silicon substrate 11... Oxide film, 12. ... Active silicon layer

Claims (3)

  In the method for manufacturing a bonded SOI substrate by hydrogen ion implantation, a method for cleaning a silicon substrate, wherein ozone water cleaning (O3 cleaning) is performed before bonding the substrates.   2. The method for cleaning a silicon substrate according to claim 1, wherein ozone water cleaning is performed subsequent to SC-1 cleaning or HF cleaning. The method for cleaning a silicon substrate according to claim 1 or 2, wherein an ozone concentration of the ozone water is 1 ppm or more.

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