JPH06350063A - Manufacture of semiconductor substrate - Google Patents

Abstract

PURPOSE:To provide substrates having good film thickness distribution with good productivity and to provide a manufacturing-by-lamination method for SOI substrates which lowers an interface level and suppresses the generation of fine voids, by making a bonding of a single crystal Si layer to an insulator layer (I layer) secure and high-quality. CONSTITUTION:This manufacture of semiconductor substrate concerns a manufacturing method for semiconductor substrates having a process of closely attaching, through the medium of an oxide film 103, two substrates 101-103 being Si substrates at least one of which has an oxide film 103, and making the bond of the said closely attached interface firmer by heat treatment, and it is characterized by implanting impurities 107 whose bond with Si is stronger than hydrogen beforehand into the oxide film 103 before the adhesion work, and by removing the above-mentioned porous part 101 of the closely-attached substrates selectively by etching.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing an SOI semiconductor substrate obtained by a bonding method.

[0002]

2. Description of the Related Art The formation of a single crystal Si semiconductor layer on an insulator is performed by using a Silicon on Insulator (SO
I) A large amount of research has been done because this substrate has a number of advantages that are widely known as technology and cannot be reached by a bulk silicon substrate for producing a normal silicon integrated circuit. That is, by using the SOI technology, 1.
1. Easy dielectric isolation and high integration 2. Excellent radiation resistance 3. Stray capacitance is reduced and high speed is possible,
4. 4. The well process can be omitted. Latch-up can be prevented, 6. Advantages such as a fully depleted field effect transistor can be obtained by thinning the film.

One of the SOI technologies that has been receiving the most attention in recent years is a technology commonly called "bonded SOI". This is because the mirror surfaces of two wafers, at least one of which has an insulating film formed by oxidation etc., are brought into close contact with each other, and heat treatment is performed to strengthen the bond at the close contact interface, and then the substrate is polished from either side. Alternatively, it is a technique of leaving a silicon single crystal thin film having an arbitrary thickness on the insulating film by etching. Since the thin film obtained by this bonded SOI is originally a single crystal substrate itself, the crystal orientation is not only controllable but also crystal defects are extremely small, and the crystal perfection is the best among many SOI technologies. It is thought that

[0004]

However, there are still problems to be solved in the conventional technique using this bonding method.

The first problem is the controllability of the film thickness in the step of uniformly thinning one side of the two bonded silicon substrates. That is, a silicon substrate having a thickness of several hundred μm must be uniformly polished or etched to a thickness of several μm or 1 μm or less, which is technically extremely difficult in terms of controllability and uniformity. Since the film thickness distribution causes variations in the electrical characteristics of the elements formed on it, there is an urgent need to solve this problem.

As a second problem, an unbonded portion (hereinafter referred to as a "void") generated at a contact interface between two substrates.
There is control of. Voids are minute (several μm) attached to the interface.
(Or less) One of the causes is dust etc., but other than that, it is simply bubbles taken in at the time of bonding, water vapor generated by a chemical reaction at the interface during heat treatment of the bonded substrates, or It has been reported that it may be caused by hydrocarbon-based contamination that was physically adsorbed on the substrate surface before bonding. The size of voids generated by these causes may range from a diameter of 1 μm or less to several cm. If a void is generated when the two substrates are stuck together,
Most of these void regions are lost during the thinning by polishing or etching, and holes are formed in the film. Naturally, the SOI device cannot be formed in the region where the thin film is missing. Even if the thin film remains in the void region, the void region is very likely to be lost due to the element forming process.

Here, the pasting mechanism according to the conventional technique will be described.

FIG. 2 shows two substrates 210, 203, 20.
It is a schematic diagram which shows the bonding state of the atom of the 3'bonding interface, (A) shows the state of the interface by each conventional heat treatment temperature, and (B) shows this invention. FIG. 6 is a schematic diagram showing a state of an interface by the manufacturing method of FIG.

As shown in (a) of FIG. 2 (A), OH groups are formed on the wafer surface before bonding. When the wafers are brought into contact with each other at room temperature, the OH groups cause a hydrogen bond and adhere to each other, as shown in FIG. As described above, the presence of minute dust attached to the interface causes poor adhesion and causes voids. The adhesion at this time is weak 3
It is about 5 kg / cm ² .

Next, heat treatment is performed in order to strengthen the bond at the tight interface. At this time, a dehydration condensation reaction first occurs and the hydrogen bond is changed to a Si—O—Si bond as shown in FIG. (C), and when the temperature is further increased, the Si atom may be directly bonded as shown in FIG. .

However, in the conventional bonding method,
As shown in (c) and (d) of FIG. 2A, there is a problem that the termination of Si atoms may form a dangling bond at the bonding interface with a high probability.

This dangling bond raises the problem of increasing the interface state between the single crystal Si of the SOI substrate and the insulating layer (I layer) and forming a void depending on the density.

Since a complete solution to these problems has not yet been found, the bonded SOI has the possibility of providing the highest quality single crystal thin film in the SOI technology, but also has high quality and high reliability. Stuff has not been produced yet.

As described above, a technique capable of providing a high-quality SOI substrate with sufficient productivity for producing a high-performance electronic device with high productivity has not been achieved yet.

[Object of the Invention] An object of the present invention is to manufacture a high-performance SOI substrate by a bonding method.
While providing a substrate with a good film thickness distribution with good productivity,
Method for manufacturing an SOI substrate in which the interface state is reduced and the generation rate of minute voids is suppressed by making the bonding between the single crystal Si and the insulating layer (I layer) more reliable and high quality To provide.

[0016]

As a means for solving the above-mentioned problems, the present invention has two substrates, at least one of which is a Si substrate having an oxide film, in which mirror surfaces having the oxide film are adhered to each other. In the method for manufacturing a semiconductor substrate, which includes a step of performing heat treatment to strengthen the bond at the adhesion interface, an impurity having a bond with Si stronger than hydrogen is introduced into the oxide film before the adhesion. A method for manufacturing a semiconductor substrate is provided.

In the method of manufacturing a semiconductor substrate having single crystal silicon on an insulator, the step of making the silicon single crystal substrate porous and the step of epitaxially growing a silicon single crystal thin film on the one surface made porous. And a step of oxidizing the surface of the epitaxial layer, a step of introducing impurities into the oxidized surface, a step of producing a first substrate, and a second substrate prepared separately from the first substrate. A method for manufacturing a semiconductor substrate, comprising: a step of bringing the substrates into close contact with each other, subjecting the adhered substrates to a heat treatment to attach the substrates, and a step of selectively removing the porous portion of the attached substrates by etching. Is the means.

Further, a first substrate obtained through a step of making the entire silicon single crystal substrate porous by anodization and a step of epitaxially growing a silicon single crystal thin film on the one surface made porous is provided. A step of producing a second substrate obtained through a step of producing, a step of oxidizing the surface of the silicon single crystal substrate, a step of introducing impurities into the oxidized surface, and a step of producing a silicon single crystal of the first substrate. A step of bringing the crystal surface and the oxide film surface of the second substrate into which the impurities are introduced into close contact with each other, and bonding them by a heat treatment; and a step of selectively etching and removing the porous portion of the bonded substrates, And a method of manufacturing a semiconductor substrate, characterized in that

The impurities are fluorine and fluorine compounds, and the impurities are carbon and carbon compounds.

[0020]

According to the present invention, two physical effects of porous silicon play an important role in solving the problem of uneven distribution of film thickness.

One is the etching characteristics of porous silicon. Normally, silicon is hardly etched with hydrofluoric acid, but it can be etched with hydrofluoric acid by making it porous. Moreover, when a mixed etching solution of hydrofluoric acid, hydrogen peroxide and alcohol is used, an etching rate ratio of about 10 5 times or more can be obtained between non-porous and porous. Therefore, even a thin layer with a thickness of about 1 μm can be selectively etched uniformly and with good controllability, so that a uniform film thickness can be obtained.

Another effect is the epitaxial growth characteristics. Porous silicon maintains a single crystal structure as a crystal structure, and it has several tens to several hundred Å from the surface to the inside.
The diameter of the holes is high. The epitaxial layer grown on this surface has the characteristic that crystallinity equivalent to that of an epitaxial layer on a non-porous single crystal substrate is obtained. Therefore, since the single crystal thin film equivalent to the epitaxial layer on the highly reliable single crystal silicon substrate is used as the active layer, it is possible to provide the SOI substrate having excellent crystallinity as compared with the conventional SOI substrate.

Further, among the above problems, in order to solve the problem of voids with respect to the device, the adhesion force at the bonding interface is enhanced, and the interface level between the single crystal Si and the insulating layer (I layer) is reduced. Therefore, the introduction of impurities into the oxide film surface of the present invention plays an important role.

That is, when impurities are not introduced, there is a high probability that the termination of Si atoms will form a dangling bond at the interface. In this case, depending on the density, the thin film is separated from the substrate without the support base, and voids are generated, causing film peeling. Further, when the terminal of Si atom forms a dangling bond, the interface state is increased and the device characteristics are changed. For this, there is a method of stabilizing the device characteristics by fixing the potential of the substrate serving as a support base, but this is not possible in the case of complementary MOS.

Therefore, in the present invention, by introducing an impurity into the oxide film surface, the impurity terminates the dangling bond at the interface, and as a result, the generation of minute voids is suppressed, the adhesion is enhanced, and the interface level is improved. Can be achieved.

FIG. 2B is a schematic diagram showing the bonding state of the bonding interface between the two substrates 210 and 203 showing the method of the present invention. As shown in FIGS. (C) and (d), in the present invention, Si is eliminated by introducing fluorine or the like, which has a larger binding force to hydrogen than Si as hydrogen, as the impurity 207 in advance on the silicon oxide surface. Termination with fluorine or the like can prevent the generation of dangling bonds.

Embodiment Example FIG. 1 shows an embodiment example of the present invention.
3 and FIG.

Description of (FIG. 1A): A single crystal silicon substrate 100 is anodized to form porous silicon 101. At this time, the region to be made porous may be only one surface layer of the substrate or the entire substrate. When only one surface layer is made porous, that region may have a thickness of 10 to 100 μm.

A method for forming porous silicon will be described with reference to FIG.

First, a P-type single crystal silicon substrate 300 is prepared as a substrate. The N type is not impossible, but in that case, it is limited to a low resistance substrate. The substrate 300 is shown in FIG.
Set in the device as shown in (a). That is, one side of the substrate is in contact with the hydrofluoric acid-based solution 304, the negative electrode 306 is provided on the solution side, and the opposite side is the positive metal electrode 3.
I am in contact with 05.

Further, as shown in FIG. 3 (b), the positive electrode side 305 'may also have a potential via the solution 304'.

In any case, porosity occurs from the negative electrode side in contact with the hydrofluoric acid solution. Hydrofluoric acid solution 304
In general, concentrated hydrofluoric acid (49% HF) is used. Diluting with pure water (H ₂ O) is not preferable because etching will occur from a certain concentration, although it depends on the value of the flowing current.

Bubbles are generated from the surface of the substrate 300 during anodization, and alcohol may be added as a surfactant for the purpose of efficiently removing the bubbles. Methanol, ethanol, propanol, isopropanol, etc. are used as alcohol. Further, a stirrer may be used instead of the surfactant, and the anodization may be performed while stirring the solution.

For the negative electrode 306, a material that is not corroded by the hydrofluoric acid solution, such as gold (Au) or platinum (Pt), is used. The material of the positive electrode 305 may be a commonly used metal material, but when the anodization is performed on all the substrates 300, the hydrofluoric acid-based solution 304 is used.
Since it reaches the positive electrode 305, it is preferable to coat the surface of the positive electrode 305 with a hydrofluoric acid solution resistant metal film.

The maximum current value for anodization is several hundred mA / c.
m ² and the minimum value should not be zero. This value is determined within a range that allows good quality epitaxial growth on the surface of porous silicon. Usually, when the current value is large, the rate of anodization increases, and at the same time, the density of the porous silicon layer decreases. That is, the volume occupied by the holes becomes large. This changes the conditions for epitaxial growth.

Description of (FIG. 1B): The porous silicon substrate or porous layer 10 formed as described above.
A non-porous single crystal silicon layer 102 is epitaxially grown on the first layer 1. Epitaxial growth is a general thermal CV
D, low pressure CVD, plasma CVD, molecular beam epitaxy, sputtering method, etc. The grown film thickness may be the same as the design value of the SOI layer.

Description of FIG. 1C: The surface of the grown epitaxial layer 102 is oxidized to form a SiO ₂ layer 103.
To form. The oxide film 103 becomes an insulator layer (I layer) having an SOI structure. Oxidizing the epitaxial layer 102 also includes reducing the interface state density of the epitaxial layer 102, which is an active layer, with the underlying insulator interface when forming a device on the completed SOI substrate.
At this time, the thickness of the epi oxide film is preferably 0.5 to 1.0 μm in order to utilize the characteristics of the SOI device.

Description of FIG. 1D: Impurity 107 is introduced into the surface of the oxide layer 103. The impurities 107 are fluorine, fluorine compounds, carbon and carbon compounds,
Any material may be used as long as it has a stronger bond with Si than hydrogen. The introduction method is also not particularly limited, and an ion implantation method, solid phase diffusion, spin coating, CVD method or the like is used. The introduction amount of the impurities 107 may be determined arbitrarily, but the larger the introduction amount, the higher the probability of terminating the dangling bond at the interface. This leads to reduction of the interface state and suppression of generation of minute voids. Therefore, the amount of impurities 10 should be within the range that can be easily produced under the normal process conditions of the equipment used.
It is sufficient to determine the introduction amount of 7. For example, when the ion implantation method is used to introduce the impurities 107, the dose amount is 1 × 1.
It is about 0 ¹⁵ to 1 × 10 ¹⁶ [atom / cm ² ].

If the impurity 107 is introduced by the ion implantation method or the like, the epitaxial growth layer 102 may become amorphous. This becomes a single crystal again when heat treatment is performed, but it is not always necessary to perform special heat treatment because there is a step of performing heat treatment after the subsequent bonding.

Further, when the impurity 107 is introduced by the CVD method, there are cases where considerable unevenness exists on the surface.
Since the flatness of the substrate surface is important when the substrates are bonded together, if unevenness occurs, the substrate surface is polished to be flattened, or after the impurity 107 is introduced, etching is performed to flatten the surface. Good to do.

The substrate obtained by the above steps is hereinafter referred to as "first substrate".

Description of (FIG. 1 (e)): A first substrate,
The separately prepared second substrates 110 are bonded to each other with their mirror surfaces, and subsequently the bonded substrates are heat-treated. The heat treatment temperature is set to a temperature at which a bonding force is obtained to the extent that peeling of the bonding interface does not occur during the next polishing or etching process. Specifically, about 100 ° C. or higher is preferable. However, when the process is performed at a relatively low temperature, the polishing or etching is completed, and after the final SOI morphology is obtained, 1000
It is preferable to perform heat treatment at about ° C. This is to prevent film peeling or the like due to thermal stress during the device process.

The second substrate 110 is completely arbitrary and may be selected from a silicon substrate, a quartz substrate, another ceramic substrate, and the like.

Description of (FIG. 1 (f)): Next, the porous portion 101 and others are selectively removed from the first substrate side, leaving the epitaxial growth layer 102. If the portion to be removed at this time is entirely porous, if the bonded substrates are immersed in a hydrofluoric acid-based solution, the entire porous portion 101 is selectively etched. When the portion to be etched includes the region of the single crystal silicon substrate 100 as it is, it is preferable to polish and remove only the region of the silicon substrate 100. Then, the polishing is terminated when the porous portion 101 is exposed, and selective etching can be performed later in a hydrofluoric acid solution. In any case, the non-porous single crystal epitaxial growth portion 102 hardly reacts with hydrofluoric acid, and thus remains as a thin film.

Further, as a matter of course, when the second substrate 110 contains SiO ₂ as a main component, it easily reacts with a hydrofluoric acid solution, so that a silicon nitride film is previously formed on the surface opposite to the bonding surface by CVD or the like. It is advisable to deposit a substance that does not easily react with or other hydrofluoric acid. Alternatively, if the porous portion 101 is made thin to some extent before the substrate is dipped in the etching solution, the time required for the porous selective etching can be shortened, so that the second substrate does not need to react much. Of course the second
There is no problem if the substrate of 1 is one that does not react with hydrofluoric acid such as silicon.

The hydrofluoric acid-based solution used for selective etching is a mixture of hydrofluoric acid, hydrogen peroxide solution (H ₂ O ₂ ) and alcohols. Selective etching of porous silicon is possible with a mixed solution of hydrofluoric acid and nitric acid or acetic acid, but in this case the epitaxial silicon film 102 to be left is also etched to some extent, so it is necessary to precisely control the time and the like. There is.

Through the above steps, an SOI substrate having the deposit 107, the silicon oxide film 103, and the epitaxial silicon layer 102 on the second substrate 110 in order is obtained.

[0048]

The present invention will be described in detail below with reference to specific examples, but the present invention is not limited to these examples.

(Example 1) Description of (FIG. 1 (a)): 4-inch P-type (100) single crystal silicon substrate (0.1) having a thickness of 200 microns.
.About.0.2 .OMEGA.cm) was prepared, and this was set in an apparatus as shown in FIG. 3 (a) for anodization to obtain porous silicon 101. A 49% HF solution was used as the solution 304 at this time, and the current density was 100 mA / cm ² . The porosification rate at this time is 8.4 μm / min. The P-type (100) silicon substrate having a thickness of 200 μm was entirely made porous in 24 minutes.

Description of (FIG. 1 (b)): The P-type (10
0) A single crystal silicon layer 102 was epitaxially grown on the porous silicon substrate 101 by the CVD method to a thickness of 1.0 μm. The deposition conditions are as follows.

Gas used: SiH ₄ / H ₂ gas flow rate: 0.62 / 140 (1 / min) Temperature: 750 ° C. Pressure: 80 Torr Growth rate: 0.12 μm / min. Description of (FIG. 1C): Epitaxial growth layer 102
Surface is oxidized in a steam atmosphere at 1000 ° C.
A silicon oxide film 103 of μm was obtained.

Description of FIG. 1D: Monovalent F ions 107 are added to the oxide film 103 at 5 × 10 ¹⁵ [atom / c].
m ² ] and 30 [keV] were used for ion implantation. The substrate thus formed was used as the first substrate.

Description of (FIG. 1 (e)): A 4-inch silicon substrate 110 was prepared as a second substrate, and was washed together with the first substrate in a CHl: H ₂ O ₂ : H ₂ O solution. After thoroughly washing with water, the mirror surfaces of the first and second substrates were bonded together. Further, this substrate was heat-treated in a nitrogen atmosphere at 1100 ° C. for 2 hours to strengthen the bonding force at the interface of the bonded substrates.

Description of (FIG. 1 (f)): The substrate adhered after the heat treatment is immersed in the selective etching solution to form the porous portion 1.
Only 01 was selectively etched. At this time, the composition of the etching solution and the etching rate for the porous silicon were HF: H ₂ O ₂ : C ₂ H ₅ OH = 5: 25: 6 1.6 μm / min. Met. Therefore, the porous part of 200 μm is about 125
All were etched in minutes. Incidentally, the etching rate of the single crystal silicon layer 102 at this time is 0.0006 μm.
m / hour, which remained without being etched.

Through the above steps, the silicon oxide film 103 and the epitaxial growth layer 10 are formed on the silicon substrate 110.
An SOI substrate having 2 was sequentially obtained.

In order to compare the effects of voids, an SOI substrate (hereinafter referred to as a comparative SOI substrate) prepared by omitting only the step of implanting F ions 107 on the silicon oxide film 103 was observed with an optical microscope. 0.1 μm
Minute voids of about 1 μm were present at a density of about 5 / cm ² , and about half of these voids caused film breakage. On the other hand, no void was observed with the same optical microscope in the SOI substrate in which the F ions 107 were ion-implanted, which was created in the above-mentioned process of this example.

Further, when the adhesion force was measured for the above two kinds of SOI substrates by using a tensile tester, the comparative SOI substrate was 600 to 700 [kg / cm ² ], and the F ion 107 produced in the above process was used. Ion-implanted SOI
The substrate had a thickness of 700 to 800 [kg / cm ² ].

Next, 100 of these two types of SOI substrates are used.
A heat treatment is performed in an oxygen atmosphere at 0 ° C. to form a 1000 [Å] gate oxide film. After this, use an EB vapor deposition machine to
(Aluminum) was formed, and this was patterned using a photolithography technique to form an electrode.

Two kinds of SO created by the above process
The C-V characteristics of the I substrate were measured, and the fixed charge density was 10 ¹² [cm ⁻² ] for the comparative SOI substrate and 10 ¹⁰ [c for the SOI substrate into which F ions 107 were ion-implanted.
m ^-2 ], and a two-digit difference occurred. This is the gate oxide and Si
Since there is no difference in the interface, it is clear that Si and the insulating film layer (I layer)
It is the difference in the interface with.

Example 2 In this example, a substrate having an oxide film on its surface was used as the second substrate, and impurities characteristic of the present invention were introduced into the oxide film surface of this second substrate.

A silicon single crystal surface of a first substrate having an epitaxial growth layer on a porous silicon substrate;
Similarly, the surface of the substrate and the surface of the oxide film having impurities introduced therein were bonded together, and the porous portion was removed by etching.

Regarding the SOI substrate obtained in this example,
When the adhesion and C-V characteristics were measured, the same effect as that of the SOI substrate obtained in Example 1 was obtained.

Note that the method of introducing impurities into the oxide film on the bonding surface to prevent dangling bonds of the present invention can be applied as long as it is a method for manufacturing a semiconductor substrate having a bonding step, and the same method can be applied. The effect can be obtained.

[0064]

As described above, according to the present invention,
Obtained by a process of forming a single crystal epitaxial growth layer and an oxide film on porous silicon, bonding a first substrate to an arbitrary second substrate, performing heat treatment, and selectively removing porous silicon. In the bonded SOI substrate, by providing a step of introducing impurities into the oxide film at the bonding interface before the bonding step, the Si atoms are terminated by the impurities, and the dangling bonds generated at the bonding interface are terminated by the impurities. Therefore, the interface state could be reduced.

Further, by strengthening the bonding at the bonding interface, it was possible to eliminate minute voids.

Therefore, until now, all the substrates having voids at the bonding interface were treated as defective products. However, the present invention allows generation of voids to some extent, and thus yield is remarkable. Improved.

Further, the effect of increasing the speed peculiar to the SOI substrate and omitting the well process can be made more ideal by reducing the interface state.

Further, by the porous selective etching, an SOI substrate having a uniform film thickness could be obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram for explaining a process of the present invention.

FIG. 2 is a schematic diagram of a bonding process and its mechanism at a bonding interface between the present invention and a conventional example.

FIG. 3 is a schematic view of an apparatus used for making a silicon substrate porous.

[Explanation of symbols]

 100,300 Single crystal silicon substrate 101 Porous silicon substrate 102 Epitaxial growth layer 103,203 Epitaxial oxide film 107,207 Impurity 110,210 Single crystal silicon substrate 304,304 'Etching solution 305,305' Positive electrode 306, 306 'negative electrode

Claims (5)

[Claims] 1. A semiconductor substrate having a step of bringing two substrates, at least one of which is an Si substrate having an oxide film, into close contact with each other through the oxide film and performing a heat treatment to strengthen the bond at the close contact interface. 2. The method for manufacturing a semiconductor substrate according to claim 1, wherein an impurity having a bond with Si stronger than hydrogen is introduced into the oxide film before the adhesion. 2. A method for manufacturing a semiconductor substrate having single crystal silicon on an insulator, the step of making a silicon single crystal substrate porous, and the step of epitaxially growing a silicon single crystal thin film on the one surface made porous. And a step of oxidizing the surface of the epitaxial layer, a step of introducing impurities into the oxidized surface, a step of producing a first substrate, and a step of preparing the first substrate and another second substrate And a step of bringing the adhered substrates into contact with each other by heat treatment, and a step of selectively etching and removing the porous portion of the adhered substrates. Method. 3. A step of making the entire silicon single crystal substrate porous by anodization, and a step of epitaxially growing a silicon single crystal thin film on the one surface made porous.
A step of producing a first substrate obtained through, a step of oxidizing the surface of the silicon single crystal substrate, a step of introducing impurities into the oxidized surface, and a step of producing a second substrate obtained through A step of bringing the silicon single crystal surface of the first substrate and the oxide film surface of the second substrate into which the impurities are introduced into close contact with each other, and attaching them by heat treatment; and And a step of selectively removing by etching. 4. The method for manufacturing a semiconductor substrate according to claim 1, wherein the impurities are fluorine and a fluorine compound. 5. The method for manufacturing a semiconductor substrate according to claim 1, wherein the impurities are carbon and carbon compounds.