JP2550353B2 - Semiconductor film forming method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for forming a Ge semiconductor thin film, and more particularly to a method for forming a Ge semiconductor thin film whose film thickness can be controlled on the atomic layer order. Is.

[Conventional technology]

Conventionally, when a thin film of Ge, which is a group IV semiconductor, is formed on a substrate, a molecular beam epitaxial growth (MBE) apparatus is used to heat and evaporate the element of Ge in vacuum and deposit it on the substrate, or A method is used in which a GeH ₄ gas or the like is introduced onto a substrate using a CVD apparatus or the like and a Ge semiconductor film is deposited by thermal decomposition or photolysis. When the Ge thin film is formed by such a method, the film thickness is generally controlled by keeping the deposition rate constant and adjusting the deposition time.

[Problems to be solved by the invention]

In such a Ge semiconductor film forming method, in order to obtain a desired film thickness without error, it is necessary to strictly control the deposition rate and keep it accurately constant. Deposition rate control is MB
When E is used, it can be performed by controlling the temperature of the vapor deposition source and the substrate temperature, and when using the CVD method or the like, it can be performed by controlling the partial pressure of the reaction gas such as GeH ₄ and the substrate temperature. In theory, the deposition rate can be kept constant. However, in practice, it is difficult to maintain the substrate temperature, the temperature of the vapor deposition source, the partial pressure of the gas, and the like accurately and accurately, and thus it is difficult to obtain a desired film thickness on the atomic layer order. As long as the conventional method is used, even if the film thickness can be controlled on the order of atomic layers, it is difficult to deposit one atomic layer of Ge atoms 2 on the substrate 1 as shown in FIG. In addition to the atomic layer region, a region that is not deposited at all or 2
A region where more than the atomic layer is deposited is generated.

[Means for solving problems]

The semiconductor film forming method of the present invention has been made in view of the above problems, and supplies a gas in which a group or an atom that is difficult to dissociate to a Ge atom and a group or an atom that is easily dissociated are supplied to the substrate surface, and A step of attaching the Ge atom to the substrate surface in a state of having a group that is difficult to dissociate by removing a group that is easily dissociated, and a step of releasing the group that is difficult to dissociate that the attached Ge atom has. Is included.

[Action]

When a gas in which a group that is difficult to dissociate with Ge atom and a group or atom that is easy to dissociate are bonded to each other is introduced to the substrate surface and the group or atom that is easily dissociated is separated from the Ge atom, the group that is difficult to dissociate remains. The Ge atoms and the substrate are bonded as they are. As a result, the group that is difficult to dissociate functions as a protective group to prevent the deposition of the next layer, so that the reaction is stopped by the formation of one atomic layer. If the group that is difficult to dissociate is eliminated after the formation of one atomic layer, the next one atomic layer is further formed by introducing the gas again.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to Examples.

Example 1 FIG. 1 is a schematic diagram showing a first example of the method for forming a semiconductor film of the present invention, showing an example using a Ge (C ₂ H ₅ ) ₂ H ₂ gas.

A (100) plane of Si, Ge, GaAs or the like is used as the substrate 1, and a Ge (C ₂ H ₅ ) ₂ H ₂ gas 3 is introduced thereon (FIG. 1 (a)). Here, “Et” in the figure is an ethyl group (C ₂ H ₅ group)
Indicates. At this time, the ethyl group and Ge do not dissociate because they are strongly bonded to each other, but the Ge and H atoms (-H groups) have weak bond, so that they dissociate at a temperature of, for example, 100 to 300 ° C. Ge (C ₂ H ₅ ) 4 is adsorbed on. At this time, since Ge is adsorbed in a form in which an ethyl group is exposed on the surface, the surface of Ge is covered with the ethyl group and the ethyl group acts as a protective group, and Ge (C ₂ H ₅ ) ₂ H The decomposition reaction of ₂ gas 3 does not occur. Therefore, the adsorption of Ge (C ₂ H ₅ ) 4 automatically stops in one atomic layer (Fig. 1 (b)).

Next, the unreacted Ge (C ₂ H ₅ ) ₂ H ₂ gas 3 is removed, and then the temperature of the substrate 1 is raised to 300 ° C. or higher, whereby the ethyl group 5 can be desorbed from the Ge surface ( FIG. 1 (c)). And when the eliminated ethyl group 5 is eliminated,
A surface with exposed Ge is obtained (FIG. 1 (d)). When Ge (C ₂ H ₅ ) ₂ H ₂ gas 3 is introduced to the surface again in this surface state, the next Ge (C ₂ H ₅ ) ₂ 4 ′ absorbs one atomic layer and Ge of the second atomic layer A semiconductor film is formed (FIG. 1 (e)). Therefore, by repeating this process, it is possible to form a Ge film for each atomic layer.

By the way, in FIG. 1 (b), a model in which two ethyl groups serving as protecting groups are bonded per Ge 1 atom is used, but as shown in FIG. 2 (a), per Ge 1 atom It goes without saying that even in the form in which one ethyl group is bonded, it is sufficient if the ethyl group can prevent the formation of the next atomic layer. Further, as shown in FIG. 2 (b), the Ge 1 atomic layer on the surface undergoes rearrangement, and two Ge forms a dimer structure,
The same applies to a form having one ethyl group each.

Next, the present embodiment will be described in more detail with specific numerical values.

The substrate is a Si (100) surface or a Ge (100) surface and is installed in a quartz reaction furnace. The inside of this reaction furnace is initially evacuated to a vacuum of 10 ^-8 Torr level or less by a turbo molecular pump. Ge (C ₂ H ₅ ) ₂ H ₂ gas 3 is introduced onto this substrate 1, the introduction is stopped after a certain time, and the substrate is evacuated to vacuum. After that, the substrate was heated by an infrared irradiation lamp from outside the reaction furnace, and the gas desorbed from the substrate 1 was monitored by a quadrupole mass spectrometer while raising the substrate temperature at a constant rate of 30 ° C./min.

FIG. 3 shows the substrate temperature dependence of the desorption amount of the ethyl group desorbed from the substrate at this time (the desorption intensity up to a certain point differentiated from the temperature rise start time). is there. It can be seen that the elimination of the ethyl group begins at a temperature near 400 ° C and is eliminated after a few minutes. The temperature at which this desorption starts depends on the initial pressure of Ge (C ₂ H ₅ ) ₂ H ₂ gas 3 introduced,
Although it does not depend on the introduction time or the temperature of the substrate at the time of introduction, the total desorption amount (corresponding to the saturation value in the graph of FIG. 3) depends on the gas introduction pressure, the introduction time, and the substrate temperature at the time of introduction.

Fig. 4 is a graph showing the gas introduction amount dependency of the total amount of desorbed ethyl groups. The gas introduction amount is represented by the product of the gas introduction pressure and the introduction time, and is 10 ^-6 Torr · sec = 1 L (Langmuir). , "L" is used as a unit. The substrate temperature T _I during gas introduction was 220 ° C. As you can see from the figure,
The total amount of desorption becomes saturated when the gas introduction amount is 10 ⁵ L or more. That is, adsorption of _{Ge (C 2 H 5) 2} 4 to the surface indicates that saturated. This adsorption of _{Ge (C 2 H 5) 2} 4 is none other than saturated with 1 atomic layer.

FIG. 5 is a graph showing the dependency of the total desorption amount on the substrate temperature T _I when the gas is introduced. The amount of gas introduced was constant at 10 ⁶ L. As is clear from the figure, the total desorption amount is constant over a wide range of T _I = 100 ° C to 300 ° C. This is Ge (C
₂ H _{5) 2} H ₂ H is dissociated from the gas 3, to the surface _{Ge (C 2 H 5) 2} 4 which means that the resulting from substrate temperature the reaction is 100 ° C. or less to adsorption. In this experiment, if the gas introduction amount is increased to more than 10 ⁶ L, it is considered that the decrease in the total desorption amount is alleviated even at a substrate temperature of 100 ° C. or less. Moreover, the total amount of desorption decreases above 300 ° C. This is because the elimination of the ethyl group shown in FIG. 3 starts slowly at 300 ° C. or higher. This temperature also corresponds to the fact that continuous Ge film formation by thermal decomposition of Ge (C ₂ H ₅ ) ₂ H ₂ gas 3 occurs at 300 ° C. or higher. It has been confirmed that at 300 ° C or less, almost no continuous Ge film formation occurs.

Based on the above results, the introduction and removal of Ge (C ₂ H ₅ ) ₂ H ₂ gas 3 (exhaustion to a pressure of 10 ^-7 Torr or less) and the sequence of raising and lowering the substrate temperature were used to calculate this. A hundred times,
An attempt was made to grow Ge atomic layer by atomic layer on a Ge substrate. FIG. 6 (a) shows the time change of the gas pressure at this time, and FIG. 6 (b) shows the time change of the substrate temperature. The time to of each process was 90 seconds, and the time required for one cycle was 360 seconds. As a result, the number of repetitions of the process cycle in FIG. 6 is proportional to the increase in the thickness of Ge, and the increase in the thickness of Ge per cycle corresponds to the thickness of one atomic layer of Ge. I was able to confirm that. In addition, it was confirmed by electron diffraction that the formed Ge film was epitaxially grown.

Considering the data in FIG. 4 and FIG.
The product (introduction amount) of the introduction time and the introduction pressure of Ge (C ₂ H ₅ ) ₂ H ₂ gas 3 used in the sequence in the figure is 9 × 10 ⁵ L, but if this is 10 ⁵ L or more It is enough. Further, the substrate temperature at the time of introducing the gas may be in the range of approximately room temperature to 300 ° C. when the introduction amount is 10 ⁶ L. On the other hand, the time required to raise the substrate temperature and eliminate the ethyl group can be easily obtained experimentally. That is, confirm that the elimination of the ethyl group is completed with a quadrupole mass spectrometer, or actually repeat the process cycle as shown in FIG.
It suffices to confirm that the increase in the Ge film thickness corresponds to one atomic layer of Ge. In the sequence of FIG. 6, it was confirmed by a quadrupole mass spectrometer that the ethyl group was completely desorbed at to = 90 seconds.

To make the time required for one cycle relatively short,
It is necessary to shorten the heating / cooling time by increasing the pumping speed of the exhaust pump and shortening the pumping time, and by using the infrared heating method capable of heating / cooling at high speed.

Needless to say, when the temperature is raised, it is necessary to keep the substrate temperature below the temperature at which Ge on the surface and Ge (or Si) on the base are broken and Ge is desorbed.

Further, according to the method of this embodiment, no Ge film is formed on SiO ₂ . This is because a decomposition reaction of Ge (C ₂ H ₅ ) ₂ H ₂ gas does not occur on SiO _{2 and} an adhesion layer of Ge (C ₂ H ₅ ) ₂ is not formed. Therefore, with the SiO ₂ film as a mask film, Ge
It can be selectively grown on Si or Ge. In addition,
This also applies to other embodiments described later.

Example 2 In the first example, the growth of Ge per atomic layer using Ge (C ₂ H ₅ ) ₂ H ₂ gas on the (100) plane of Si, Ge, GaAs, etc. will be described. did. On the other hand, in the present embodiment, the growth of each atomic layer of Ge on the (110) plane is performed. In an ideal Ge (110) plane, there is only one bond coming out from the surface Ge. Therefore, as shown in FIG. 7, Ge (C ₂
By using H ₅ ) H ₃ gas 7, it is possible to grow Ge for each atomic layer.

When Ge (C ₂ H ₅ ) H ₃ gas 7 is introduced to the surface of the substrate 1, three H are dissociated to form one atomic layer of Ge (FIG. 7 (b)). At this time, since the ethyl group remains on the surface, the Ge does not adhere due to the decomposition of the Ge (C ₂ H ₅ ) H ₃ gas 7 on the ethyl group. The temperature at which the formation of one atomic layer on this surface occurs, the gas pressure, the gas introduction time, etc. can be determined by using Ge (C
₂ H ₅ ) It is almost the same as when introducing ₂ H ₂ gas.

Next, by raising the temperature of the substrate 1 to 300 ° C. or higher, the ethyl groups are dissociated from the surface (FIG. 7 (c)), and the dissociated ethyl groups are evacuated by a vacuum pump or inert. It is excluded from the atmosphere by flowing a different gas (Fig. 7 (d)). Here, the temperature characteristic of the reaction of leaving the ethyl group from the surface is almost the same as the characteristic of FIG. 3 in the first embodiment. Then again Ge (C
_{If 2} H ₅ ) H ₃ gas 7 is introduced, the next Ge1 atomic layer can be formed.

Example 3 In the first and second examples, the ethyl group is used as the protecting group, but other alkyl groups can be used.

For example, in the first embodiment, when Ge (CH ₃ ) ₂ H ₂ gas is used instead of Ge (C ₂ H ₅ ) ₂ H ₂ gas 3, CH is generated on the (100) plane of the substrate such as Si or Ge. _{With the use of 3} (methyl group) as a protecting group, the same monoatomic layer of Ge as in FIG. 1 (b) can be formed. Further, the methyl group on the surface can be dissociated by raising the substrate temperature similarly to the ethyl group. For example, when a temperature programmed desorption experiment was conducted under the same conditions as in FIG. 3, it was found that methyl groups were desorbed from the surface. However, in comparison with the results shown in Fig. 3, Ge (CH ₃ ) ₂ H ₂
When gas is used, the onset temperature of elimination of methyl group is 50
It is 0 ° C, which is higher than that of ethyl group by almost 100 ° C. Therefore, it is necessary to set the temperature condition for eliminating the methyl group higher by 100 ° C. than in the first embodiment.

Further, the relationship between the amount of Ge (CH ₃ ) ₂ H ₂ gas introduced and the total amount of desorption corresponding to FIG. 4 is almost the same as in FIG. Therefore, Ge (CH ₃ ) ₂ for the formation of one atomic layer
Delivery conditions of H ₂ gas may be considered to be substantially the same as using _{Ge (C 2 H 5) 2} H 2 gas in the first embodiment. However, since the desorption temperature of the methyl group is high by 100 ° C, the upper limit of the gas introduction temperature can be increased by 100 ° C.

Similarly, Ge (CH ₃ ) H ₃ gas may be used on the (110) plane. Also, judging from the tendency of the desorption temperature from the surface of the ethyl group and the tendency of the desorption temperature from the surface of the methyl group,
When a (C _n H _{2n + 1} ) group is used as a protecting group, the larger the value of n, the more likely it is to be eliminated at a lower temperature.

In addition, part of the hydrogen of the (C _n H _{2n + 1} ) group is replaced with halogen (Cl,
F, Br, etc.)-Substituted (C _n H _m X _{2n + 1-m} ) group may be used.

Furthermore, when a long alkyl group or the like is used as a protecting group,
It may have a double bond or the like. Further, for example, Ge (CH ₃ ) (C ₂ H ₅ ) H ₂ gas such as 2
You may use the gas of the structure which has a different kind of alkyl group.

Example 4 In the first to third examples, the removal of the protective group is performed by raising the temperature, but in the present example, this is performed by irradiating the surface of the substrate with light and photoexciting it.

Here, an example using Ge (CH ₃ ) ₂ H ₂ gas as a reaction gas will be described with reference to FIG. The formation condition of one atomic layer, that is, the formation condition of the Ge (CH ₃ ) ₂ layer 8 is as described in the third embodiment, and the structure of FIG. 8A can be realized (here, the eighth structure). “Me” in the figure indicates a methyl group.When the surface on which such a Ge (CH ₃ ) ₂ layer 8 is formed is irradiated with ultraviolet rays emitted from an ultra-high pressure mercury lamp, the methyl group is desorbed from the surface (Fig. 8). (B)), the surface methyl groups are removed and the Ga surface appears (Fig. 8 (c)), where again Ge (CH ₃ ) ₂ H ₂
When the gas is introduced, the next Ge (CH ₃ ) ₂ layer is formed. That is, by repeating this process, it is possible to form a film for each atomic layer of Ge.

Next, experimental results for confirming the elimination of the methyl group due to such photoexcitation will be shown. On the Ge (100) surface, Ge (C
H ₃ ) ₂ H ₂ gas at 0.04 Torr for 60 seconds (2.4 ×
10 ⁶ L) is introduced to form one atomic layer of Ge (CH ₃ ) ₂ . As described above, the introduction amount of 1 × 10 ⁵ L or more is sufficient. Then exhaust the gas,
After applying a high vacuum of 10 ^-8 Torr, the surface was irradiated with ultraviolet rays emitted from an ultra-high pressure mercury lamp, and methyl groups desorbed from the surface were measured by a quadrupole mass spectrometer. As a result, it was confirmed that the methyl group was desorbed from the surface simultaneously with the light irradiation.

Next, using the method of this example, the growth of Ge for each atomic layer was actually performed. Fig. 9 shows an example of on / off sequence of introduction and exhaust of Ge (CH ₃ ) ₂ H ₂ gas used for growth and irradiation of ultraviolet rays (ultra high pressure mercury lamp). The figure (a) shows how the gas pressure changes with time, and the figure (b) shows the ultraviolet irradiation timing. Ge (CH ₃ ) ₂ H ₂
Gas is introduced at an introduction pressure of 4 × 10 ^-2 Torr for time t1 and exhausted. In the reactor used in this experiment, when the gas introduction is stopped and the gas is exhausted, a vacuum of 10 ^-7 Torr or less is reached after a few seconds. After evacuation for t2 hours, ultraviolet rays from the ultra-high pressure mercury lamp were irradiated for t3 hours, Ge (CH ₃ ) ₂ H ₂ gas was introduced again after an interval of t4 hours, and the same process was repeated thereafter. The substrate used was a Si (100) substrate, and a natural oxide film having a thin surface was removed by a known method before introduction of gas in an ultrahigh vacuum to obtain a clean surface.

In this way, the Ge film formed on the substrate by repeating the process of FIG. 9 was measured by the known X-ray photoelectron spectroscopy (XPS) method, and the film thickness of the formed Ge was obtained. Figure 10 shows the relationship between the number of formed Ge layers (one atomic layer of Ge corresponds to about 1.41Å) and the number of cycles. Here, t1 = 20 sec, t2 = 30 sec, t3 = 5 sec, t4 = 25 sec. As is clear from the figure, the number of formed Ge layers is proportional to the number of cycles, and the Ge per cycle is
The increase in film thickness is one atomic layer. Further, from the figure, it can be seen that the Ge layer does not increase by one atomic layer or more even if the cycle of gas insertion and exhaust is repeated without light irradiation. This result was exactly the same even when the light irradiation time t3 was 15 sec. Regarding the substrate temperature during gas introduction,
The same result is obtained at about 400 ° C. When the substrate temperature during gas introduction exceeds 400 ° C., thermal desorption of the methyl group begins to occur slowly, so desorption of the methyl group occurs within the gas introduction period, and film formation of two or more atomic layers occurs. The area appears.

In addition, here, an example of growing one atomic layer of Ge on the (100) surface of Si is shown, but as an initial substrate, Ge (10
0) Even if the surface is used, Ge can be grown on the surface one atomic layer at a time.

[Embodiment 5] As in the case of the second embodiment, when Ge is formed on the (110) plane of Ge or Si one atomic layer at a time, for example, Ge (C
H ₃ ) H ₃ gas may be used. In this case, a Ge layer of one atomic layer is formed leaving a methyl group, and has a structure in which the ethyl group (Et group) in FIG. 7B is replaced with a methyl group. And
As described in the fourth embodiment, this methyl group is desorbed by irradiating with ultraviolet rays, so that a surface similar to that shown in FIG. 7 (d) is obtained. Therefore, Ge (C
If H ₃ ) H ₃ gas is introduced, the following one atomic layer can be formed.

Example 6 In the fourth example, Ge (CH ₃ ) ₂ H ₂ gas was used instead of Ge.
It was confirmed that almost the same results as in the third example were obtained even when the (C ₂ H ₅ ) ₂ H ₂ gas was used. Therefore, in the fourth and fifth examples, similar results can be obtained by using other alkyl groups (C _n H _{2n + 1} group) such as ethyl group and C ₃ H ₇ group instead of the methyl group. It is considered possible. This is because it is unlikely that the wavelength of light for causing photoexcited desorption will change so much even if the length of the alkyl group is slightly changed. Similarly, a halogenated hydrocarbon (C _n H _m X _{2n-m + 1} ) group may be used.

"Embodiment 7" In the fourth to sixth embodiments, the Ge film formation stops at one atomic layer without irradiation of light, and the Ge film formation does not increase further.
It is clear from the results in Figure 10. Therefore, using a mask that transmits laser light or light only in a specific area,
By irradiating light only on a desired region of the substrate, the next monoatomic layer of Ge is formed only in that region. That is, it is possible to form a Ge film one atomic layer at a time only in a desired region without going through a normal process such as photolithography or etching. At this time, a monoatomic layer of Ge is also formed in the region not irradiated with light, but it is practically sufficiently thin and there is no problem.

Further, as described above, in the SiO _2, Ge (C ₂
No surface reaction of H ₅ ) ₂ H ₂ gas or Ge (CH ₃ ) ₂ H ₂ gas occurs,
No Ge adhesion layer such as Ge (C ₂ H ₅ ) ₂ layer or Ge (CH ₃ ) ₂ layer is formed on the surface. Therefore, the Ge film is not formed on the SiO ₂ regardless of the presence or absence of light irradiation.

[Advantages of the Invention] As described above, according to the method for forming a semiconductor film of the present invention, Ge growth is caused by the presence of a protective group (a group that is difficult to dissociate).
By automatically stopping at the atomic layer and then removing this protecting group, the next Ge 1 atomic layer can be grown.
Therefore, by repeating this process, it is possible to grow the film for each atomic layer of Ge, and the film thickness controllability of the Ge film formation is significantly improved.

[Brief description of drawings]

FIG. 1 is a schematic process diagram showing a first embodiment of the present invention, and FIG.
The figure is a schematic surface diagram other than that shown in Fig. 1 (b). Fig. 3 shows the substrate after Ge (C ₂ H ₅ ) ₂ H ₂ gas is introduced to the surface of the Ge (100) substrate and the unreacted gas is removed. temperature-programmed desorption spectrum of ethyl group desorbed from the surface when the temperature was increased at 30 ° C. / min, the introduced amount of the fourth figure Ge desorption amount of ethyl _{(C 2 H 5) 2 H} 2 gas Fig. 5 is a characteristic diagram showing the dependence, and Fig. 5 is a characteristic diagram showing the substrate temperature dependence of the total amount of desorbed ethyl groups when Ge (C ₂ H ₅ ) ₂ H ₂ gas is introduced again. FIG. 7 is a diagram showing a sequence of gas introduction / exhaust and substrate temperature raising / lowering in an embodiment, FIG. 7 is a schematic process diagram showing a second embodiment of the present invention, and FIG. 8 is a fourth embodiment of the present invention. FIG. 9 is a schematic process chart showing the sequence of gas introduction / exhaust and substrate temperature raising / lowering in the fourth embodiment, and FIG. 10 is a relation between the number of repeated cycles and the number of Ge layers. By drawing, FIG. 11 is a conventional method shown
It is a schematic diagram which shows the film formation of Ge. 1 ... Substrate, 2 ... Ge atom, 3 ... Ge (C ₂ H ₅ ) ₂ H ₂ gas molecule, 4, 4 '... Ge (C ₂ H ₅ ) ₂ , 5 ... Ethyl group, 7 ...
… Ge (C ₂ H ₅ ) ₃ gas molecules, 8 …… Ge (CH ₃ ) ₂ .

Claims (5)

(57) [Claims] 1. A group that is difficult to dissociate by supplying a gas in which a group that is difficult to dissociate with a Ge atom and a group or an atom that is easy to dissociate are supplied to the surface of a substrate and the group that is easy to dissociate is eliminated. A method of forming a semiconductor film, comprising: attaching the Ge atoms to the surface of the substrate in a state of having the above; and removing the hard-to-dissociate groups of the attached Ge atoms. 2. The method for forming a semiconductor film according to claim 1, wherein a hydrocarbon group or a halogenated hydrocarbon group is used as the group that is difficult to dissociate, and hydrogen is used as the group or atom that is easily dissociated. 3. The method for forming a semiconductor film according to claim 1, wherein the group that the attached Ge atom does not easily dissociate is desorbed by light irradiation. 4. The method for forming a semiconductor film according to claim 1, wherein the group of the attached Ge atom which is difficult to dissociate is desorbed by raising the substrate temperature. 5. A Ge deposited on the substrate surface in a region irradiated with light by irradiating the desired region on the substrate surface with light.
The method for forming a semiconductor film according to claim 3, wherein a group of an atom which is difficult to dissociate is eliminated.