METHOD OF GAS-PHASE DEPOSITION BY EPITAXY

Abstract

A gas phase epitaxial deposition method deposits silicon, germanium, or silicon-germanium on a single-crystal semiconductor surface of a substrate. The substrate is placed in an epitaxy reactor swept by a carrier gas. The substrate temperature is controlled to increase to a first temperature value. Then, for a first time period, at least a first silicon precursor gas and/or a germanium precursor gas introduced. Then, the substrate temperature is decreased to a second temperature value. At the end of the first time period and during the temperature decrease, introduction of the first silicon precursor gas and/or the introduction of a second silicon precursor gas is maintained. The gases preferably have a partial pressure adapted to the formation of a silicon layer having a thickness smaller than 0.5 nm.

Claims

1. A method of gas phase epitaxial deposition of a semiconductor material made of one of silicon, germanium, or silicon-germanium on a single-crystal semiconductor surface of a substrate, the method comprising successive steps of: placing the substrate in an epitaxy reactor swept by a carrier gas; bringing the substrate temperature to a first temperature value; introducing, for a first time period, at least a first precursor gas selected from the group consisting of: a silicon precursor gas and a germanium precursor gas; and decreasing the substrate temperature down to a second temperature value, after the first time period, maintaining the introduction of at least the first precursor gas having a partial pressure adapted to the forming of a silicon layer having a thickness smaller than 0.5 nm.

2. The method of claim 1, wherein a surface of the substrate is made of silicon.

3. The method of claim 1, wherein the carrier gas is an inert gas.

4. The method of claim 3, wherein the carrier gas is selected from the group consisting of: hydrogen, dinitrogen, helium, and a rare gas.

5. The method of claim 1, wherein the silicon precursor gas is selected from the group consisting of: silane, disilane, dichlorosilane, trichlorosilane, and silicon tetrachloride.

6. The method of claim 1, wherein the germanium precursor gas is selected from the group consisting of: germane and digermane.

7. The method of claim 1, further comprising depositing by selective epitaxy during which a gas capable of etching silicon is introduced during the first time period.

8. The method of claim 7, wherein the gas capable of etching silicon is selected from the group consisting of: hydrogen chloride and gaseous chlorine.

9. A method of gas phase epitaxial deposition of a semiconductor material made of one of silicon, germanium, or silicon-germanium on a surface of a silicon single-crystal semiconductor substrate, said surface having a lateral dimension smaller than 40 nm formed on a silicon region, the method comprising successive steps of: placing the silicon single-crystal semiconductor substrate in an epitaxy reactor swept by hydrogen; bringing the silicon single-crystal semiconductor substrate temperature to a first temperature value; introducing, after a first time period, dichlorosilane, germane, and hydrogen chloride; and decreasing the silicon single-crystal semiconductor substrate temperature down to a second temperature value, and at the end of the first time period, maintaining the introduction of dichlorosilane.

10. The method of claim 9, wherein the silicon-germanium has a germanium concentration greater than 35%.

11. The method of claim 9, wherein the silicon-germanium deposit has a thickness in the range from 4 to 25 nm.

12. The method of claim 9, wherein the hydrogen is introduced into the epitaxy reactor, at a flow rate in the range from 40 to 50 standard liters per minute, the dichlorosilane is introduced at a flow rate in the range from 0.06 to 0.3 standard liter per minute, the germane is introduced at a flow rate in the range from 0.006 to 0.03 standard liter per minute, and the hydrogen chloride is introduced at a flow rate in the range from 0.01 to 0.1 standard liter per minute.

13. The method of claim 12, wherein the dichlorosilane is introduced at a flow rate in the order of 0.1 standard liter per minute.

14. The method of claim 12, wherein germane is introduced at a flow rate in the order of 0.01 standard liter per minute.

15. The method of claim 12, wherein hydrogen chloride is introduced at a flow rate in the order of 0.06 standard liter per minute.

16. The method of claim 9, wherein the first temperature value is in the range from 650 to 750 C.

17. The method of claim 9, wherein the second temperature value is in the range from 400 to 650 C.

18. The method of claim 9, wherein the silicon or silicon-germanium is boron-doped in situ by using diborane.

19. The method of claim 9, wherein the silicon or the silicon-germanium is doped in situ with a negative-type dopant by using phosphine or arsine.

20. The method of claim 9, wherein a silicon-germanium-carbon alloy is deposited by epitaxy.

21. A structure obtained by implementing the method of claim 1.

22. The structure of claim 21, wherein the structure comprises a silicon-germanium deposit on a silicon surface having a lateral dimension smaller than 40 nm of a substrate, said deposit having a lateral dimension smaller than 40 nm and being faceted, with no rounding of the facet angles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, wherein:

[0030] FIGS. 1A and 1B, previously described, show timing diagrams illustrating a heteroepitaxy deposition method;

[0031] FIG. 2, previously described, is a cross-section view of a heteroepitaxial structure;

[0032] FIG. 3, previously described, is a cross-section view of another heteroepitaxial structure;

[0033] FIGS. 4A and 4B show two timing diagrams illustrating an embodiment of a heteroepitaxy deposition method; and

[0034] FIG. 5 is a graph illustrating the shape of the deposition formed with the method of FIG. 4.

DETAILED DESCRIPTION

[0035] The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those steps and elements which are useful to the understanding of the described embodiments have been shown and are detailed.

[0036] In the following description, unless otherwise specified, expression in the order of means to within 10%, preferably to within 5%.

[0037] An embodiment of a method of gas-phase epitaxial deposition of silicon, of germanium, or of silicon-germanium on a semiconductor substrate, for example, silicon or silicon-germanium is here provided. This method comprises the same steps as the method described in relation with FIGS. 1A and 1B, but for the fact that certain deposition gases are kept in the epitaxy reactor after the actual epitaxy phase. Gases 24 comprise, for example, precursor gases for the deposition of silicon, precursor gases for the deposition of germanium, and gases capable of etching silicon. The gases which are desired to be kept are, for example, called active gases hereafter. The active gases comprise precursor gases for the deposition of silicon and gases capable of etching silicon. It will be within the abilities of those skilled in the art to determine the partial pressures of active gases to be introduced so as to form a silicon layer having a thickness which remains lower than 0.5 nm.

[0038] Precursor gases for the deposition of silicon are, for example, silane (SiH.sub.4), disilane (Si.sub.2H.sub.6), trisilane (Si.sub.3H.sub.8), dichlorosilane (SiH.sub.2Cl.sub.2), trichlorosilane (SiHCl.sub.3), silicon tetrachloride (SiCl.sub.4), or any other known precursor. Precursor gases for the deposition of germanium are for example germane or digermane (Ge.sub.2H.sub.6), or any other known precursor. Gases capable of etching silicon are for example hydrogen chloride (HCl) or gaseous chlorine (Cl.sub.2).

[0039] As an example, for a case of epitaxial deposition of silicon-germanium on silicon in the presence of dichlorosilane (SiH.sub.2Cl.sub.2), of germane (GeH.sub.4), and of hydrogen chloride (HCl), the carrier gas being hydrogen (H.sub.2), the active gases are dichlorosilane and possibly hydrogen chloride.

[0040] FIG. 4A shows, in a timing diagram 50, the temperature variation during the process. FIG. 4B shows, in a timing diagram 60, the different gases flowing through the epitaxy reactor during the process.

[0041] This embodiment comprises the successive steps of: [0042] between times t0 and t1, increasing the susceptor temperature up to deposition temperature Td; [0043] between times t1 and t2, introducing deposition gases 24; [0044] between time t2 and a time t3, maintaining the above-mentioned active gases 62 and decreasing the temperature down to a temperature Tdu at which the surface mobility of silicon or germanium atoms becomes negligible and the shape of the epitaxial structure is no longer capable of deforming under the action of temperature; and [0045] after time t3, purging the reactor and ventilating when the wafer temperature reaches a sufficiently low temperature.

[0046] As an example, to obtain a silicon-germanium deposit having, for example, a germanium concentration greater than 35%, the following pressure and flow rate values are selected. The total pressure of the gases in the epitaxy reactor is in the order of 2,600 Pa (20 torr). The hydrogen may be introduced into the epitaxy reactor at a flow rate in the range from 30 to 40 slm (standard liters per minute, liter at standard pressure and temperature conditions, that is, for a 1-bar pressure and a 25 C. temperature). The dichlorosilane is introduced, for example, at a flow rate in the order of 0.1 slm. The germane is introduced, for example, at a flow rate in the order of 0.01 slm. The hydrogen chloride is introduced, for example, at a flow rate in the order of 0.05 slm. Deposition temperature Td is in the range from 650 to 750 C., for example, 620 C. The duration of the deposition phase t2-t1 is, for example, in the order of 300 s for a deposit having a thickness in the order of 20 nm. Temperature Tdu is in the range from 400 to 650 C., for example, in the order of 500 C.

[0047] In the case where a silicon-germanium deposit doped with boron atoms is desired to be formed, a gas containing boron atoms, such as diborane (B.sub.2H.sub.6), is added to deposition gases 24. The diborane may be introduced into the epitaxy reactor at a flow rate selected according to the flow rates of the other deposition gases, such a selection being within the abilities of those skilled in the art. In this case, a deposition temperature Td in the order of 610 C. is for example selected. In these conditions, a deposition of boron-doped silicon-germanium is performed with a dopant atom concentration in the range from 10.sup.19 to 510.sup.20 atoms/cm.sup.3, for example, in the order of 410.sup.20 atoms/cm.sup.3.

[0048] FIG. 5 shows profiles of studs 72 and 74 respectively obtained by the method of FIGS. 1A and 1B and by that of FIGS. 4A and 4B, in the case of studs having lateral dimensions smaller than 30 nm. The axis of abscissas represents a lateral dimension L of the stud and the axis of ordinates represents thickness H of the stud. These two dimensions are expressed in nm. Profile 72 has a more or less semi-circular shape like the stud described in relation with FIG. 3. Profile 74 has a substantially planar upper surface like the large stud described in relation with FIG. 2. This upper surface has a radius of curvature greater than 4 times the width of the pattern and/or a RA roughness smaller than 0.5 nm rms (root mean square) after correction of the main curvature.

[0049] Such a satisfactory result can be expressed as follows. The thermal rounding phenomenon would be the result of the surface tension of the silicon (or silicon-germanium or germanium) surface and of the mobility of silicon (and/or germanium) atoms after the actual deposition phase. The effect of this phenomenon very strongly increases when dimension L becomes smaller than 30 nm. There would seem that after time t2, once the epitaxial deposition phase is over, the shape of the deposition is identical to that described in relation with FIG. 2, whatever the value of dimension L. It is considered that the degradation of the stud shape appears during the third phase of the method. The silicon (and/or germanium) atoms of the silicon-germanium stud would have a certain surface mobility once the deposition is completed, that is, after time t2. Since the surface mobility decreases as the temperature decreases, the stud would stop deforming once a temperature Tdu has been reached. The introduction of the active gases during this phase would generate a phenomenon of adsorption of atoms of the active gases at the surface of the deposit. The silicon atoms of the deposit would be immobilized by the atoms, generally chlorine and/or hydrogen, originating from the active gases coupling to their dangling bonds. Thus, the stud can no longer degrade. However, since the presence of germanium favors the desorption of chlorine and hydrogen atoms and decreases the quantity of adsorbed radicals, germane thus does not belong to the active gases. The rearrangement of the semiconductor crystal atoms, at high deposition temperatures, by surface mobility, would decrease the surface energy of epitaxial structures of small dimensions. This same surface mobility at high temperature would further be implemented during the forming of Stranski-Krastanov islands which affect planar epitaxial surfaces in the presence of mechanical stress. These islands are local unevennesses of the deposit thickness.

[0050] The presence of precursor gases for the deposition of silicon may favor the deposition of a silicon layer, having a thickness smaller than 0.5 nm, at the surface of the deposit. The layer will be removed by different cleanings which conventionally follow epitaxial deposition methods.

[0051] Specific embodiments have been described. Various alterations and modifications will occur to those skilled in the art. In particular, this method is also efficient to suppress Stranski-Krastanov islands.

[0052] Further, the silicon or the silicon-germanium may be doped in situ with a negative-type dopant by using phosphine or arsine.

[0053] Further, the epitaxial deposit may be made of an alloy of silicon-germanium-carbon (SiGeC).