Patent Application
20090272976
1. Field of the Invention
The present invention is related to a method for integrating CMOS devices on a semiconductor substrate.
2. Description of the Related Technology
Integrated CMOS processing involves the production of pMOSFET and nMOSFET devices (hereafter generally called ‘pMOS’ and ‘nMOS’) on a semiconductor substrate. Current research has revealed promising results for Ge pMOS devices, i.e. p-channel MOSFET transistors produced in a layer of Ge, whereas Ge nMOS devices have shown inferior performance. It is advantageous therefore to combine Si nMOS with Ge pMOS, but present research has encountered a number of technical difficulties in this area. Standard dopant activation anneals for Si are performed at temperatures typically higher than 1000° C. for high performance logic. However, the melting temperature of Ge is 937° C. Consequently, with a conventional process flow, the Si nMOS needs to be fabricated through dopant activation prior to depositing Ge. Essentially the entire front end process flow needs to be repeated for the Ge pMOS, with the associated increase in fabrication cost and the risk of adversely affecting the existing Si nMOS. Lowering the anneal temperature and annealing for longer times would be an option, but for short channel devices (channel width <˜0.5 μm) such a cooler, longer anneal would result in low dopant activation and an excessive diffusion of dopants in the tail of the junction profile. This makes it difficult to produce short channel Ge pMOS and Si nMOS in one process flow, while controlling dopant activation and diffusion.
The document “Integration of Germanium-on-Insulator and Silicon MOSFETs on a Silicon Substrate”, Jia Feng et al, IEEE Elect. Dev. Lett. 27 911 (2006) describes a CMOS flow with Si nMOS and Ge pMOS. They report fabricating the Si nMOSFET using a conventional activation anneal (1050° C. RTP), then depositing the Ge and fabricating the Ge pMOSFET.
The document “Strained Si and Ge MOSFETs with High-K/Metal Gate Stack for High Mobility Dual Channel CMOS”, O. Weber et al, IEEE IEDM Tech Dig, 137 (2005) discusses CMOS, but actually describes fabrication of nMOS and pMOS on separate samples. For the CMOS emulation, a 600° C. activation anneal is done for Ge and Si. However, Weber et al. only present long channel (10 μm) device results for Si, and no indication is given as to how the method can be scaled to short channel devices. In fact, there is no indication of Ge implantation, as an amorphization process, for NMOS devices. Such an amorphization process is essential for allowing deep short channel transistor scaling with a low thermal budget. An anneal time of 15 min is specified for this work in “Etude, Fabrication et Propriétés de Transport de Transistors CMOS associant un Diélectrique Haute Permittivité et un Canal de Conduction Haute Mobilité”, O. Weber, PhD Thesis, CEA-LETI and INSA de Lyon (2005).
It is thus desirable to have an improved method for integrating Ge pMOS and Si NMOS devices in CMOS processing.
One inventive aspect is related to a method as disclosed in the appended claims. Preferred embodiments are disclosed in combinations of the independent claim with one or more dependent claims.
In particular, one inventive aspect is related to a method for producing one or more nMOSFET devices and one or more pMOSFET devices on the same semiconductor substrate. The method comprises creating at least one crystalline Si active area and at least one crystalline Ge active area on the substrate, the active areas being separated by insulator areas; forming a gate on the Si active area and a gate on the Ge active area; after formation of the gates, implanting the Ge and Si areas with dopants to form source and drain regions; activating the dopants, wherein after gate formation, and prior to the implantation process, at least the crystalline Si active area is subjected to an amorphization process, so as to form an amorphous Si layer on top of a crystalline Si layer, wherein activation of dopants in the amorphous Si layer takes place by a process of activation-annealing the substrate at a temperature and during a time interval suitable for obtaining Solid Phase Epitaxial Regrowth of the amorphous layer, wherein activation of dopants in the Ge area takes place by the same activation-annealing process.
The process of implanting the Ge and Si areas with dopants to form source and drain regions is typically done by applying the following processes in the Si active area and, in the Ge active area, respectively: (a) a first implantation process to form halo and extension areas in the Si, respectively, Ge area not covered by the gate stack, followed by (b) formation of spacers on both sides of the gate, followed by (c) a second implantation process to form highly doped areas in the Si, respectively Ge area not covered by the gate stack and spacers, having a higher dopant density than the halo/extension areas.
The amorphization process may take place by implanting the Si active area with Si ions, Ge ions, or impurity ions.
The Ge active area may be implanted with the ions in the same implanting process, so as to obtain an amorphous Ge layer, on top of a crystalline Ge layer.
The activation-annealing process may take place at a temperature between about 500° C. and 900° C., more preferably between about 550° C. and 700° C. Even more preferably, the activation-annealing process takes place at a temperature between about 600° C. and 900° C., and during a time interval of less than about 5 minutes.
The Si active area may include or consist of one or more layers of SixGe1-x, with x>about 80%. The Ge active area may include or consist of one or more layers of SiyGe1-y, with y<about 20%.
One inventive aspect utilizes a single activation anneal that serves for both Si nMOS and Ge pMOS. By use of a solid phase epitaxial regrowth (SPER) process for Si nMOS, the thermal budget for the Si nMOS can be lowered to be compatible with Ge pMOS. Activation of Ge n-type dopants and reasonable SPER rates for Si provide a practical lower limit on temperature of about 500° C. and the melting of Ge provides an upper limit of about 937° C. According to the preferred embodiment, an activation anneal temperature is applied between about 500 and 900° C., more preferably between about 550 and 700° C. Within these temperature ranges, times that are sufficient to crystallize the Si are sufficient to activate the dopants in Ge.
The Si-area 2 comprises a Si p-well, delimited by the dotted line, and the Ge-area 3 likewise comprises a Ge n-well (delimited by the dotted line). These wells may be formed, for example, by a doping implantation process and/or during a doped selective epitaxial growth of all or part of areas 2 and/or 3, prior to the gate formation. Gate stacks 7,7′ and spacers 8,8′ are present both in the nMOS and the pMOS devices. Gate stacks 7, 7″ are hereafter called ‘gates’, while the expression ‘gate stack’ is reserved for the stack of layers formed before formation of the gate by etching the gate stack. The n-doped regions 5,6 are the source and drain of the nMOS device, whereas the p-doped regions 5′,6′ are the source and drain of the pMOS device. These source and drain regions are typically formed by implantation processes which take place after the formation of the gate, in order to form the typical junction profiles (Highly doped drain (HDD), Lowly doped drain (LDD), halos). Such implantation to form source and drain areas can take place by applying the following processes in the Si active area and, separately, in the Ge active area:
These dopants need to be activated by an anneal process. In one embodiment, the dopant activation takes place simultaneously in the Si and Ge active areas, by an anneal process which activates the dopants in the Si-active area through Solid Phase Epitaxial Regrowth (SPER) of an amorphous Si layer. The anneal may take place at temperatures of around 600° C., and in any case below the melting temperature of Ge, so that activation of the dopants in the Ge-active area is performed without danger of melting the Ge.
To obtain the amorphous Si layer, the Si-active area is subjected to an amorphization process, after formation of the gate 7 (and before spacer formation, and before the first implantation process forming the extension and/or halo regions). The amorphization process can be performed prior to the doping implant applied for forming source and drain areas (5,6).
When performed as a separate process, amorphization of a crystalline Si-layer may be obtained by implanting the layer with high energy ions, e.g. impurity ions or Si or Ge ions. Ge and Si ions with a dose of about 1×1014 to 1×1015 ions/cm2 and with energies of about 5 to 100 keV may be used to guarantee amorphization and good short channel transistor scaling. The result is an amorphous Si-layer on top of a crystalline Si substrate. Later, the p and n dopants are added to the amorphous Si to form the p-channel and n-source/drain areas. By annealing the amorphous layer at temperatures of around 600° C., a short anneal time (˜5 min) suffices to activate the dopants which are present in the amorphous Si.
In order to prevent deep S/D transistor punch-through, the amorphous layer depth needs to include not only the junction dopant profile but also the underneath halo profile. During the regrowth, junction and halo dopants will be highly activated and this will ensure good short channel transistor properties. The energy of the amorphous implant has to be carefully chosen to avoid dopant penetration through the metal gate stack.
The thermal budget for the SPER flow is limited to ensure minimal dopant diffusion. Conceptually, the anneal time is chosen to substantially crystallize the amorphised semiconductor. The exact thermal budget will depend upon the thickness to be crystallized and the rate of solid phase epitaxial regrowth. For undoped Si, the solid phase epitaxial regrowth rate is about (3.68×108 cm/s)exp[−(2.76 eV)/kBT], where kB is the Boltzmann constant and T is the absolute temperature, per G L Olson and J A Roth, Materials Science Reports 3, 1 (1988). As an example, at 600° C., the SPER rate is 26 nm/min, which means that it would take slightly less than 4 min to crystallize a 100 nm amorphous Si layer. At 550° C., the required time for the same 100 nm layer increases to ˜40 min and at 650° C., the time drops to less than a minute. While the exact rate of SPER will vary with impurity concentration and crystal orientation, the concepts are the same.
One of the benefits of the SPER regrowth associated with deep Ge or Si pre-amorphization, is the high activation of junction and halo dopant below the junction profile ensuring good short channel transistor properties. A second benefit of the dopant activation by SPER over the more conventional hotter activation anneals is the much higher rate of dopant activation at a much lower thermal budget. At typical SPER temperatures for Si of about 550 to 750° C., the diffusion of dopants is very slow, so as to be essentially diffusionless during the typical anneal times. As the amorphous/crystalline interface moves past the dopant atom, the dopant is incorporated into substitutional sites in higher amount compared to the dopant solid solubility in Si. For this reason the dopant activation reached with SPER is named ‘metastable’. Any additional thermal budget, after re-crystallization is completed, will induce dopant de-activation. In other words, the system will relax from a metastable activation state towards an activation limited by the dopant solid solubility in silicon. For conventional (non-SPER) activation in Si, a much higher temperature must be used for the dopant to go into electrically active sites, and furthermore, these high temperatures can lead to unwanted diffusion.
For germanium, the typical dopants are B and Ga for p-type and P, As, and Sb for n-type, see for example, D P Brunco et al, “Germanium: The Past and Possibly A Future Material for Microelectronics, ECS Trans 11 (4), 479 (2007). For Ge pMOS devices, B is typically used for extensions (aka lowly doped drains (LDD)) and for source/drain implants (aka highly doped drains (HDD)). At technologically relevant doses and conditions, B is essentially diffusionless and is well activated, even for temperatures as low as 400° C. Arsenic and/or phosphorus are used for wells, including implants known as deep well, shallow well, anti-punchthrough and VT-adjust, as well as for halos. While the problem of high activation/minimal diffusion of n-type dopants for Ge has not been solved for high doses (>about 5×1019 cm−3), the required doses for wells are typically less than ˜1×108 cm−3 and for halos are typically less than about 2×1019 cm−3. At these concentrations, both P and As activate fairly well with essentially no diffusion at temperatures as low as about 550 or 600° C.
According to one embodiment, amorphization of Si is performed by implanting the Si active area, e.g. with Ge ions, while protecting the Ge active area so that the Ge active area is not influenced by the amorphization implant. Alternatively, the Ge active area is not protected and the amorphization implant is performed on the Ge area as well, leading additionally to amorphization of the Ge-areas. In that case, under the right annealing conditions, SPER-activation takes place not only in the Si area, but also in the Ge areas.
For estimation purposes, the following table shows the calculated SPER rates in Si and the estimated SPER anneal times for activating an undoped or lightly doped 00 nm amorphous Si layer. For highly doped layers, the growth rates may be different due to other influences. In one embodiment, activation annealing at temperatures between 600° C. and 900° C. allows dopant activation in times of less than 5 min.
For estimation purposes, the following table shows the calculated SPER rates in Ge and the estimated SPER anneal times for activating a 100 nm amorphous Ge layer. For undoped Ge, the SPER rate is estimated (4.45×1018 nm/min)exp[−(2.19 eV)/kBT], where kB is the Boltzmann constant and T is the absolute temperature, per T E Haynes et al, Physical Review B 51, 7762 (1995). As can be seen, these rates are much higher than for Si such that any anneal time sufficient to crystallize the Si will also crystallize the Ge.
The following table provides an overview of the processes according to certain embodiments of the invention, with some comments or alternatives in the right hand column. The invention is not limited by this exemplary process flow. These embodiments are characterized only by the presence of an amorphization process, and by the activation anneal under SPER-conditions.
In one of the foregoing embodiments, the method relates to the use of a single activation anneal that serves for both Si nMOS and Ge pMOS. By use of a solid phase epitaxial regrowth (SPER) process for the Si nMOS, the thermal budget for the Si nMOS can be lowered to be compatible with Ge pMOS. Activation of Ge n-type dopants and reasonable SPER rates for Si provide a practical lower limit on temperature of about 500° C. and the melting of Ge provides an upper limit of about 937° C. According to one embodiment, an activation anneal temperature is applied between about 500 and 900° C., more preferably between 550 and 700° C. Within these temperature ranges, times that are sufficient to crystallize the Si are sufficient to activate the dopants in Ge.
The foregoing description details certain embodiments of the invention. It will be appreciated, however, that no matter how detailed the foregoing appears in text, the invention may be practiced in many ways. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the invention with which that terminology is associated.
While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the technology without departing from the spirit of the invention. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
| Number | Date | Country | Kind |
|---|---|---|---|
| EP 08155510.4 | Apr 2008 | EP | regional |
