Abstract:
A method for making a semiconductor device may include forming, on a first semiconductor layer of a semiconductor-on-insulator (SOI) wafer, a second semiconductor layer comprising a second semiconductor material different than a first semiconductor material of the first semiconductor layer. The method may further include performing a thermal treatment in a non-oxidizing atmosphere to diffuse the second semiconductor material into the first semiconductor layer, and removing the second semiconductor layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of electronic devices and, more particularly, to semiconductor devices and related methods. 
     BACKGROUND OF THE INVENTION 
     Some semiconductor devices utilize semiconductor-on-insulator (SOI) technology, in which a thin layer of a semiconductor (typically having a thickness of a few nanometers), such as silicon, is separated from a semiconductor substrate by a relatively thick electrically insulating layer (typically featuring a thickness of a few tens of nanometers). Integrated circuits using SOI technology offer certain advantages compared to traditional “bulk” technology for Complementary Metal Oxide Semiconductor (CMOS) integrated circuits. For example, SOI integrated circuits typically provide a lower power consumption for a same performance level. 
     SOI circuits may also feature a reduced stray capacitance, allowing an increase of commutation speeds. Furthermore, the latch-up phenomena encountered in bulk technology may be mitigated. Such circuits are commonly used in System on Chip (SoC) and Micro electro-mechanical systems (MEMS) applications. SOI circuits may also be less sensitive to ionizing radiations, making them more reliable than bulk-technology circuits in applications where radiation may induce operating problems (e.g., aerospace applications). SOI integrated circuits may include memory components such as Static Random Access Memory (SRAM), as well as logic gates. 
     One SOI implementation is for enhanced scaling in Ultra-thin Body and BOX (UTBB), or UTBOX, devices. UTBB cells may include an NMOS transistor and a PMOS transistor, both formed in the thin silicon layer which overlies the buried insulating oxide layer. One example UTBOX integrated circuit is disclosed in U.S. Pat. No. 8,482,070 to Flatresse et al., which is hereby incorporated herein in its entirety by reference. The integrated circuit has cells placed in a cell row having a PMOS transmitter including a ground beneath the PMOS transmitter, and an n-doped well beneath the ground and configured to apply a potential thereto. An NMOS transmitter includes a ground beneath the NMOS transmitter, and a p-doped well beneath the ground and configured to apply a potential thereto. 
     Despite the existence of such configurations, further enhancements in SOI or UTBB devices may be desirable in some applications. 
     SUMMARY OF THE INVENTION 
     A method for making a semiconductor device may include forming, on a first semiconductor layer of a semiconductor-on-insulator (SOI) wafer, a second semiconductor layer comprising a second semiconductor material different than a first semiconductor material of the first semiconductor layer. The method may further include performing a thermal treatment in a non-oxidizing atmosphere to diffuse the second semiconductor material into the first semiconductor layer, and removing the second semiconductor layer. 
     More particularly, removing the second semiconductor layer may comprise removing the second semiconductor layer using a wet etch. Furthermore, performing the thermal treatment may comprise performing the thermal treatment to provide a non-linear diffusion profile of the second semiconductor material within the first semiconductor layer. 
     In one example embodiment, a thermal treatment may also be performed in an oxidizing atmosphere prior to removing the second semiconductor layer. More particularly, performing the thermal treatment in the non-oxidizing atmosphere may be for a shorter duration than performing the thermal treatment in the oxidizing atmosphere. Additionally, performing the thermal treatment in the non-oxidizing atmosphere may be at a higher temperature than performing the thermal treatment in the oxidizing atmosphere. 
     By way of example, performing the thermal treatment may comprise performing the thermal treatment in the non-oxidizing atmosphere for 50 seconds or less. Also by way of example, performing the thermal treatment may comprise performing the thermal treatment in the non-oxidizing atmosphere at a temperature of 1000° C. or less. 
     The method may further include forming a nitride layer overlying the second semiconductor layer prior to performing the thermal treatment in the non-oxidizing atmosphere. The second semiconductor material may comprise germanium, and the first semiconductor material may comprise silicon, for example. The method may also include forming source and drain regions defining a channel in the first layer after the diffusion of the second semiconductor material therein, and forming a gate above the channel. By way of example, the SOI wafer may comprise an ultra-thin body and buried oxide (UTBB) wafer. 
     A related semiconductor device may include a semiconductor substrate, and an n-channel metal-oxide semiconductor field-effect transistor (NMOS) comprising a first buried oxide (BOX) layer on the semiconductor substrate, a first semiconductor layer on the first BOX layer, a first pair of source and drain regions defining an n-type channel therebetween in the first semiconductor layer, and a first gate above the n-type channel. The semiconductor device may further include a p-channel MOSFET (PMOS) including a second BOX layer on the semiconductor substrate, a second semiconductor layer on the second BOX layer, a second pair of source and drain regions defining an p-type channel therebetween in the second semiconductor layer, and a second gate above the p-type channel. A shallow trench isolation (STI) region may be between the NMOS and the PMOS. Furthermore, the second BOX layer may have a planar upper surface devoid of oxide protrusions adjacent the STI region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is cross-sectional diagram of a semiconductor device in accordance with an example embodiment. 
         FIGS. 2-4  are a series of cross-sectional diagrams illustrating an example method of making the semiconductor device of  FIG. 1 . 
         FIG. 5  is a graph illustrating depth vs. percent concentration for germanium within a PMOS device formed using the method of  FIGS. 2-4 . 
         FIGS. 6 ,  6 A,  7 , and  8  are a series of cross-sectional diagrams illustrating another example method of making a semiconductor device. 
         FIG. 9  is a cross-sectional diagram of a PMOS cell of a prior art SOI device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     By way of background, one potential problem that may arise with SOI devices, and more particularly UTBB devices, is that the BOX thickness may be undesirably increased during the silicon germanium p-channel formation using typical condensation techniques. Referring more particularly to  FIG. 9 , a partially completed PMOS cell  90  of a UTBB device formed using conventional techniques is shown. The PMOS cell  90  illustratively includes a substrate  91 , a BOX layer  92  on the substrate, an SiGe channel layer  93  on the BOX layer, and shallow trench isolation (STI) regions  94  which insulate the PMOS cell from adjacent cells. Typically, the silicon germanium channel layer  93  is formed by depositing a germanium-rich layer above the thin silicon layer of the UTBB wafer, and then performing a rapid thermal oxidation (RTO) at a sufficiently high temperature (e.g., 1050° C. or higher) to cause germanium condensation into the thin silicon layer. This is done for a given time to oxidize an appropriate depth of material, which is in turn removed to provide the desired channel thickness. 
     Yet, one difficulty which may arise when performing germanium condensation using the RTO in this manner is that the BOX layer  92  also oxidizes, leaving undesirable protrusions or wedges of oxidation  95  that create a non-uniform thickness of the BOX layer. These additional oxidation regions  95  tend to be more pronounced adjacent the STI regions  94 , as shown. This variation in the thickness of the BOX layer  92  may undesirably impact the threshold voltage, V T , of the device. For example, a 1 nm growth of the BOX layer  92  results in an approximate  10  mV increase in the threshold voltage for next generation devices (e.g., 15-20 nm BOX thicknesses). 
     Referring now to  FIG. 1 , a semiconductor device  30  is first described. In the illustrated example, the semiconductor device  30  is a UTBB CMOS device, although other types of devices and SOI configurations may also be provided using the techniques set forth herein. The semiconductor device  30  illustratively includes a substrate  31  (e.g., silicon) with a PMOS transmitter  32  and an NMOS transmitter  33  thereon, which are separated by an STI region  34 . The NMOS transmitter  33  includes a first BOX layer  35  on the substrate  31 , a first semiconductor layer  36  on the first BOX layer, a first pair of source and drain regions  37 ,  38  defining an n-type channel  39  therebetween in the first semiconductor layer, and a first gate  40  above the n-type channel. The gate  40  illustratively includes a gate dielectric layer  41 , a gate electrode layer  42 , and sidewall spacers  43 , as will be appreciated by those skilled in the art. 
     The PMOS  32  illustratively includes a second BOX layer  45  on the semiconductor substrate  31 , a second semiconductor layer  46  on the second BOX layer, a second pair of source and drain regions  47 ,  48  defining a p-type channel  49  therebetween in the second semiconductor layer, and a second gate  50  above the p-type channel. The gate  40  illustratively includes a gate dielectric layer  51 , a gate electrode layer  52 , and sidewall spacers  53 . As will be discussed further below, the second BOX layer  45  may have a planar upper surface devoid of oxide protrusions adjacent the STI region  34 . 
     One example method for making the semiconductor device  30  is now described with reference to  FIGS. 2-4 . A nitride (mask) layer  55  is formed over the NMOS region (the right half of  FIG. 2 ) on a UTBB wafer to protect the NMOS during formation of the SiGe channel layer on the PMOS region. To this end, an SiGe layer  56  is grown on the second semiconductor layer  46  (e.g., silicon on a silicon UTBB wafer). In the present example, the SiGe layer  56  has a concentration of about 40% germanium, although other concentrations may be used in different embodiments, as will be discussed further below. 
     An optional nitride liner  57  may be deposited overlying the SiGe layer  56  prior to heat treating ( FIG. 2A ), which may help reduce germanium loss, although the nitride liner need not be used in all embodiments. A rapid thermal anneal (RTA) step is then performed to cause diffusion of the germanium from the SiGe layer  56  into the second silicon layer  46 . More particularly, in the present embodiment, the RTA step replaces the above-described RTO condensation process and instead causes diffusion of the germanium from the SiGe layer  56  into the second silicon layer  46  in a non-oxidizing (or un-oxidized) atmosphere, as will be appreciated by those skilled in the art. By way of example, an example non-oxidizing atmosphere may have less than ten parts-per-million (PPM) of oxygen atoms therein. The original boundary between the second silicon layer  46  and the SiGe layer  56  is indicated by a dashed line  58 . 
     Besides being performed in a non-oxidizing atmosphere, the RTA may also be performed at a relatively lower temperature, which also helps prevent any further oxidation of the second BOX layer  45  if any stray oxygen were present. For example, the RTA may be performed at a temperature of 1000° C. for 90 seconds, although other temperatures and times may be used in different embodiments. 
     While the typical RTO process may undesirably add unwanted oxidation to the second BOX layer  45 , one advantage of RTO processing is that it is relatively easy to control the depth of oxidation (and therefore the final thickness of the second semiconductor layer  46 ), and it provides a relatively smooth upper surface to the second semiconductor layer. With the RTA processing, a wet etch (e.g., SC1, etc.) may instead be used to pattern the second semiconductor layer  46  to the desired thickness, which may ordinarily be more difficult to control in terms of depth and smoothness of the second semiconductor layer  46  upper surface. To provide enhanced control of the etching, the initial quantity of germanium in the SiGe layer  56 , as well as the temperature and duration of the anneal, may be selected to provide a non-linear diffusion profile of germanium in the second semiconductor layer  46 , as will be understood further with respect to the simulated germanium profiles shown in the graph  60  of  FIG. 5 . In accordance with one example wet etching process, an SC1 clean may be performed with a 1:1:5 solution of NH 4 OH (ammonium hydroxide 27%)+H 2 O 2  (hydrogen peroxide 30%)+H 2 O (water) at 75 to 80° C. This allows for etching of the SiGe with an etch rate that depends on the Ge concentration present. That is, the more Ge that is present, the higher the etch rate. 
     In the illustrated example, a first plot line  61  represents the concentration of germanium in the SiGe layer  56  prior to the RTA. The SiGe layer  56  has a thickness of 35 nm (X-axis), and a liner or uniform concentration of 40% germanium (Y-axis) throughout. There is no germanium in the second semiconductor (i.e., silicon) layer  46 , which extends from 0 to −5 nm in the illustrated example), and the second BOX layer  45  (SiO 2 ) extends from −5 nm to −10 nm in the graph  60 . 
     The second plot line  62  is a modeled non-linear profile of the germanium within the SiGe layer  56  and the second semiconductor layer  46  after 90 seconds of RTA at a temperature of 1000° C. This creates a germanium concentration of about 27% at the Si/SiGe layer interface, and about 21% at the Si/SiO 2  interface. As such, by selecting an appropriate etch formulation and time, etching can be performed to the desired depth, i.e., where the germanium concentration is about 27% in the present example, to thereby provide the desired thickness for the second semiconductor layer  46 . The germanium within the second semiconductor layer  46  will typically be distributed evenly (i.e., with a liner distribution profile) during later thermal processing (e.g., gate oxide  51  formation, etc.), providing a density of about 25% germanium throughout the second semiconductor layer  46 . However, it will be appreciated that other starting and finishing concentrations of germanium may be used in different embodiments. 
     Once etching has been performed to provide the desired depth for the second semiconductor layer  46  ( FIG. 4 ), the protective nitride layer  55  may be removed and further processing steps may be performed to provide the structure shown in  FIG. 1 , as will be appreciated by those skilled in the art. 
     Turning to  FIGS. 6-8 , another process for making the semiconductor device  30  is now described. In this approach, the conventional high-temperature RTO condensation approach described above with reference to  FIG. 9  is instead replaced by a first RTA diffusion step in a non-oxidizing atmosphere, followed by a subsequent lower temperature RTO condensation step in an oxidizing atmosphere. Yet, because of the first RTA step to diffuse germanium within the second semiconductor layer  46 , the second RTO condensation step may be performed with lower thermal budget to thereby help prevent oxidation of the second BOX layer  45 . 
     More particularly, as previously described with reference to  FIG. 2  above, a SiGe layer  56  is first formed above the second semiconductor (silicon in the present example) layer  46 . However, in this embodiment, the germanium content of the SiGe region may be the same as the final germanium content desired in the second semiconductor layer  46 , e.g., 25%. This is because the subsequent RTO processing step may be used to remove the appropriate material thickness without a wet etch, and thus the non-linear germanium profile need not be used to control the etch depth as described above. By way of example, the first RTA step ( FIG. 7 ) may be performed at a temperature of 1050° C. for 50 sec., while the second RTO step ( FIG. 8 ) may be performed at a temperature of 1000° C. for 60 sec., although different temperatures and durations may be used as appropriate in different embodiments. As seen in  FIG. 6 , the RTO step oxidizes the SiGe layer  56  into an oxide layer  60 , which may be removed during further processing steps to provide the structure shown in  FIG. 1 , as will again be appreciated by those skilled in the art. With this approach, the SiGe oxidation occurs with a lower thermal budget compared to silicon oxidation, which helps prevent oxidation of the second BOX layer  45 . This may be done because of the initial germanium diffusion via RTA. 
     Both of the above-described approaches may advantageously provide for reduced BOX oxidation on PMOS devices, to accordingly help facilitate next generation scaling, such as for UTBB devices, for example. More particularly, scaling of UTBB devices at 14 and 10 nm nodes may potentially be achieved using reduced BOX and channel thickness, which may be particularly beneficial for future CMOS devices. In an example UTBB embodiment, the BOX may be less than about 30 nm, and more particularly about 25 nm. Also by way of example, the semiconductor layer over the BOX may be less than about 10 nm, and more particularly about 7 nm, although other dimensions are also possible in different embodiments. 
     While silicon and germanium have been described in the illustrated embodiments, other semiconductor materials could also be used. Moreover, in view of the above, it will be appreciated that a variety of different transistor structures may be implemented, including but not necessarily limited to: planar CMOS, high-k metal gate CMOS, PD-SOI, FD-SOI, UTBB, vertical double gate, buried gate, FinFET, tri-gate, multi-gate, 2D, 3D, raised source/drain, strained source/drain, strained channel, and combinations/hybrids thereof, for example. 
     Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.