Patent Application: US-201414764606-A

Abstract:
the invention provides a method of forming at least one metal germanide contact on a substrate for providing a semiconducting device by providing a first layer of germanium and a second layer of metal . the invention provides a step of reacting the second layer with the first layer with high energy density pulses for obtaining a germanide metal layer having a substantially planar interface with the underlying first layer .

Description:
there will now be described by way of example a specific mode contemplated by the inventors . in the following description numerous specific details are set forth in order to provide a thorough understanding . it will be apparent however , to one skilled in the art , that the present invention may be practiced without limitation to these specific details . in other instances , well known methods and structures have not been described in detail so as not to unnecessarily obscure the description . laser thermal annealing ( lta ) is used to form germanide contacts on n - doped ge , which are systematically compared to results generated by conventional rapid thermal annealing ( rta ). surface topography , interface quality , crystal structure , and material stoichiometry are explored for both annealing techniques . for electrical characterization , specific contact resistivity and thermal stability are extracted . it is shown that lta can produce a uniform contact with a remarkably smooth substrate interface , with specific contact resistivity 2 to 3 orders of magnitude lower than with the rta technique . it is shown that a specific contact resistivity of 2 . 84 × 10 − 7 ω · cm 2 is achieved for optimized lta energy density conditions . fig1 summarizes the process undertaken , from which the following results are discussed in this study . after cleaning , high - resistivity (& gt ; 40 ω · cm ) n - type ( 100 ) wafers received well implants , namely p with the dose of 4 × 10 12 cm − 2 and energy of 180 kev , followed by a b implant with the dose of 1 × 10 13 cm − 2 and energy of 40 kev to create a semi - insulating layer . the wafers then received a shallow p implant with the dose of 1 × 10 15 cm − 2 and energy of 12 kev . dopant activation was performed using an rta at 500 ° c . for 10 seconds in an n 2 ambient . thereafter 20 nm of ni was deposited using thermal evaporation . tlm patterning and dry etch was then carried out to minimize leakage currents . nickel is described herein to illustrate operation of the invention , it will be appreciated that other metals can be used such as fe , co , ni , pd , pt , cu or yb . one set of samples received rta treatment at either 250 , 275 , 300 , 325 or 350 ° c . in n 2 for 30 seconds . another set of samples received lta processing ( λ = 308 nm , single - pulse ) with laser densities ranging from 0 . 25 to 0 . 55 j / cm 2 and time durations ranging from 144 to 165 ns . the laser beam area was approximately 10 × 10 mm 2 . it is noted that these energy densities are significantly lower than those required for proper lta assisted dopant activation in ge [ 8 ]. various material characterization techniques were applied to inspect nige layer surface topography and crystalline quality , including scanning electron microscopy ( sem ), atomic force microscopy ( afm ), x - ray diffraction ( xrd ), and cross - sectional transmission electron microscopy ( xtem ). xtem was carried out using the jeol 2100 high - resolution tem . in particular , afm shows smoother surface layer , and cross - sectional tem shows a sharp , non - undulating interface after lta . for electrical characterization , the transfer length method ( tlm ) was used to extract ρ c and the keithley 37100 and keithley 2602 were used . in particular , contact resistance is determined as approximately 100 times lower using lta , relative to rta . in order to study the respective effects of rta and lta , firstly surface roughness was evaluated . afm was performed in tapping / non contact mode at room temperature in air . fig2 shows representative afm images of nige surface topography which were formed by ( a ) rta at 275 ° c . and ( b ) lta at the energy of 0 . 35 j / cm 2 . surface roughness ( rms ) for the rta sample is approximately 1 . 28 nm whereas the lta treated sample exhibits a roughness of approximately 0 . 39 nm . the table in fig2 shows the rms data extracted for all the samples . rms is larger for the rta set , except for the highest energy density lta . much like there is a process window for nige formation by rta [ 11 , 12 ] where , at high temperatures , the thin film agglomerates into islands , this data indicates that lta also has a process window for ni x ge y formation , above which the film degrades . 0 . 55 j / cm 2 appears too elevated a value for this application . mazzocchi et al . also reported a change in afm rms versus energy density in their lta dopant activation study in ge , which was attributed to the transition from non - melt , to sub - melt , to melt conditions [ 8 ]. from fig2 therefore , it can be concluded that ni x ge y layers formed by lta , at an energy density of 0 . 25 to 0 . 45 j / cm 2 , are much smoother that those formed by rta . a representative xtem image from a germanide contact formed by rta at 350 ° c . for 30 seconds in n 2 is shown in fig3 a . as seen in the figure , large grains of nige are formed , with a rough undulating interface between the alloy and the ge substrate . this result is expected , as non - smooth nige interfaces are commonplace when rta is used for the formation anneal [ 5 , 7 , 13 ]. with comparative reference to fig3 b now , the representative xtem image of a sample treated by lta , in this case with an energy density of 0 . 35 j / cm 2 stands in stark contrast , as lta results in smaller polycrystalline grains of germanide and a very a flat interface between the ge substrate and the alloy . a representative high - resolution ( hr ) xtem image of the germanide - substrate interface in the sample of fig3 a is next shown in fig4 a . as seen in the figure , there is no sharp transition from ge to nige . with comparative reference to fig4 b now , the representative ( hr ) xtem image of a sample treated by lta , in this case with an energy density of 0 . 35 j / cm 2 stands in stark contrast , showing a flat and uniform interface between ge and the germanide . the rows of ge atoms are clearly observable in the substrate . in terms of the interface quality , the interface is substantially planar and the term “ atomically - flat ” may be used , because there are unbroken horizontal rows in the ge ( 100 ) substrate transitioning immediately to germanide above it , without any detectable interfacial region or transition zone . furthermore , the evidence of ( hr ) tem indicates that this germanide layer is not necessarily lattice matched , nor epitaxially grown on top of the ge substrate . if one follows any row of ge substrate atoms diagonally upwards in fig4 b , this sequence does not continue into the germanide layer . the rows of atoms in the germanide layers are arranged in different directions to those in the ( 100 ) ge substrate . in some regions of the ge substrate - germanide interface the is evidence of an epitaxial relationship between the two crystalline materials , but this is highly localised to certain places along the horizontal interface . furthermore due to lattice mismatch between the two materials and the small size of crystals in the germanide layer , this does not extend vertically throughout the entire germanide layer . lattice - matched nisi growth on si has been reported by gao et al ., where ultra - thin ni layers were deposited on si [ 14 ], and nisi 2 preferentially formed as it has a similar lattice spacing to si . the dramatic improvement in interface roughness is linked to the thermal gradient and shallow heat distribution associated with ultra - short - pulse lta . unlike rta , wherein substantially the entire sample is at the target temperature without significant thermal gradients , lta generates intense thermal gradients , linked to the wavelength of the incident energy pulse , and the thermal diffusivity of the target material . the lta pulse heats the surface locally , and may melt the surface layers depending on the energy density applied . the two inset pictures in fig4 b show the electron diffraction pattern of ge substrate ( bottom right ) and the ni x ge y ( top right ). with reference to fig5 next , xtem images are presented for lta at a high energy density of 0 . 55 j / cm 2 . the left section ( a ) of fig5 is a wide view , and the right section ( b ) is a high resolution view , of ge and the contact interface . in this case , larger grains of germanide are formed and the interface is coarser relative to those observable in fig3 b and 4b . turning now to electrical characterisation , using the fabricated tlm test structures , ρ c of the germanide / n - type ge interface and the sheet resistance r sh of the underlying p doped ge layer were then extracted . in the tlm test structure , each nige bar was 380 × 100 μm 2 and the spacings were 4 , 16 , 36 , 64 , 100 , 144 , and 196 μm . the layout consisted of a repeated array of this tlm design . approximately 40 tlm structures within each array were electrically measured in order to extract reliable values for ρ c and r sh . fig6 shows the output from a tlm measurement in the above context . the inset shows current versus voltage as a function of contact spacing of a typical tlm structure fabricated using lta ( 0 . 45 j / cm 2 ). the resistance between contacts increases as the spacing increases . in the main part of fig6 , resistance versus contact spacing is plotted for the nige formed by rta at 275 °, 300 °, 350 ° c . and lta at 0 . 35 , 0 . 45 , and 0 . 55 j / cm 2 . straight lines are fitted to the data . intercepts of the line with vertical and horizontal axes are used to calculate ρ c and r sh according to theory [ 3 ]. the following table shows the results of ρ c and r sh extracted from all the tlm measurements . in the rta samples , r sh and ρ c decrease as the formation temperature increases from 275 to 350 ° c ., except at 325 ° c . for which there is currently no physical explanation . in an overall sense , the rta samples produce ρ c & gt ; 10 − 4 ω · cm 2 . in general , r sh and ρ c are lower in the lta samples . the best ρ c value is 2 . 84 × 10 − 7 ω · cm 2 obtained for the tlm sample lta - annealed at 0 . 45 j / cm 2 , and ρ c = 1 . 33 × 10 − 6 ω · cm 2 obtained for the tlm sample lta - annealed at 0 . 35 j / cm 2 is also a significant result . these ρ c values are 2 to 3 orders of magnitude lower than the equivalent rta cases . it should be noted that the only process variable in this experimental work was the nige formation anneal , and it is interesting to see that increasing the lta energy density to 0 . 55 j / cm 2 results in higher ρ c , which may be attributed to the degradation of the interface quality seen as shown in fig5 . it is well - known that ρ c is a strong function of active doping in the substrate below the contact , thus any boost in dopant activation will yield a similar improvement in ρ c . with reference to the above results the 0 . 35 and 0 . 45 j / cm 2 lta , it may be argued that the lta is merely improving the p activation which is generating these ρ c results . from the above table , the r sh values suggest that lta is a benefit for p activation . however , with reference now to fig7 , plotting ρ c versus r sh shows that , for a fixed r sh , lta can still produce better ρ c , if the correct energy density condition is selected . thermal stability of the ni x ge y layers was also analysed . the ultra - short time and highly - localized energy densities of lta processing may form highly non - equilibrium metastable conditions in the semiconductor materials and substrates . in such a case , the thermal budget in the processes which follow the lta process step , may cause any metastable condition to revert back to a more equilibrium state . in order to evaluate germanide thermal stability , a sample prepared at 300 ° c . rta and a sample prepared by 0 . 45 j / cm 2 lta were subjected to “ post - processing ” rta treatments from 100 to 500 ° c . the anneal times were 30 seconds each . only one sample was post - processed for both rta and lta , whereby the post - processing thermal - budget should be considered as cumulative in this analysis . fig8 provides tlm measurements of the lta sample after post - processing rta steps , which show that the slope and intercept of the fitted lines are changing after each rta treatment , indicating that r sh and ρ c are deteriorated . some tlm measurements are not shown in fig8 , however , for purposes of not obscuring the figure unnecessarily . extracted ρ c results are shown in fig9 . in the lta sample , ρ c increases gradually , and at 250 ° c . there is a significant increase in resistivity . by 500 ° c ., the ρ c value is similar to the rta cases . in the rta sample , ρ c shows a slight decrease at 150 ° c ., then follows an increasing trend . both samples were inspected by sem ( data not shown ), and it was observed at the end of this post - processing anneal sequence that the germanide had agglomerated . it is well known that nige layers annealed at 500 ° c . become agglomerated [ 11 , 12 ]. there are various known methods for altering thermal stability of silicide or germanide layers . one recent report highlighted the benefit of cosputtering ni and pt prior to alloy formation [ 14 ]. in that reference , the addition of pt improved the thermal stability of r sh in the germanide layers , and alternative embodiments of the present method may therefore include the addition of pt to achieve the same benefit . the present invention thus provides an improved method of forming germanide ( ni x ge y ) contacts on n - type germanium ( ge ) substrates for use with semiconducting devices , with a substantially planar , or regular , interface relative to the rough , uneven interface of prior art techniques . the quality of germanide contacts formed by state - of - the - art lta on n - type ge was investigated and compared systematically with conventional rta . lta resulted in smoother layers , smaller polycrystalline grains , and greater content of ni - rich germanide phases . the germanide - substrate interface was dramatically sharpened without any detectable interfacial region or transition zone in hrxtem . ρ c of the contacts was also extracted from tlm structures . the best contact resistivity obtained was 2 . 84 × 10 − 7 ω · cm 2 using a 1 × 10 15 cm − 2 12 kev p implant followed by 500 ° c . 10 - second activation anneal and lta of 0 . 45 j / cm 2 energy density for germanide formation . it will therefore be readily understood by the skilled reader that the lta annealing technique disclosed herein is particularly advantageous for any semiconductor device having a ge or gaas component such as , typically , but non - exhaustively , cmos devices , photodiodes and imagers . with reference to fig1 , a semiconductor device 100 is shown by way of example , with which the invention may be practiced . the semiconductor device 100 has a gate electrode 110 formed over a ge substrate 120 , having an n - or p - well 130 with a gate insulating film 140 formed between the well and the gate . a sidewall 150 is formed about the gate insulting film 140 and the gate electrode 110 . an ni x ge y layer 160 is formed on both sides of the gate electrode 110 as respective junctions , on the sides of which the sidewall 150 is formed and , in the figure and by way of illustration only , the left layer 160 a is an ni x ge y layer formed by lta according to the invention , and the right layer 160 b is an nige layer formed by rta according to the prior art . it will be appreciated that use of any of the above methods can be employed in manufacturing germanium or iii - iv semiconductor devices , such as transistor devices . it will be appreciated that use of any of the above methods can be employed in manufacturing of contact structures of germanium or iii - iv semiconductor devices . in the specification the terms “ comprise , comprises , comprised and comprising ” or any variation thereof and the terms include , includes , included and including ” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa . the invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail . a . dimoulas , p . tsipas , a . sotiropoulos , and e . k . evangelou , “ fermi - level pinning and charge neutrality level in germanium ,” applied physics letters , vol . 89 , p . 252110 , 2006 . y . zhou , w . han , y . wang , f . xiu , j . zou , r . kawakami , and k . l . wang , “ investigating the origin of fermi level pinning in ge schottky junctions using epitaxially grown ultrathin mgo films ,” applied physics letters , vol . 96 , p . 102103 , 2010 . m . shayesteh , c . l . l . m . daunt , d . o &# 39 ; connell , v . djara , m . white , b . long , and r . duffy , “ nige contacts and junction architectures for p and as doped germanium devices ,” electron devices , ieee transactions on , vol . 58 , pp . 3801 - 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