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'''Feedback to this page''': '''[mailto:labadviser@danchip.dtu.dk?Subject=Feed%20back%20from%20page%20http://labadviser.danchip.dtu.dk/index.php/Specific_Process_Knowledge/Thin_film_deposition/Deposition_of_Gold/Adhesion_layers click here]'''
'''This work was performed by PhD student Matteo Todeschini during period 2014-2017'''
= Adhesion layers =
= Adhesion layers =
The deposition of metal thin-film structures on dielectric or semiconductor substrates is important in a wide range of micro/nanofabrication applications (plasmonics, metamaterials, organic transistors, substrates for graphene growth, field effect devices).
The deposition of metal thin-film structures on dielectric or semiconductor substrates is important in a wide range of micro/nanofabrication applications (plasmonics, metamaterials, organic transistors, substrates for graphene growth, field effect devices).
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Fig. 1 represents the simplified adhesion layer model usually used in the micro and nanofabrication community for a generic bilayer system. In this model, the adhesion layer and the noble metal over-layer are deposited sequentially by PVD and are considered to be completely uniform and in perfect contact. Ti and Cr are known to be more chemically reactive than noble metals and thereby increase the adhesion as they chemically bind to the substrate. However, the model does not include any layer-layer interaction. Moreover, it is considered to be independent of the thin-film thickness. Due to the continuous miniaturization of structures and devices, the total metal thin-film thicknesses have reached tens of nanometers, with the thickness of the over-layer approaching the one of the adhesion layer. In these conditions it is unclear if the simplified model is still adequate.
Fig. 1 represents the simplified adhesion layer model usually used in the micro and nanofabrication community for a generic bilayer system. In this model, the adhesion layer and the noble metal over-layer are deposited sequentially by PVD and are considered to be completely uniform and in perfect contact. Ti and Cr are known to be more chemically reactive than noble metals and thereby increase the adhesion as they chemically bind to the substrate. However, the model does not include any layer-layer interaction. Moreover, it is considered to be independent of the thin-film thickness. Due to the continuous miniaturization of structures and devices, the total metal thin-film thicknesses have reached tens of nanometers, with the thickness of the over-layer approaching the one of the adhesion layer. In these conditions it is unclear if the simplified model is still adequate.


Here the characterization work carried out on two main thin-film systems, Cr/Au and Ti/Au stacks, is presented.
[[File:Picture00.png|650px|center|thumb|Fig. 1: Schematic representation of a substrate, adhesion layer and overlayer system. The substrate is either a dielectric or semiconductor material and the over-layer is a metal.]]


[[File:Picture00.png|650px|center|thumb|Fig. 1: Schematic representation of a substrate, adhesion layer and overlayer system. The substrate is either a dielectric or semiconductor material and the over-layer is a metal.]]
Here the first recommendations on the adhesion layers best suited for specific nanodevice applications are presented. Below the background and characterization work carried out on two main thin-film systems, Cr/Au and Ti/Au stacks, to arrive at these recommendations is presented.
 
= Recommendations for nanodevice fabrication =
 
== Nano-optic devices ==
 
For nano-optic devices there are two implications:
 
1) the grain size values of the Au thin-films are comparable with the sizes of plasmonic nanostructures, which are 70-200 nm. Hence, each nanostructure might have multi-grain structures that are different. This might affect its interaction with the light and its optical response, which leads to broadening of the optical resonance peaks.
 
2) during light and plasmonics device interactions, the temperature can locally raise to above 795 K, further enhancing the alloying of Cr and Au already present at room temperature and thus deteriorating the electrical properties of the thin-film stack.
 
Ti does not inter-diffuse with Au at room temperature, supported by the ''µ''4PP measurements which show that the electrical properties appear to be similar to pure Au thin-films. The formation of a stable Ti layer under the Au prevents diffusion of Au into the underlying substrate and improves the performance-time and temperature stability of the devices. The Ti-Au inter-diffusion starts at temperatures higher than 175°C. If the devices have to be used for prolonged time above this temperature, Pd and Pt diffusion barriers must be used. Comparative studies show a localized surface plasmon damping in plasmonic nanostructures due to the presence of a very thin adhesion layer, and overall better performances of Ti over Cr for the optical resonance of such nanostructures.
 
Therefore, if the choice is limited to metallic adhesion layers, '''Ti is preferred over Cr for nano-optics applications'''. A perhaps even better alternative is to use organosilane-based adhesion layers. Comparative measurements of Ti vs Mercaptopropyltrimethoxysilane (MPTMS) and Cr vs Aminopropyltrimethoxysilane (APTMS) show overall better performances for the organosilane-based adhesion layers over the metallic ones. A considerable disadvantage of these molecular adhesion layers is their lack of compatibility with the lithographic and lift-off processes, which still play an important role in the fabrication of nanostructures.
 
== Nano-electronic devices ==
 
For nano-electronics applications, the reported results show that Ti and Cr form oxides with surface adsorbed water and free oxygen in the vacuum chamber of the physical vapor deposition system. To avoid oxidation of the adhesion layer, the chamber vacuum needs to be in UHV conditions and the sample must be baked to remove the surface adsorbed water. However, these baking temperatures are not compatible with e.g. lift-off nanofabrication of nano-electronic devices that often utilize photo- or electron beam lithography resists which do not tolerate high baking temperatures.
 
To avoid oxidation of Ti or Cr that is in physical contact with the nano-electronic materials, one solution is to avoid these materials completely. Indeed, some of the best performing CNT devices are made without the use of adhesion layers, as e.g. Pd which is directly used. '''If an adhesion layer is required for mechanical stability, a less than 2nm thin Cr layer is recommended and Ti must be avoided'''. This is because the partially oxidized Ti might form a barrier between the nano-electronic material and the Au over-layer, with a consequent deterioration of the electron transport performances. Because of the single-layer morphology due to the Cr-Au alloy formation, the alloy will make electrical and physical contact to the nano-electronic material, despite the chrome oxide content. Furthermore, a low temperature annealing will enhance inter-diffusion of Au and Cr and improve electrical contact between the nano-electronic material and the Au over-layer.


= Adhesion layer effect on Au thin films =
= Characterization of adhesion layer effect on Au thin films =


== Effect on Au film morphology, grain size and texture ==
== Effect on Au film morphology, grain size and texture ==
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The samples with the adhesion layers and 20 nm Au layer were also analyzed. The addition of the adhesion layer had in both cases a profound impact on grain size and orientation of the Au film, as visible in Fig. 7a and 7c. The image shows small grains mainly oriented in the [111] crystal direction, with an average grain size of 40 nm for Ti (Fig. 7b) and 36 nm for Cr (Fig. 7d). During the collection of the TKD maps, only the Kikuchi patterns produced by the electron scattering from the Au layer were recorded and indexed. Cr and Ti layers did not contribute to the pattern formation due to two main reasons: i) they are too thin to produce enough scattered electrons and ii) they are
The samples with the adhesion layers and 20 nm Au layer were also analyzed. The addition of the adhesion layer had in both cases a profound impact on grain size and orientation of the Au film, as visible in Fig. 7a and 7c. The image shows small grains mainly oriented in the [111] crystal direction, with an average grain size of 40 nm for Ti (Fig. 7b) and 36 nm for Cr (Fig. 7d). During the collection of the TKD maps, only the Kikuchi patterns produced by the electron scattering from the Au layer were recorded and indexed. Cr and Ti layers did not contribute to the pattern formation due to two main reasons: i) they are too thin to produce enough scattered electrons and ii) they are
worse electron scattering centers tahn Au because they have a lower atomic number.
worse electron scattering centers than Au because they have a lower atomic number.


[[File:Picture8.png|300px|center|thumb|Fig. 7: TKD IPFZ maps of the growth direction and grain size distributions of the 2-Ti/20-Au sample (a-b) and of the 2-Cr/20-Au sample (c-d).]]
[[File:Picture8.png|300px|center|thumb|Fig. 7: TKD IPFZ maps of the growth direction and grain size distributions of the 2-Ti/20-Au sample (a-b) and of the 2-Cr/20-Au sample (c-d).]]
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For the chemical distribution of the elements in the 2nm Ti/2nm  Au sample, the STEM-EDX measurements at the Ti/Au interface showed the presence of a continuous Ti layer below the Au layer (Fig. 8a). The analysis of the 2nm Cr/2nm Au sample showed instead the presence of Cr throughout the whole thickness of the Au layer (Fig. 8b). Since Cr and Au were deposited sequentially and not by a co-deposition process, this result suggests a strong inter-diffusion. The 2nm Cr/20nm Au sample was also investigated, to understand the degree of inter-diffusion for a thicker Au layer (Fig. 8c). In this case the inter-diffusion between the elements was incomplete, being present for a thickness of 2-3 nm. This suggests that the diffusion process is limited to such a thickness when Cr and Au are deposited at room temperature.
For the chemical distribution of the elements in the 2nm Ti/2nm  Au sample, the STEM-EDX measurements at the Ti/Au interface showed the presence of a continuous Ti layer below the Au layer (Fig. 8a). The analysis of the 2nm Cr/2nm Au sample showed instead the presence of Cr throughout the whole thickness of the Au layer (Fig. 8b). Since Cr and Au were deposited sequentially and not by a co-deposition process, this result suggests a strong inter-diffusion. The 2nm Cr/20nm Au sample was also investigated, to understand the degree of inter-diffusion for a thicker Au layer (Fig. 8c). In this case the inter-diffusion between the elements was incomplete, being present for a thickness of 2-3 nm. This suggests that the diffusion process is limited to such a thickness when Cr and Au are deposited at room temperature.


[[File:Picture29.png|350px|center|thumb|Fig. 8: STEM-EDX maps of the 2nm Ti/2nm Au sample (a), 2nm Cr/2nm Au sample (b) and 2nm Cr/20nm Au sample (c). The Au L-alpha signal is acquired at 9713 eV, the Ti K-alpha signal at 4510.9 eV and the Cr K-alpha signal at 5414.7 eV.]]
To verify the chemical composition of the samples, in particular to investigate a possible presence of oxygen in the adhesion layer, STEM-EELS analysis was used. A line scan across the layer interfaces of the 2nm Ti/2nm Au sample shows the presence of a Ti core loss L3 edge at 460 eV and an L2 edge at 465 eV. A SiO2 O-K edge is visible at 538 eV, while the O-K edge of O bounded to Ti is found at 532 eV (Fig. 9a). The same investigation was performed on the 2nm Cr/2nm Au sample (Fig. 9b). The analysis showed a Cr L3 edge at 585 eV and an L2 edge at 594 eV. The SiO2 O-K edge is visible at 545 eV, while at 540 eV a weak OCr-K edge of O bounded to Cr is visible for a limited thickness below Au. Furthermore, the Cr edge presents a compositional tail along the scan direction, which confirms diffusion into the Au layer. For the length of the tail there is no presence of OCr-K edge, indicating that Cr inside Au is in metallic form. This is in good agreement with the observed diffusion, which involves only metallic Cr.
 
<gallery widths="400px" heights="400px" perrow="2" halign="center"> image:Picture29.png|Fig. 8: STEM-EDX maps of the 2nm Ti/2nm Au sample (a), 2nm Cr/2nm Au sample (b) and 2nm Cr/20nm Au sample (c). The Au L-alpha signal is acquired at 9713 eV, the Ti K-alpha signal at 4510.9 eV and the Cr K-alpha signal at 5414.7 eV.  
image:Picture12.png|Fig. 9: (a) STEM-EELS linear scan of the 2nm Ti/2nm Au sample, showing the presence of oxygen in the Ti layer. (b) STEM-EELS linear scan of the 2nm Cr/2nm Au sample, which shows the presence of oxygen that is bounded to Cr and Cr diffusion into the Au layer. </gallery>


To verify the chemical composition of the samples, in particular to investigate a possible presence of oxygen in the adhesion layer, STEM-EELS analysis was used. A line scan across the layer interfaces of the 2nm Ti/2nm Au sample shows the presence of a Ti core loss L3 edge at 460 eV and an L2 edge at 465 eV. A SiO2 O-K edge is visible at 538 eV, while the O-K edge of O bounded to Ti is found at 532 eV (Fig. 9a). The same investigation was performed on the 2nm Cr/2nm Au sample (Fig. 9b). The analysis showed a Cr L3 edge at 585 eV and an L2 edge at 594 eV. The SiO2 O-K edge is visible at 545 eV, while at 540 eV a weak OCr-K edge of O bounded to Cr is visible for a limited thickness below Au. Furthermore, the Cr edge presents a compositional tail along the scan direction, which confirms diffusion into the Au layer. For the length of the tail there is no presence of OCr-K edge, indicating that Cr inside Au is in metallic form. This is in good agreement with the observed diffusion, which involves only metallic Cr.
The source of oxidation of the adhesion layer could have three origins: i) oxidation due to migration of oxygen from the SiO2 substrate, ii) oxidation during the e-beam deposition process, or iii) oxidation due to substrate contamination with water and oxygen molecules. To investigate the oxidation origin, XPS depth profile analysis were performed on 2nm Ti/20nm Au and 2nm Cr/20nm Au as-deposited samples.


[[File:Picture12.png|350px|center|thumb|Fig. 9: (a) STEM-EELS linear scan of the 2nm Ti/2nm Au sample, showing the presence of oxygen in the Ti layer. (b) STEM-EELS linear scan of the 2nm Cr/2nm Au sample, which shows the presence of oxygen that is bounded to Cr and Cr diffusion into the Au layer.]]
The first analysis was performed to clarify if oxygen was originating from source i). To rule out possible oxygen migration from the oxygen-rich SiO2 substrate, the metal thin-films were deposited on amorphous Si3N4. For both samples, after Ar ion milling of 20 nm of Au, the Au 4f signal intensity decreased and Cr 2p and Ti 2p signals started to appear together with the O 1s signal (Fig. 10a for Ti and 10b for Cr, respectively).


[[File:Picture13.png|500px|center|thumb|Fig. 10: XPS depth profiling of 2nm Ti/20nm Au (a) and 2nm Cr/20nm Au (b) samples deposited on Si3N4 substrate. Both samples show the presence of a Au 4f signal from the surface to a depth of 20 nm. For a depth deeper than 20 nm, both Ti 2p and Cr 2p signals appear together with the O 1s signal.]]


The source of oxidation of the adhesion layer could have three origins:
Peak fits for the Ti and Cr signals were performed, and are reported in Fig. 11. The Ti 2p peak is a convolution of three components: a TiN doublet at 455.0 and 461.1 eV, a TiO2 doublet at 457.7 and 464.0 eV and a TiOx signal, which forms the descending background tail. The fit gives the following information: 1) Ti has formed Ti-N bonds with the Si3N4 substrate; 2) the adhesion layer was partially oxidized during deposition and not from source i). Metallic Ti could not be detected, but its presence cannot be excluded: metallic Ti is highly reactive with respect to oxygen and nitrogen, and the destructive sputtering process used for the depth profiling could have enhanced the mixing between Ti, O and N, catalyzing the reaction of the metallic Ti bound to Au to form an oxide or a nitride. The Cr 2p peak fit is formed by three components: a metallic Cr doublet at 574.4 and 583.6 eV, a Cr2O3 doublet at 576.3 and 585.6 eV and a CrO3 doublet at 580 and 589.2 eV. The result of the fit indicates a partial oxidation of Cr, as in the Ti case.
i) oxidation due to migration of oxygen from the SiO2 substrate, ii) oxidation
during the e-beam deposition process, or iii) oxidation due to substrate
contamination with water and oxygen molecules. To investigate the oxidation
origin, XPS depth pro�le analysis were performed on 2-Ti/20-Au and
2-Cr/20-Au as-deposited samples.
The �rst analysis was performed to clarify if oxygen was originating from
source i). To rule out possible oxygen migration from the oxygen-rich SiO2
substrate, the metal thin-�lms were deposited on amorphous Si3N4. For
both samples, after Ar ion milling of 20 nm of Au, the Au 4f signal intensity
decreased and Cr 2p and Ti 2p signals started to appear together with the
O 1s signal (Fig. 5.11a for Ti and 5.11b for Cr, respectively). For the Cr
case, due to the lower spatial resolution of XPS with respect to STEM-EDX,
it was not possible to verify the thickness of Cr-Au inter-di�usion in this
case. The XPS electron escape depth at 1486.7 eV for Au is around 0.15
nm [114], meaning that it is not possible to obtain information about interdi
�usion for layers thinner than this value without collecting a signal from
the underlying material.
Peak �ts for the Ti and Cr signals were performed, and are reported in
Fig. 5.12. The Ti 2p peak is a convolution of three components: a TiN
doublet at 455.0 and 461.1 eV, a TiO2 doublet at 457.7 and 464.0 eV and
a TiOx signal, which forms the descending background tail [115]. The �t
gives the following information: 1) Ti has formed Ti-N bonds with the Si3N4
substrate; 2) the adhesion layer was partially oxidized during deposition
and not from source i). Metallic Ti could not be detected, but its presence
cannot be excluded: metallic Ti is highly reactive with respect to oxygen
and nitrogen, and the destructive sputtering process used for the depth
pro�ling could have enhanced the mixing between Ti, O and N, catalyzing
the reaction of the metallic Ti bound to Au to form an oxide or a nitride.


The Cr 2p peak �t is formed by three components: a metallic Cr doublet
[[File:Picture14.png|500px|center|thumb|Fig. 11: Left: Ti 2p XPS peak fit of the 2-Ti/20-Au sample. Right: Cr 2p XPS peak fit of the 2-Cr/20-Au sample.]]
at 574.4 and 583.6 eV, a Cr2O3 doublet at 576.3 and 585.6 eV and a CrO3
doublet at 580 and 589.2 eV [115]. The result of the �t indicates a partial
oxidation of Cr, as in the Ti case.


The second analysis was performed to understand whether the oxygen
The second analysis was performed to understand whether the oxygen was originating from source ii) or iii). A 2nm Ti/20nm Au/2nm Ti/20nm Au sandwich structure was deposited and analyzed. Both layers of Ti were partially oxidized: the Ti 2p peak signals (Fig. 12a) are present at the same depth together with the O 1s signals (Fig. 12b). The O 1s signal in the Ti layer in contact with the Si3N4 substrate has higher intensity than the one of the Ti layer between the Au layers. Hence, the Ti layer in contact with the substrate is more oxidized, which suggests that Ti reacted with water adsorbed on the substrate surface. The conclusion is that the oxygen originated from oxidation during the e-beam deposition process and from oxidation due to substrate contamination with water and oxygen molecules.
was originating from source ii) or iii). A 2-Ti/20-Au/2-Ti/20-Au sandwich
structure was deposited and analyzed. Both layers of Ti were partially oxidized:
the Ti 2p peak signals (Fig. 5.13a) are present at the same depth
together with the O 1s signals (Fig. 5.13b). The O 1s signal in the Ti layer
in contact with the Si3N4 substrate has higher intensity than the one of the
Ti layer between the Au layers. Hence, the Ti layer in contact with the substrate
is more oxidized, which suggests that Ti reacted with water adsorbed
on the substrate surface. The conclusion is that the oxygen originated from
sources ii) and iii).


Water is present in most thin-�lm deposition systems and it is very
[[File:Picture15.png|450px|center|thumb|Fig. 12: XPS depth profiling of a 2nm Ti/20nm Au/2nm Ti/20nm Au multilayer sample deposited on Si3N4 substrate. For each Ti 2p signal (a) there is an O 1s signal (b) at the same depth. The Ti layer in contact with the Si3N4 substrate is more oxidized than the Ti layer between the Au layers, due to its reaction with oxygen and water adsorbed on the substrate surface.]]
di�cult to remove because of its capacity to form hydrogen bonds with
many materials. Ti reacts with water and oxygen and forms titanium oxides.
Furthermore, the use of the same deposition conditions and of the same type
of substrate with respect to Ti and a similar chemical reactivity of Ti and
Cr suggest that also the Cr layer gets further oxidized by the water and
oxygen molecules present on the substrate surface.


== Adhesion layer effect on bilayer thin-film electrical resistivity ==
== Adhesion layer effect on bilayer thin-film electrical resistivity ==


The change in nanostructure of the Au thin �lm due to the presence of the
The change in nanostructure of the Au thin film due to the presence of the adhesion layers observed above, could have an important impact on the film macroscopic properties such as electrical resistivity. Electrical resistivity in polycrystalline films is dependent on electron scattering at surfaces and grain boundaries, and it was expected that the grain size change measured with TKD to be reflected in the electrical properties. Furthermore, for the case of Cr, a Cr-Au alloy was formed, which also was expected to have an impact on the thin-film electrical resistivity.
adhesion layers observed above, could have an important impact on the �lm
macroscopic properties such as electrical resistivity. Electrical resistivity
in polycrystalline �lms is dependent on electron scattering at surfaces and
grain boundaries, and it was expected that the grain size change measured
with TKD to be re
ected in the electrical properties. Furthermore, for the
case of Cr, a Cr-Au alloy was formed, which also was expected to have an
impact on the thin-�lm electrical resistivity.
To verify these hypotheses, the sheet resistance (R) of the three samples
(20-Au, 2-Ti/20-Au, and 2-Cr/20-Au) investigated with TKD above
was measured using micro 4-point probe (�4PP). Fig. 5.14a shows that
the Ti/Au sample had a lower sheet resistance than pure Au, which can
be attributed to the two layers acting as parallel resistors [124]. Data normalization
was performed with respect to the average sheet resistance (R)
measured at the 20-Au sample. To exclude tip wear e�ects of the �4PP, the
measurements were performed measuring with the same probe alternatively


on the 20-Au and 2-Ti/20-Au sample, respectively.
To verify these hypotheses, the sheet resistance (R) of the three samples (20nm Au, 2nm Ti/20nm Au, and 2nm Cr/20nm Au) was measured using micro 4-point probe (''µ''4PP). Fig. 13a shows that the Ti/Au sample had a lower sheet resistance than pure Au, which can be attributed to the two layers acting as parallel resistors. Data normalization was performed with respect to the average sheet resistance (R) measured at the 20-Au sample. To exclude tip wear effects of the ''µ''4PP, the measurements were performed measuring with the same probe alternatively on the 20nm Au and 2nm Ti/20nm Au sample, respectively. Fig. 13b shows that the Cr/Au system had a higher sheet resistance than pure Au. In this case, the formation of a single layer due to Cr-Au inter-diffusion compromised the multilayer assumption. The sheet resistance increase is in line with the general resistivity increase of Cr-Au alloys, which increases linearly with the Cr concentration. Data acquisition and normalization were done as in the Ti/Au case.
Moreover, to rule out thin-�lm thickness variation e�ects, Au TEM crosssection
thickness measurements were performed on 36 points along the 20-
Au and the 2-Ti/20-Au samples. They revealed a slightly thicker Au �lm
thickness in the Ti/Au sample compared to the pure Au �lm (23.6 ±0.5
nm vs 21.9 ±0.5 nm, respectively). Since electrical resistivity is inversely
proportional to �lm thickness, the thicker Au �lm contributes to decrease
the sheet resistance in the Ti/Au sample with respect to pure Au.


Fig. 5.14b shows that the Cr/Au system had a higher sheet resistance
The Ti/Au parallel behavior and Cr/Au inter-diffusion seem to have a larger impact on the electrical properties of the multilayer systems than the nanostructure change observed by TKD. For both samples, the increase of grain boundary scattering due to the higher density of grain boundaries, compared to pure Au, could not be measured with setup used, but cannot be excluded a priori.
than pure Au. In this case, the formation of a single layer due to Cr-Au
 
inter-di�usion compromised the multilayer assumption. The sheet resistance
[[File:Picture16.png|550px|center|thumb|Fig. 13: (a) Normalized sheet resistance of 20-Au vs 2-Ti/20-Au samples. The Ti/Au bilayer system has lower sheet resistance than pure Au due to parallel resistors behavior. (b) Normalized sheet resistance of 20-Au vs 2-Cr/20-Au samples. The Cr/Au bilayer system has higher sheet resistance than pure Au due to Cr-Au
increase is in line with the general resistivity increase of Cr-Au alloys, which
alloy formation.]]
increases linearly with the Cr concentration [125]. Data acquisition and normalization
were done as in the Ti/Au case above, and TEM cross-section
thickness measurements of the Au layer were done on 29 points of the Cr/Au
sample, giving a mean thickness of 22.7 ±0.4 nm. Compared with the thickness
of pure Au, this value is slightly higher. However, the sheet resistance
decrease due to this thickness di�erence was not enough to compensate the
increase due to the Cr-Au alloy formation.
The Ti/Au parallel behavior and Cr/Au inter-di�usion seem to have a
larger impact on the electrical properties of the multilayer systems than the
nanostructure change observed by TKD. For both samples, the increase of
grain boundary scattering due to the higher density of grain boundaries,
compared to pure Au, could not be measured with setup used, but cannot
be excluded a priori.


= Adhesion layer model =
= Adhesion layer model =
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Based on the results obtained above, a revised adhesion layer model for Ti/Au and Cr/Au thin-film systems is proposed. The model is intended as an improvement in the description of the layer-layer interaction compared to the model described in Fig. 1, with the aim of helping the thin-film engineering of nanodevices.
Based on the results obtained above, a revised adhesion layer model for Ti/Au and Cr/Au thin-film systems is proposed. The model is intended as an improvement in the description of the layer-layer interaction compared to the model described in Fig. 1, with the aim of helping the thin-film engineering of nanodevices.


The adhesion layer model is shown in Fig. 5.15. The growth of both adhesion layers starts with the deposition of the atoms on the substrate (1) and the subsequent formation of an amorphous layer (2). During the deposition, both Ti and Cr get partially oxidized by oxygen and water molecules present on the substrate surface and in the deposition chamber. The adhesion layer acts as a wetting layer for Au, reducing the nucleation energy barrier and increasing the number of nucleation sites compared to the case where Au is directly evaporated onto the SiO2 surface. The enhanced wetting is due to the formation of Ti-Au and Cr-Au chemical bonds. This leads to the formation of a continuous film having i) monodisperse grain size and ii) the energetically most favorable [111] crystal orientation for Au (3). For the Cr/Au system there is an extra step: the inter-diffusion between Cr and Au to form a Cr-Au alloy (4). Such diffusion is limited to a thickness of 2-3 nm for samples prepared at room temperature.
The adhesion layer model is shown in Fig. 14. The growth of both adhesion layers starts with the deposition of the atoms on the substrate (1) and the subsequent formation of an amorphous layer (2). During the deposition, both Ti and Cr get partially oxidized by oxygen and water molecules present on the substrate surface and in the deposition chamber. The adhesion layer acts as a wetting layer for Au, reducing the nucleation energy barrier and increasing the number of nucleation sites compared to the case where Au is directly evaporated onto the SiO2 surface. The enhanced wetting is due to the formation of Ti-Au and Cr-Au chemical bonds. This leads to the formation of a continuous film having i) monodisperse grain size and ii) the energetically most favorable [111] crystal orientation for Au (3). For the Cr/Au system there is an extra step: the inter-diffusion between Cr and Au to form a Cr-Au alloy (4). Such diffusion is limited to a thickness of 2-3 nm for samples prepared at room temperature.


[[File:Picture17.png|550px|center|thumb|Fig. 3. Revised adhesion layer model for Ti/Au and Cr/Au multilayer systems. (1) Adhesion layer nucleation; (2) growth of a partially oxidized adhesion layer; (3) growth of Au grains with [111] crystal orientation on top of the adhesion layer; (4) only for Cr: Cr/Au inter-diffusion and alloy formation during/after Au deposition.]]
[[File:Picture17.png|550px|center|thumb|Fig. 14. Revised adhesion layer model for Ti/Au and Cr/Au multilayer systems. (1) Adhesion layer nucleation; (2) growth of a partially oxidized adhesion layer; (3) growth of Au grains with [111] crystal orientation on top of the adhesion layer; (4) only for Cr: Cr/Au inter-diffusion and alloy formation during/after Au deposition.]]


= Recommendations for nanodevice fabrication =
= Adhesion layer impact on Au film stability with temperature =
 
Control of temperature is very important in several steps during micro and nanofabrication processes. For example, the general sequence of processing steps for a typical photolithography process is as follows:
 
1) substrate preparation,
 
2) photoresist spin coating,
 
3) prebake, 4) photoresist exposure,
 
5) post-exposure bake,
 
6) photoresist development,
 
7) postbake,
 
8) photoresist strip after pattern transfer.
 
Of the whole process, three steps (3, 5 and 8) involve the use of a heat treatment on the sample for periods varying from one to 30 minutes (and even hours for the case of very thick resists) and using temperatures ranging from 100°C to 200°C.The modification of the nanostructure of pure Au in this temperature range is already quite pronounced and can have a big impact on the fabrication and performances of nanodevices if this material is used without adhesion layers. Furthermore, also the use of Ti and Cr adhesion layers to enhance adhesion of Au on the substrate led to a change of the nanostructure and electrical performances of the bilayer systems; however such in uence did not seem as dramatic as
the change due to the increase in temperature. It is currently unclear if the use of adhesion layers can have a positive impact on the stability of the Au nanostructure at elevated temperatures: this is the main point addressed in the following.
 
Figure 15 shows the TKD maps of film nanostructure evolution for a different set of temperatures of a 20 nm pure Au film (left column), 2 nm Ti/20 nm Au (center column) and 2 nm Cr/20 nm Au (right column) bilayer systems. The nanostructure of pure Au at room temperature consisted of small grains having [100] and [110] crystal orientations with respect to the sample surface orientation and larger grains having [111] orientation, as already described above. The film started to dewet between 100°C and 200°C. When Ti was used as adhesion layer, the nanostructure of Au at room temperature had a smaller grain size and complete [111] orientation. When the sample was treated with the same annealing conditions as pure Au, the effect of the temperature on the nanostructure was very different: the annealing did not affect the continuity of the film up to 500°C and up to 200°C there was little grain growth. From 400°C the grain size started to increase, but not at the levels of pure Au. For the Cr/Au bilayer system, the Au film had a slightly smaller average grain size than in the Ti/Au case. Due to the low average grain size, a lot of grains were not properly indexed and are displayed as black areas. The Au layer maintained a continuous morphology up to 500°C also in this case, but the increase of the grain size was more limited than in the Ti/Au sample.


== Nano-optic devices ==
[[File:PictureA.png|400px|center|thumb|Fig. 15: IPFZ maps showing the nanostructure evolution with temperature of a 20 nm pure Au film (left column), a 2nm Ti/20 nm Au bilayer (center column) and a 2nm Cr/20 nm Au bilayer system (right column).]]


== Nano-electronic devices ==
A more quantitative analysis of the grain size increase for all three samples was performed between the map at room temperature and the one after the annealing at 200°C, since at such temperature the nanostructure of Au was still continuous and the dewetting was not affecting the analysis significantly. The results are reported in Fig. 16. The average grain size of the pure Au sample increased from 97 nm to 105 nm in the evaluated temperature range (Fig. 16a), the one of the Ti/Au sample from 45 nm to 56 nm (Fig. 16b) and the one of the Cr/Au sample almost did not change, increasing from 34 nm to 36 nm (Fig. 16c).


= Adhesion layer impact on Au �film stability with temperature =
[[File:PictureB.png|600px|center|thumb|Fig. 16: Histogram plot of the average grain size value before annealing (black) and after the annealing at 200°C (red) for the Au (a), Ti/Au (b) and the Cr/Au (c) samples.]]


[[Image:section under construction.jpg|150px]]
At higher temperatures, the Au grain size increase was very different between the two adhesion layers. Figure 17a shows the variation for Ti/Au between room temperature and 500°C: the plotted average grain size value increased from 45 nm to 113 nm after the annealing. Figure 17b shows the variation for Cr/Au: in this case the grain size increase is lower, varying from 34 nm to 44 nm, highlighting the higher nanostructure stability guaranteed by Cr respect to Ti.


allic bond with gold. 5 nm to 10 nm thick of Cr or Ti is commonly used and it is important to deposit Cr or Ti and then immediately Au. If the vacuum chamber is opened in between, the surface of Cr or Ti will get oxidized and that will give a poor adhesion. If a gold layer needs to be deposited directly on Silicon, then native oxide has to be removed by deep in diluted HF and immediately load the evaporation chamber. And after the deposition, the wafer has to be heated op to get some Au-Si diffusion which improves the adhesion.
[[File:PictureC.png|400px|center|thumb|Fig. 17: Histogram plot of the average grain size value before annealing (black) and after the final annealing at 500°C (red) for the Ti/Au (a) and the Cr/Au (b) samples.]]
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