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Cryogenic processing of thin metal films
Surface and Coatings Technology 150 (2002) 76–79
Cryogenic processing of thin metal films L. Hea,*, J.E. Siewenieb a
San Jose State University, Department of Electrical Engineering, San Jose, CA, USA Northern Illinois University, Department of Electrical Engineering, DeKalb, IL, USA
b
Received 5 July 2001; accepted in revised form 11 August 2001
Abstract Cryogenic processing has been proven to be efficient in increasing Schottky contact barrier height, and significantly reducing device reverse leakage current. Metal-semiconductor-metal (MSM) photodetectors are widely used in optoelectronic integrated circuit (OEIC) receivers because of their compatibility with the preamplifier for their planar integration scheme, the minimum number of processing steps, high performance, and low cost. InGaAsyInP is usually chosen for the long wavelength application. However, there is always a potential problem of high dark current in an InGaAsyInP MSM photodetector due to the lower barrier height for such a material. In a recent study on AgyInGaAsyInP contacts fabricated by a low temperature (LT) processing the Schottky barrier height was found to be as high as 0.64 eV. This value is more than double that of the same contact obtained by room temperature (RT) processing. Therefore, it is believed that the LT processing could greatly enhance the performance of a MSM photodetector. In this work, the effect of LT processing on the properties of thin metal films was investigated. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and in-situ resistivity measurements were conducted. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cryogenic processing; Metal-semiconductor-metal (MSM) photodetectors
1. Introduction Cryogenic processing of semiconductor material has been used for enhancing the Schottky contact performance for metalyInP, InGaAs contacts in recent years w1– 3x. Later study has been carried out on the properties of cryogenically processed thin metal films alone. Fundamentally, thin metal films are of considerable interest for microelectronic device fabrication. Electrical connections between circuit elements are made by thin metal films patterned into micron-wide lines. Schottky contact made by metalysemiconductor contact is especially important for device application such as in metalsemiconductor field effect transistors (MESFETs). The high electrical resistivity of thin metal films is a drawback in these applications. Compared to the film deposited at room temperature (RT), the resistivity of the LT * Corresponding author. E-mail address: [email protected] (L. He).
film can be four to five orders lower in magnitude w4,5x. In the meantime, Schottky barrier heights are significantly increased in materials such as InP, GaAs and InGaAs w3–5x. The work studied the electrical and micro-structural properties of several frequently used metal films, including Au, Ag, Al, Pd and Ni, formed at LT. The results presented here, however, are mainly from the analysis of Au. Data for other materials are either still under investigation, or have been reported elsewhere. Transmission electron microscopy (TEM) diffraction is of particular interest. Atomic force microscopy (AFM) and in-situ resistivity measurements were used to coordinate the TEM results. 2. Experimental Thin metal films were deposited by thermal evaporation on glass substrates in a vacuum of 10y7 torr. The substrate temperatures were 77 K (liquid nitrogen temperature) and 300 K (room temperature). The substrate
0257-8972/01/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 5 0 4 - 3
L. He, J.E. Siewenie / Surface and Coatings Technology 150 (2002) 76–79
77
Fig. 2. Resistance (R) vs. thickness relation for Au films.
Fig. 1. The I–V characteristics of AgyInGaAs Schottky diodes processed at RT and LT.
temperature of 77 K was achieved by putting the substrate on the top of the sample holder cooled by continuous liquid nitrogen flow. The schematic description of the sample holder was reported elsewhere w6x. A thermocouple attached on the sample holder was used to measure the substrate temperature. The increase of sample temperature was realized by heating the sample with two 150 W heaters mounted on the same sample holder. A thickness monitor is used to measure the thickness of the thin films during deposition. The calibration, performed on a Sloan Dektak IIA Thickness Profiler, indicated that the thickness reading through the monitor was in good agreement with the actual thickness. The deposition rate varied with the metal. It was controlled so that the data could be properly recorded. A metal mask was used to obtain electrical connection electrodes by first depositing Au at room temperature. The mask was designed so that the later-deposited thin film would be a square, the measured resistance value would then be independent of the square size, and depend only on resistivity and thickness. This is known as sheet resistance and normally measures the resistance of a thin metal film: RsŽryd.Žlyb., where l is the film width, b is the length and d is the thickness. If lsb, this then becomes RsrydsRs, which is independent of the size of the square, and depends only on resistivity and thickness. In this work, the deposited thin film was designed as a square (i.e. lsb). An electrical connection was prepared to in-situ
to measure the sheet resistance. For simplicity, in the following discussion the measured resistance R is referred to as sheet resistance Rs. Atomic force microscope (AFM) was used for surface morphology analysis. All AFM measurements were carried out at room temperature. In addition, metal grids on alumina as well as glass and silicon substrates were attached to allow the simultaneous deposition of films for the TEM and AFM analysis. 3. Results and discussion Fig. 1 shows the I–V characteristics of AgyInGaAs Schottky diodes processed at RT and LT. It is known that the InGaAs is one of the semiconductor materials with low Schottky barrier height when a metalysemiconductor contact is fabricated. The barrier height, FB is calculated from the following equation: FBsŽkTyq.lnŽA*T2 yJ0., where A* is the Richardson constant, and J0 is the saturation current density. The ideality factor, n, is determined from the forward characteristics using the relation: nsŽqykT.w≠Vy≠ŽlnJ.x. The I–V results shown can be used to calculate the Schottky barrier height. The contact formed by ordinary room temperature processing had a very low value at approximately 0.3 eV. The same contact formed by LT processing the barrier height was calculated as high as 0.63 eV. The higher barrier height plays an important role in reducing leakage current; the LT contact could lead to high performance devices for electronics and opto-electronics. Further studies were carried out for the electrical properties of the LT thin film. Fig. 2 shows the results
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L. He, J.E. Siewenie / Surface and Coatings Technology 150 (2002) 76–79
Fig. 3. A typical samples of TEM diffraction patterns for (a) an amorphous film, and (b) a polycrystalline film.
of in-situ resistance measurements for LT metal films, the resistance (R) vs. thickness relation for Au films. For comparison, the measurements were also performed for RT films. Firstly, the LT film becomes continuous at a lower thickness than its RT counterpart. Secondly, the RT film always overtakes the LT film at some point, becoming a better conductor at higher film thickness. Thirdly, while the LT curve has a more or less constant curvature up until it reaches its peak conductivity, the RT films have a positive curvature at lower thickness. It is not until they make the transition to a negative curvature that their resistances are able to drop below the LT curve. It is believed that the primary difference between LT and RT films lies in the mobility of the adatom w6x. While adatoms on a cryogenically cooled substrate tend to stay close to where they land, adatoms on a room temperature substrate are more likely to diffuse across the substrate or to re-evaporate. The higher the substrate temperature, the more quickly the metal clusters will grow to the point of exhibiting liquidlike behavior, coalescing and leaving voids on the substrate. It is not until secondary nucleation occurs in the channels and begins filling in between the clusters that RT films become continuous w7x. This extra step could account for the initial positive curvature in the RT graphs. For the LT films, which are simply filling in as they go, this liquid-like stage is not so prevalent, and this is reflected in the resistivity vs. thickness data. Additional resistivity vs. thickness experiments demonstrated that the Al, Au and Ag have the same characteristics as stated above. It was also shown that Ni films behave typically, although there was little difference between the RT and LT resistance at all temperatures. The Pd films were more unusual since the LT films were higher in resistivity for all temperatures. The resistivity vs. temperature graph for Au is shown in Fig. 2b. The figure shows the resistance of the LT film decreasing as the substrate is brought back up to room temperature. As the film temperature is raised, surface and bulk diffusion increase, filling lattice voids
Fig. 4. TEM surface scan and electron diffraction images for RT Pd (a) and RT Ag (b) thin films at 10 nm (1:30 000 magnification).
or otherwise rearranging atoms to correct some of the faults made during deposition w8x. Fig. 3 shows samples of TEM diffraction patterns for (a) an amorphous film, and (b) a polycrystalline film. The diffraction patterns of an amorphous solid contain only a few broad rings. A film consisting of very small crystallites has the same pattern since there is no longrange structural order w9x. Continuous changes in resistance vs. temperature are due to an increase in grain size, and this can be observed in the decrease in the half-width of the electron diffraction lines. The angular width of the diffraction maximum follows the relationship: Bs2lytr Where B is the angular width, and tr the size of the crystal in the direction radial to the electron beam w10x. The diffraction pattern of a single crystal film would result in a Laue pattern array of dots, and very large grains in a polycrystalline film result in a dotted appearance to the rings, an indication of the higher degree of
Fig. 5. TEM surface scan and electron diffraction images for RT (a) and LT (b) Al thin films at 11.8 nm (1:30 000 magnification).
L. He, J.E. Siewenie / Surface and Coatings Technology 150 (2002) 76–79
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Fig. 6 shows some typical AFM images for the LT and RT Al films at 20-nm-thickness. The LT film shows much large size of crystal formation, while the RT film is formed by smaller sized structures. Less crystal boundaries are expected from the LT film, which corresponds to the lower resistance for the LT films. It was observed in the early growth stage, e.g. at 10-nmthickness, the LT and RT films do not have as obvious differences in surface morphology w5x. As the film becomes thicker, as shown in the 20-nm-thickness, the surface roughness in LT film becomes significant. When the films become increasingly thick, e.g. thicker than 100 nm in most films, the LT and RT films show no difference. This observation also supports the resistance measurement result. 4. Summary The LT-processed thin metal film shows important advantages over the RT film. The low resistivity at very thin thickness for the LT films has a promising potential in various applications for microelectronic and optoelectronic devices. The increased Schottky barrier height is especially important to enhance device performance. The TEM diffraction study indicates the possible formation of amorphous-like material in the surface of LT film, which may be responsible for the barrier height increase (an MIS-like structure). The TEM and AFM surface scan images show that the LT thin films have a rougher surface, and larger sized crystal grains, which results in the lower resistance of the LT films. Acknowledgements Fig. 6. A typical AFM surface spectrum of LT (a) and RT (b) Au films at 20 nm.
This work was sponsored by the National Science Foundation under grant No. ECS-0096190.
order in these films. Fig. 4 shows the TEM surface scan and electron diffraction images for RT Pd (a), and Al (b) films. These two films had the same thickness with different degree of crystallization. This result shows the crystallization variation with metals. Fig. 5 shows the effect of substrate temperature. With the same thickness, the surface scan showed a much rougher surface for the LT Al film. Also, in the diffraction pattern, less distinctive rings were observed from the LT film, and no pronounced dotting. The results indicated that the LT film is less crystallized, i.e. an amorphous-like material is formed. For the discussion of crystallization, more evidence is necessary for the LT film structure. More results are expected later.
References w1x Z.Q. Shi, W.A. Anderson, J. Vac. Sci. Technol. A 11 (1993) 985. w2x L. He, Z.Q. Shi, J. Vac. Sci. Technol. A 14 (1996) 704. w3x J.E. Siewenie, L. He, J. Vac. Sci. Technol. A 17 (1999) 1799. w4x Z.Q. Shi, W.A. Anderson, Appl. Phys. Lett. 66 (1991) 448. w5x L. He, Solid-State Electron. 39 (1811) 1996. w6x L. He, M. Costello, K.Y. Cheng, D.E. Wohlert, J. Vac. Sci. Technol. A 16 (1998) 1646. w7x M. Ohring, The Material Science of Thin Films, Academic Press, New York, 1992, p. 452. w8x D. Smith, Thin Film Deposition, McGraw-Hill, 1995, p. 170. w9x K. Behrndt, J. Vac. Sci. Technol. 7 (1969) 388. w10x K. Lark-Horovitz, V. Johnson, Methods of Experimental Physics 6: Solid State Physics, Prentice-Hall, 1959, p. 255.
Cryogenic processing of thin metal films L. Hea,*, J.E. Siewenieb a
San Jose State University, Department of Electrical Engineering, San Jose, CA, USA Northern Illinois University, Department of Electrical Engineering, DeKalb, IL, USA
b
Received 5 July 2001; accepted in revised form 11 August 2001
Abstract Cryogenic processing has been proven to be efficient in increasing Schottky contact barrier height, and significantly reducing device reverse leakage current. Metal-semiconductor-metal (MSM) photodetectors are widely used in optoelectronic integrated circuit (OEIC) receivers because of their compatibility with the preamplifier for their planar integration scheme, the minimum number of processing steps, high performance, and low cost. InGaAsyInP is usually chosen for the long wavelength application. However, there is always a potential problem of high dark current in an InGaAsyInP MSM photodetector due to the lower barrier height for such a material. In a recent study on AgyInGaAsyInP contacts fabricated by a low temperature (LT) processing the Schottky barrier height was found to be as high as 0.64 eV. This value is more than double that of the same contact obtained by room temperature (RT) processing. Therefore, it is believed that the LT processing could greatly enhance the performance of a MSM photodetector. In this work, the effect of LT processing on the properties of thin metal films was investigated. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and in-situ resistivity measurements were conducted. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cryogenic processing; Metal-semiconductor-metal (MSM) photodetectors
1. Introduction Cryogenic processing of semiconductor material has been used for enhancing the Schottky contact performance for metalyInP, InGaAs contacts in recent years w1– 3x. Later study has been carried out on the properties of cryogenically processed thin metal films alone. Fundamentally, thin metal films are of considerable interest for microelectronic device fabrication. Electrical connections between circuit elements are made by thin metal films patterned into micron-wide lines. Schottky contact made by metalysemiconductor contact is especially important for device application such as in metalsemiconductor field effect transistors (MESFETs). The high electrical resistivity of thin metal films is a drawback in these applications. Compared to the film deposited at room temperature (RT), the resistivity of the LT * Corresponding author. E-mail address: [email protected] (L. He).
film can be four to five orders lower in magnitude w4,5x. In the meantime, Schottky barrier heights are significantly increased in materials such as InP, GaAs and InGaAs w3–5x. The work studied the electrical and micro-structural properties of several frequently used metal films, including Au, Ag, Al, Pd and Ni, formed at LT. The results presented here, however, are mainly from the analysis of Au. Data for other materials are either still under investigation, or have been reported elsewhere. Transmission electron microscopy (TEM) diffraction is of particular interest. Atomic force microscopy (AFM) and in-situ resistivity measurements were used to coordinate the TEM results. 2. Experimental Thin metal films were deposited by thermal evaporation on glass substrates in a vacuum of 10y7 torr. The substrate temperatures were 77 K (liquid nitrogen temperature) and 300 K (room temperature). The substrate
0257-8972/01/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 5 0 4 - 3
L. He, J.E. Siewenie / Surface and Coatings Technology 150 (2002) 76–79
77
Fig. 2. Resistance (R) vs. thickness relation for Au films.
Fig. 1. The I–V characteristics of AgyInGaAs Schottky diodes processed at RT and LT.
temperature of 77 K was achieved by putting the substrate on the top of the sample holder cooled by continuous liquid nitrogen flow. The schematic description of the sample holder was reported elsewhere w6x. A thermocouple attached on the sample holder was used to measure the substrate temperature. The increase of sample temperature was realized by heating the sample with two 150 W heaters mounted on the same sample holder. A thickness monitor is used to measure the thickness of the thin films during deposition. The calibration, performed on a Sloan Dektak IIA Thickness Profiler, indicated that the thickness reading through the monitor was in good agreement with the actual thickness. The deposition rate varied with the metal. It was controlled so that the data could be properly recorded. A metal mask was used to obtain electrical connection electrodes by first depositing Au at room temperature. The mask was designed so that the later-deposited thin film would be a square, the measured resistance value would then be independent of the square size, and depend only on resistivity and thickness. This is known as sheet resistance and normally measures the resistance of a thin metal film: RsŽryd.Žlyb., where l is the film width, b is the length and d is the thickness. If lsb, this then becomes RsrydsRs, which is independent of the size of the square, and depends only on resistivity and thickness. In this work, the deposited thin film was designed as a square (i.e. lsb). An electrical connection was prepared to in-situ
to measure the sheet resistance. For simplicity, in the following discussion the measured resistance R is referred to as sheet resistance Rs. Atomic force microscope (AFM) was used for surface morphology analysis. All AFM measurements were carried out at room temperature. In addition, metal grids on alumina as well as glass and silicon substrates were attached to allow the simultaneous deposition of films for the TEM and AFM analysis. 3. Results and discussion Fig. 1 shows the I–V characteristics of AgyInGaAs Schottky diodes processed at RT and LT. It is known that the InGaAs is one of the semiconductor materials with low Schottky barrier height when a metalysemiconductor contact is fabricated. The barrier height, FB is calculated from the following equation: FBsŽkTyq.lnŽA*T2 yJ0., where A* is the Richardson constant, and J0 is the saturation current density. The ideality factor, n, is determined from the forward characteristics using the relation: nsŽqykT.w≠Vy≠ŽlnJ.x. The I–V results shown can be used to calculate the Schottky barrier height. The contact formed by ordinary room temperature processing had a very low value at approximately 0.3 eV. The same contact formed by LT processing the barrier height was calculated as high as 0.63 eV. The higher barrier height plays an important role in reducing leakage current; the LT contact could lead to high performance devices for electronics and opto-electronics. Further studies were carried out for the electrical properties of the LT thin film. Fig. 2 shows the results
78
L. He, J.E. Siewenie / Surface and Coatings Technology 150 (2002) 76–79
Fig. 3. A typical samples of TEM diffraction patterns for (a) an amorphous film, and (b) a polycrystalline film.
of in-situ resistance measurements for LT metal films, the resistance (R) vs. thickness relation for Au films. For comparison, the measurements were also performed for RT films. Firstly, the LT film becomes continuous at a lower thickness than its RT counterpart. Secondly, the RT film always overtakes the LT film at some point, becoming a better conductor at higher film thickness. Thirdly, while the LT curve has a more or less constant curvature up until it reaches its peak conductivity, the RT films have a positive curvature at lower thickness. It is not until they make the transition to a negative curvature that their resistances are able to drop below the LT curve. It is believed that the primary difference between LT and RT films lies in the mobility of the adatom w6x. While adatoms on a cryogenically cooled substrate tend to stay close to where they land, adatoms on a room temperature substrate are more likely to diffuse across the substrate or to re-evaporate. The higher the substrate temperature, the more quickly the metal clusters will grow to the point of exhibiting liquidlike behavior, coalescing and leaving voids on the substrate. It is not until secondary nucleation occurs in the channels and begins filling in between the clusters that RT films become continuous w7x. This extra step could account for the initial positive curvature in the RT graphs. For the LT films, which are simply filling in as they go, this liquid-like stage is not so prevalent, and this is reflected in the resistivity vs. thickness data. Additional resistivity vs. thickness experiments demonstrated that the Al, Au and Ag have the same characteristics as stated above. It was also shown that Ni films behave typically, although there was little difference between the RT and LT resistance at all temperatures. The Pd films were more unusual since the LT films were higher in resistivity for all temperatures. The resistivity vs. temperature graph for Au is shown in Fig. 2b. The figure shows the resistance of the LT film decreasing as the substrate is brought back up to room temperature. As the film temperature is raised, surface and bulk diffusion increase, filling lattice voids
Fig. 4. TEM surface scan and electron diffraction images for RT Pd (a) and RT Ag (b) thin films at 10 nm (1:30 000 magnification).
or otherwise rearranging atoms to correct some of the faults made during deposition w8x. Fig. 3 shows samples of TEM diffraction patterns for (a) an amorphous film, and (b) a polycrystalline film. The diffraction patterns of an amorphous solid contain only a few broad rings. A film consisting of very small crystallites has the same pattern since there is no longrange structural order w9x. Continuous changes in resistance vs. temperature are due to an increase in grain size, and this can be observed in the decrease in the half-width of the electron diffraction lines. The angular width of the diffraction maximum follows the relationship: Bs2lytr Where B is the angular width, and tr the size of the crystal in the direction radial to the electron beam w10x. The diffraction pattern of a single crystal film would result in a Laue pattern array of dots, and very large grains in a polycrystalline film result in a dotted appearance to the rings, an indication of the higher degree of
Fig. 5. TEM surface scan and electron diffraction images for RT (a) and LT (b) Al thin films at 11.8 nm (1:30 000 magnification).
L. He, J.E. Siewenie / Surface and Coatings Technology 150 (2002) 76–79
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Fig. 6 shows some typical AFM images for the LT and RT Al films at 20-nm-thickness. The LT film shows much large size of crystal formation, while the RT film is formed by smaller sized structures. Less crystal boundaries are expected from the LT film, which corresponds to the lower resistance for the LT films. It was observed in the early growth stage, e.g. at 10-nmthickness, the LT and RT films do not have as obvious differences in surface morphology w5x. As the film becomes thicker, as shown in the 20-nm-thickness, the surface roughness in LT film becomes significant. When the films become increasingly thick, e.g. thicker than 100 nm in most films, the LT and RT films show no difference. This observation also supports the resistance measurement result. 4. Summary The LT-processed thin metal film shows important advantages over the RT film. The low resistivity at very thin thickness for the LT films has a promising potential in various applications for microelectronic and optoelectronic devices. The increased Schottky barrier height is especially important to enhance device performance. The TEM diffraction study indicates the possible formation of amorphous-like material in the surface of LT film, which may be responsible for the barrier height increase (an MIS-like structure). The TEM and AFM surface scan images show that the LT thin films have a rougher surface, and larger sized crystal grains, which results in the lower resistance of the LT films. Acknowledgements Fig. 6. A typical AFM surface spectrum of LT (a) and RT (b) Au films at 20 nm.
This work was sponsored by the National Science Foundation under grant No. ECS-0096190.
order in these films. Fig. 4 shows the TEM surface scan and electron diffraction images for RT Pd (a), and Al (b) films. These two films had the same thickness with different degree of crystallization. This result shows the crystallization variation with metals. Fig. 5 shows the effect of substrate temperature. With the same thickness, the surface scan showed a much rougher surface for the LT Al film. Also, in the diffraction pattern, less distinctive rings were observed from the LT film, and no pronounced dotting. The results indicated that the LT film is less crystallized, i.e. an amorphous-like material is formed. For the discussion of crystallization, more evidence is necessary for the LT film structure. More results are expected later.
References w1x Z.Q. Shi, W.A. Anderson, J. Vac. Sci. Technol. A 11 (1993) 985. w2x L. He, Z.Q. Shi, J. Vac. Sci. Technol. A 14 (1996) 704. w3x J.E. Siewenie, L. He, J. Vac. Sci. Technol. A 17 (1999) 1799. w4x Z.Q. Shi, W.A. Anderson, Appl. Phys. Lett. 66 (1991) 448. w5x L. He, Solid-State Electron. 39 (1811) 1996. w6x L. He, M. Costello, K.Y. Cheng, D.E. Wohlert, J. Vac. Sci. Technol. A 16 (1998) 1646. w7x M. Ohring, The Material Science of Thin Films, Academic Press, New York, 1992, p. 452. w8x D. Smith, Thin Film Deposition, McGraw-Hill, 1995, p. 170. w9x K. Behrndt, J. Vac. Sci. Technol. 7 (1969) 388. w10x K. Lark-Horovitz, V. Johnson, Methods of Experimental Physics 6: Solid State Physics, Prentice-Hall, 1959, p. 255.