Specific contact resistivity of Al-NiSi contacts using Cross Kelvin Resistor test structure chains

Anthony S. Holland*, Madhu Bhaskaran, Sharath Sriram, Geoffrey K. Reeves,

Vykundh Ravichandran, Vishal Dodhu Borase, Shreekkanth Bhaskaran

Microelectronics & Materials Technology Centre, School of Electrical & Computer Engineering, RMIT University, GPO Box 2476V, Melbourne, Victoria 3001, Australia

ABSTRACT Silicide contacts are used in semiconductor devices because of their relatively low sheet resistance as thin films and because they form contacts with relatively low values of specific contact resistivity leading overall to low values of contact resistance. Determining the true values of the specific contact resistivity of metal-to-silicide interfaces is a challenge that requires suitable test structures. The Cross Kelvin Resistor (CKR) structure is a commonly used test structure for the extraction of the specific contact resistance of ohmic contacts. Analysis using this structure has errors associated with it and the challenge is often in determining this error. This paper demonstrates a technique that uses several Cross Kelvin Resistor structures connected in a chain and determines the specific contact resistance of aluminium to nickel silicide contacts using extrapolation rather than determining the error. The formation of the nickel silicide films and the fabrication and testing results for the Cross Kelvin Resistor structures are presented. Keywords: Cross Kelvin Resistor, Ohmic Contact, Nickel Silicide, Specific Contact Resistance, Test Structures

1. INTRODUCTION Cross Kelvin Resistor (CKR) test structure presented by Proctor, Linholm, and Mazer1 is commonly used to determine the specific contact resistance ρc (Ω.cm2) of a metal-semiconductor ohmic contact. Fig. 1 demonstrates how the CKR is used and the parameter determined is ρc’. This is larger than the true value ρc. A detailed description of the errors occurring in CKR test structures is presented in published literature2. With regard to Fig.1, the value of Rk is made up of resistance components due to the contact interface (contact area A and specific contact resistivity ρc) and the non-metal material surrounding the contact. For a CKR test structure, the contact area is related to the contact size ‘d’ (the diameter in the case of circular contacts) and the non-metal (silicide) line width ‘w’. It is this second contribution (the material surrounding the contact) that increases the voltage Va measured using the voltage tap, increasing the value of Rk and hence ρc’ is larger than ρc. However, if the contact becomes infinitesimally small then the contact component of Rk becomes extremely larger and for such a contact ρc’ is equal to ρc. Such a contact cannot be fabricated, but by extrapolating ρc’ to d/w = 0, using values of d/w of 0.1, 0.2, 0.3, etc. the value at d/w = 0 can be determined. The extrapolated y-axis intercept of the line gives the value of ρc (Fig. 2). Details of this method are described in published literature3, 4. Fig. 3 is an example of a finite element model of a CKR test structure which shows the equipotentials in a non-metal layer in contact with a metal layer e.g. nickel silicide (NiSi) to aluminium (Al). The top layer should be metal in order for the equipotential of the top of the contact interface to be the same as measured on the top voltage tap, Vb. The layers shown in this example are aluminium to nickel silicide. Fig. 4(a) show the extrapolation method developed further by connecting three CKR test structures together, which allows for less probing in order to determine ρc. In this structure the electrical current (I) travels through five contacts as shown in Fig. 4(b). The three values of Rk required to determine three values of ρc’ for the d/w values of 0.1, 0.2, and 0.3 are determined by six voltage measurements. Plotting this data will give the true value of specific contact resistance. In order to get more accurate results, such a chain structure could be increased in length to include other values of d/w such as 0.05, 0.15, etc. The linearity of the extrapolation method is described in literature describing the technique3, 4. It * anthony.holland@rmit.edu.au; phone +61-3-99252150; fax +61-3-99252007

Microelectronics: Design, Technology, and Packaging II, edited by Alex J. Hariz,Proc. of SPIE Vol. 6035, 60350X, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.650699

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4 =RkxH0 = contact area, A

Data froiii Testing— — — — Extrapolation

0 0.1 0.2 0.3 0.4

(11w (110 IIlj

is relatively linear and the use of circular contacts rather than square contact leads to more reliable data. For contacts with small dimensions it is generally observed that circular contacts remain as circles but square contacts become rounded at the corners. It is easier to fabricate test structures with circular contacts to replicate those modeled using finite element modelling.

Figure 1: Schematic of a Cross Kelvin Resistor test structure with d/w = 0.2

Figure 2: Extrapolation method for ρc’ and d/w data for determining the true value of specific contact resistance ρc using a chain of CKR test structures3, 4

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I

30. 27.25 245 2175 19. 16.25 135 lOTS 8.

(a)

(b)

Legend: Voltage (Arbitrary Units)

Figure 3: (a) Finite element model of a Cross Kelvin Resistor test structure showing the equipotentials in the non-metal layer. In (b) the metal layer has been lifted up to show the equipotentials. The contact layers are typically separated by a thin oxide layer with a contact opening. The top layer is metal and therefore Vb is practically the value of the equipotential at the top of the contact interface. Va is the voltage measured on the tap and is used to determine the average voltage at the bottom of the contact interface

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R1 "a] ISimilarly

I torR,andR

a = R x A

Contact diu'1fl,tjr.

= Ri:i A3

Ma

d

Ma

p'R xA

ci

YE,

A'Iicon dioxide

(a)

(b) Figure 4: (a) The Cross Kelvin Resistor test structure chain for determining the specific contact resistivity of a metal to non-metal contact type. A1 is the area of the contact for d/w = 0.1, A2 for d/w = 0.2, and A3 for d/w = 0.3; (b) shows the cross-section of the arrangement to illustrate the flow of the electrical current through the CKR test structure chain from point A to point A’

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2. FORMATION AND ANALYSIS OF NICKEL SILICIDE 2.1 Introduction Nickel silicide is known to have a low resistivity of about 14-18 µΩ.cm, similar to titanium and cobalt silicides5, 6. Nickel silicide (NiSi) formation can be achieved with lower process temperatures than titanium silicide (TiSi2) and cobalt silicide (CoSi2). Also, NiSi consumes lesser silicon to form the same amount of silicide when compared to TiSi2 and CoSi2

5. These advantages make NiSi attractive for shallow junction CMOS process technologies. Most published literature report nickel silicide formation by Rapid Thermal Processing (RTP)7, 8, 9. With regard to the CMOS process flow, the self-aligned silicide process would be performed after defining the transistor terminals and junctions. RTP will possibly affect the nature of these junctions, whereas a conventional anneal technique would be highly suitable with relation to the CMOS process flow, as the anneal conditions can be compensated for during the earlier oxidation and diffusion stages. 2.2 Nickel silicide formation The presence of a layer of native oxide (2-4 nm) on silicon surface generally prevents a reaction between nickel and silicon10, 11. Considering this, all samples were subjected to a buffered hydrofluoric acid (BHF) dip for one minute to strip native oxide, after standard solvent clean. Formation of nickel silicide was initially tried using electron-beam evaporated nickel, on BHF cleaned silicon samples. It was found that the native oxide growth in the short interval between the BHF dip and evaporation was sufficient to prevent a nickel-silicon reaction. This was overcome by sputtering nickel onto the substrates, ensuring that highly energetic nickel atoms penetrated the native oxide layer. Sputtering was done at a DC power of 80 W, resulting in nickel atoms being accelerated by energies in the range of 430-450 V. Table 1 lists the detailed sputtering conditions used. Under these conditions, for the sputtering time duration of one minute, the average nickel thickness obtained was 50 nm.

Table 1: DC magnetron sputtering conditions used for nickel deposition Target Nickel (99.99 %) Target Diameter 100 mm DC Power 80 W Target to Substrate Distance 50 mm Gas Mixture Argon 5.0 Base Pressure 1.0 x 10-5 Torr Sputtering Pressure 1.0 x 10-2 Torr Sputtering Duration 1 min

The sputtered nickel film was patterned to study the nature of nickel silicide growth, particularly the consumption of nickel and silicon in silicide formation. Etching of nickel was done using a spin-coated AZ 5206-E photoresist mask, in dilute aqua regia (3 parts 37 % hydrochloric acid, 1 part 69 % nitric acid, and 2 parts de-ionized water). Patterned nickel samples on p-type (111) silicon were subjected to a two-step contact anneal process. The two-step process was conceived to (i) ensure the complete consumption of nickel in formation of the silicide, as most published literature5, 12 specify the necessity of etching unreacted nickel in a 4:1 sulphuric acid and hydrogen peroxide solution; and (ii) make the process of NiSi formation gradual, through the formation of Ni2Si after the first anneal stage, which is generally formed around 250 °C13. The samples were placed on a substrate heater in a vacuum chamber. The anneal process was started under vacuum of 1.0 x 10−5 Torr. The first stage of anneal was carried out at 300 °C for two hours, followed by the second stage at 450 °C for three hours. The samples were then allowed to cool in vacuum. The temperature was ramped up for both anneal stages at 15 °C/min and cooled at 15 °C/min. 2.3 Analysis of resulting nickel silicide The nickel silicide formed was analyzed using Auger Electron Spectroscopy (AES) depth profile and the amount of metal and silicon consumed. AES was conducted using a VG Instruments Model 310 Auger/XPS spectrometer, for a sputter-etching time of 1560 seconds. Thickness measurements were done using an Ambios Instruments XP-2 profilometer.

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4 EInitial Silicon- I

Nickel Iiterface___________________________________________________

* ,N,keI

(111W-type (111W-typeSilicon Silicon

The atomic percentage composition versus the nickel silicide depth was obtained using the combination of sputter etch and AES. The resulting composition of the silicide was found to be 43 % nickel and 57 % silicon (Fig. 5). The surface exhibited traces of contamination by oxygen as expected, due to the oxygen sensitivity of nickel during silicide formation12. The surface also showed the all the nickel deposited had reacted with silicon in silicide formation. At the silicide-silicon interface, presence of 2 % oxygen was detected, an indication that this technique of nickel sputter deposition and two-step anneal can overcome thin layers of native oxide and form nickel silicide. Nickel on formation of nickel silicide consumes about 1.83 nm of silicon for every nm of metal, resulting in 2.54 nm of silicide6. We observed that the metal step height of 50 nm had reduced to 27.8 nm on nickel silicide formation. Using the region of silicide etched during the AES depth profile analysis, the total silicide thickness was found to be approximately 130 nm, and Fig. 6 depicts the observed manner in which nickel and silicon were consumed. The sheet resistance of the silicide formed was found to be 0.59 Ω/ using the four-point probe measurement method.

0

20

40

60

80

100

0 240 480 840 1320

Etch Time (seconds)

Atom

ic P

erce

nt

SiliconNickelOxygen

Figure 5: Depth profile of the atomic percentage composition of nickel silicide using Auger Electron Spectroscopy

Figure 6: Observed nickel and silicon consumption for nickel silicide formation

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3. DETERMINATION OF SPECIFIC CONTACT RESISTIVITY 3.1 Fabrication of Test Structures Samples sputter coated with 50 nm nickel were patterned to form the silicide arms of CKR test structures. After the two-step contact anneal to form nickel silicide, 100 nm of SiO2 was deposited on the samples and patterned using buffered hydrofluoric acid (BHF) to form circular contacts of a variety of diameters. The samples were then patterned in preparation for the metal layer, but with an inverted mask to perform lift-off. This mask defined the metal arms of the CKR test structure. A one minute dip in chlorobenzene was performed to improve lift-off features. On this pattern, 250 nm of aluminium was deposited by electron beam evaporation, and lifted off by ultrasonic agitation in acetone. These steps defined the CKR structures of varying contact sizes and two different silicide line widths. These Al-NiSi contacts were further annealed, in nitrogen at 500 °C for 30 minutes followed by 600 °C for 30 minutes, to convert them from Schottky to ohmic contacts. 3.2 Electrical test results Cross Kelvin Resistor (CKR) test structures and test structure chains described in Section 2 of this paper were used to obtain the following results. The following figures (Figs. 7 & 8) were plotted with data obtained for silicide line widths of 6 µm and 17 µm. Many different CKR structures with same contact diameters gave identical results as shown by overlapping data-points, reflecting the consistency of the silicide formed over the area of the sample.

1.0E-07

1.0E-06

1.0E-05

1.0E-04

0.0 0.1 0.2 0.3 0.4

d/w (no units)

ρc'

( Ω.c

m2 )

Figure 7: Specific contact resistivity obtained for different Al-NiSi contact diameters for silicide line width of 6 µm

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1.0E-07

1.0E-06

1.0E-05

1.0E-04

0.0 0.1 0.2 0.3 0.4

d/w (no units)

ρc'

( Ω.c

m2 )

Figure 8: Specific contact resistivity obtained for different Al-NiSi contact diameters for silicide line width of 17 µm By the extrapolation technique discussed in Section 2 of this paper, the specific contact resistivity of aluminium to nickel silicide is about 2.0 x 10–7 Ω.cm2.

4. CONCLUSIONS The continued downscaling of semiconductor device geometry requires the use of lower resistivity materials as well as contacts to these materials which have low specific contact resistivity. Nickel silicide is a material that has such properties. In this paper, we have demonstrated a technique for determining the specific contact resistivity using a chain of Cross Kelvin Resistor test structures. The process for forming nickel silicide is presented. The value of the aluminium to nickel silicide specific contact resistivity (ρc) was determined to be 2 x 10–7 Ω.cm2, for nickel silicide formed on p-type (111) silicon. This silicide is sensitive to the presence of oxide at the interface of the silicon and nickel metal layer and careful preparation and processing is required in order for the silicon and nickel to react fully and form a uniform nickel monosilicide (NiSi) layer.

REFERENCES 1. S. J. Proctor, L. W. Linholm, and J. A. Mazer, "Direct Measurement of Interfacial Contact Resistance, End

Contact Resistance and Interfacial Contact Layer uniformity", IEEE Transactions on Electron Devices, vol. ED-30, No. 11, pp.1535-1542, 1983.

2. A. S. Holland, G. K. Reeves, and P. W. Leech, "Universal Error Corrections for Finite Semiconductor Resistivity in Cross-Kelvin Resistor Test Structures", IEEE Transactions on Electron Devices, vol. ED-51, no. 6, pp.914-919, 2004.

3. A. S. Holland and G. K. Reeves, "New Challenges to the Modelling and Electrical Characterisation of Ohmic Contacts for ULSI Devices", Microelectronics Reliability, 40, pp. 965-971, 2000.

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4. G. K. Reeves, A. S. Holland, H. B. Harrison, and P. W. Leech, "Electrical Modelling of Kelvin Structures for the Derivation of Low Contact Resistivity," Proceedings of ESSDERC, Stuttgart, pp. 303-306, 1997.

5. J. Chen, J.-P. Colinge, D. Flandre, R. Gillon, J. P. Raskin, and D. Vanhoenacker, "Comparison of TiSi2, CoSi2, and NiSi for thin-film silicon-on-insulator applications," Journal of The Electrochemical Society, vol. 144, pp. 2437-2442, 1997.

6. J. D. Plummer, M. D. Deal, and P. B. Griffin, Silicon VLSI Technology: Fundamentals, Practice and Modeling. New Jersey: Prentice Hall, Inc. 2000.

7. O. Chamirian, A. Lauwers, J. A. Kittl, M. Van Dal, M. De Potter, C. Vrancken, R. Lindsay, and K. Maex, "Ni Silicide Morphology On Small Features," presented at MRS Spring Meeting, San Francisco, California, 2004.

8. M. van Dal, A. Akheyar, J. A. Kittl, O. Chamirian, M. De Potter, C. Demeurisse, A. Lauwers, and K. Maex, "Effects of Alloying on Properties of NiSi for CMOS Applications," presented at MRS Spring Meeting, San Francisco, California, 2004.

9. A. Vengurlekar, S. Balasubramanian, S. Ashok, N. D. Theodore, and D. Z. Chi, "Influence of Atomic Hydrogen on Nickel Silicide Formation," presented at MRS Spring Meeting, San Francisco, California, 2004.

10. T. Morimoto et. al, "Self-Aligned Nickel-Mono-Silicide Technology for High-Speed Deep Submicrometer Logic CMOS ULSI," IEEE Transactions on Electron Devices, vol. 42, pp. 915-922, 1995.

11. C.-J. Choi, S.-A. Song, Y.-W. Ok, and T.-Y. Seong, "Nickel-silicidation process using hydrogen implantation," IEE Electronics Letters, vol. 40, 2004.

12. B. Y. Choi, "Experimental Study on Self-Aligned Nickel Silicide Technology," School of Electrical Engineering, Seoul National University, Seoul 2003.

13. L. A. Clevenger and R. W. Mann, "Formation of epitaxial TM silicides," in Properties of Metal Silicides, K. Maex and M. Van Rossum, Eds. London, United Kingdom: INSPEC, the Institution of Electrical Engineers, 1995, pp. 61-70.

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