Chapter 3

Electrical Contacts

 

3.0 Outline

 

          This chapter describes the electrical contacts between the diamond electrodes and the copper connecting wires.

 

          The requirement for Ohmic contacts is discussed. A schematic illustration of the energy levels at the interface between a p-type semiconductor and a metal is used to illustrate a Schottky barrier.

 

          Details are given for the fabrication processes for two types of Ohmic contact: three layer metal contacts and titanium underlayer contacts. The practicality of the two processes for use in the laboratory is compared.

 

          Current-voltage plots are presented for the four types of contacts described in chapter 2. Silver epoxy resin contacts are considered to be sufficient for highly doped diamond samples when used over a small voltage range. For low doped samples, titanium underlayer contacts give the best performance.

 

3.1 Introduction

 

As the low doped diamond films exhibited semiconducting behaviour, it was necessary to study the metal-diamond contacts to ensure that they obeyed Ohms law. For highly doped diamond films, satisfactory contacts could be fabricated by using silver dag to attach copper wires to the diamond surface. However, this simple technique was not possible for low doped films, as Schottky barriers would form at the semiconductor-metal interface. Figure 3.1 shows a schematic energy diagram of a Schottky barrier.


Figure 3.1 - The Metal-Semiconductor Interface

 

The properties of two types of contact were studied: three layer metalisation (3LM) and titanium underlayers (TiUL).

 

Ohmic contacts are defined as metal-semiconductor contacts that have negligible contact resistance to the bulk or series resistance of the semiconductor. A satisfactory Ohmic contact should not significantly degrade device performance and pass the required current with a voltage drop that is small compared with the drop across the active region of the device. 113

 

The simplest techniques used to reduce the Schottky barrier height involve the use of diamond with a hydrogen terminated surface (to decrease c) and electronegative metals (to increase f). 101 Silver dag contacts were easy to fabricate but gave poor results, as shown in figure 3.3. Evaporating gold onto the surface of diamond gave inconsistent results as the gold layers readily delaminated.

 

Titanium has been used to fabricate low-resistance contacts 72,114-116 and it has been shown that annealing at temperatures greater than 400 C leads to the formation of a titanium carbide layer at the interface. 114

 

The reactivity of titanium posed a practical problem as the metal had a tendency to oxidise.

 

 

3.2 Fabrication of Three Layer Metal Contacts

 

An overview of the design of the three layer metal (3LM) contacts is presented in section 2.9.3.

 

To prepare the sample for titanium deposition, a pre-treatment was required to oxidise the surface and remove hydrogen and non-diamond carbon from the surface. To achieve this, the film as exposed to chromic acid at 95 C for a period of about two hours.

 

The diamond samples were then loaded into the evaporator. It was necessary to heat the sample to remove physisorbed species from the diamond surface prior to titanium deposition to prevent oxidation of the metal. The samples were placed on a heating stage inside the vacuum chamber of the evaporator. A mask was then placed over the sample and a tungsten basket (Alfa Aesar) positioned above the mask and loaded with titanium crystals (Alfa Aesar). The bell jar was placed over the assembly and the system was pumped down to less than 2  10-5 Torr. The sample was then heated to 200 - 250 C.

 

The masks were made from 0.5 mm thick mild steel foil and contained circular holes (diameter = 2 mm, spacing = 8 mm).

 

Figure 3.2 - A schematic diagram of the evaporator


An alternating current was passed through the tungsten basket to evaporate the titanium. A layer of between 100 - 150 nm of titanium was deposited at a rate of approximately 0.4 nm/min as monitored by using a quartz crystal microbalance.

 

After the titanium deposition, the sample was allowed cool slowly under vacuum and it was often left overnight to reach room temperature. The cooling time could have been substantially reduced by flowing a dry oxygen-free gas through the chamber but cooling in vacuum proved sufficient for these studies.

 

To obtain the required depth of titanium (100 - 150 nm), it was sometimes necessary to repeat the loading, heating, deposition and cooling process with a second load of titanium crystals.

 

The sample was then transferred to a sputter coater and a 40 nm thick platinum spot was sputtered over the titanium. A mask was used with a 3 mm diameter spot (a diameter greater than that used for the titanium deposition). This ensured complete coverage of the titanium by platinum.

 

The sample was then returned to the evaporator and a gold spot with a depth of approximately 100 nm was deposited over the platinum. A spot with a diameter of 4 mm was used to ensure complete coverage of the platinum.

 

After each step of the process, the three layer metal structure could be examined by eye and with an optical microscope to check the alignment, coverage and quality of the metal layers.

 

The final step of the process was to anneal the sample. A survey of the literature suggested that an anneal in vacuum at a temperature of 500 C would be sufficient to allow the titanium carbide layer to form. 114,117 This was achieved by a two stage heating process. The first stage provided the necessary vacuum and the second stage reached the necessary annealing temperature. The samples were loaded in a quartz cell, designed with two taps to allow the contents to be pumped down to vacuum with a rotary pump and then flushed with oxygen-free nitrogen (N2) or helium (He) (BOC Speciality Gases). The vessel was then routinely flushed several times with nitrogen to remove oxygen before a cement oven was used to heat the cell to 200-250 C. This heating was performed while the cell was being continuously evacuated by a rotary pump. This removed species as they desorbed from the sample and the walls of the vessel. While still being heated, the cell was sealed and it was then transferred to a high temperature furnace for annealing at 500 C.

 

A number of anneals were performed under a static atmosphere of helium to investigate the effect on the sub-surface hydrogen in the films. These helium experiments did not yield useful results.

 

The fabrication process for the 3LM contacts is summarised in table 3.1 and the electrical characteristics are described in section 3.5.

 

The Ti/Pt/Au metallization scheme represents the simplest of a range of titanium based metallurgical systems. Other proposed schemes include the Ti/W/W(Ni3Sn4)/Ni3Sn4/Au scheme proposed by Katz et al. 118

 


The fabrication process for the 3LM contacts had a number of drawbacks:

 

          The complicated design required a large number of steps which increased variation between samples and the likelihood of flaws.

 

          A hot acid pre-treatment was required which prevented study of hydrogen terminated diamond. However, the process could, in principle, be modified by designing a cell to expose only a selected area of diamond to the acid.

 

          The process was time consuming. A minimum of six working days were required to grow the film but restricted availability of the platinum sputter coater and the high temperature furnace resulted in practical lead times of over a fortnight.

 

          Despite modifications to reduce the effects of vibrations in the evaporator, the alignment of the three concentric circles was not always sufficiently accurate.

 

The sample had to be heated to form the carbide layer which led to oxidation of the titanium and possible changes to the diamond surface.

 


 



Step

Description

1

abrasion of the silicon substrate

abrade with diamond powder to provide nucleation sites

2

cleaning substrate to remove excess diamond powder

(a)    wipe with IPA soaked cotton buds

(b)   place in a beaker of IPA in an ultrasonic bath

3

diamond deposition

(a)    load substrate into chamber

(b)   evacuate chamber

(c)    pre-heat substrate

(d)   deposit diamond for several hours

(e)    initially cool in a hydrogen atmosphere

(f)     cool to room temperature in vacuum

4

surface pretreatment

(a)       heat sample in chromic acid

(b)       rinse sample with ultrapure water

5

titanium deposition

(a)      load substrate into evaporator

(b)      align mask over sample

(c)      evacuate chamber

(d)      heat sample to remove physisorbed species

(e)      evaporate titanium on to diamond surface

(f)        cool sample to room temperature in vacuum

(g)      repeat of steps (c) to (f) with more titanium

6

platinum deposition

(a)    load substrate into evaporator

(b)   align mask over sample

(c)    evacuate chamber

(d)   sputter platinum over titanium spot

7

gold deposition

(a)      load substrate into evaporator

(b)      align mask over sample

(c)      heat sample to remove physisorbed species

(d)      evaporate gold over platinum spot

(e)      cool sample to room temperature in vacuum

8

annealing

(a)      place sample into quartz cell

(b)      evacuate cell

(c)      flush cell with nitrogen

(d)      repeat (b) and (c) several times

(e)      evacuate cell

(f)        heat sample to 200 - 250 C

(g)      seal vessel and transfer to high temperature annealing furnace

(h)      high temperature anneal at 500 C

(i)        cool to room temperature in vacuum

9

attachment of wires

(a)       attach copper wire with silver dag and allow to dry

(b)       cover silver dag with epoxy resin to protect contact

Table 3.1 - Summary of the fabrication procedure for 3LM contacts

 

 

3.3 Fabrication of Titanium Underlayer Contacts

 

The drawbacks of the 3LM contact design led to the development of a simpler design, the TiUL contact. An overview of the design of the titanium underlayer (TiUL) contacts is presented in section 2.9.4.

 

The advantages of the TiUL contacts are detailed below:

 

          A reduction in the number of processing steps increased the reliability of the finished device and reduced the time required processing time.

 

          As the diamond film was grown over the titanium, a carbide layer formed without the need for annealing or any other post-treatments. Electrochemistry could therefore be performed on an as grown surface and the titanium was much less likely to be oxidised.

 

          Platinum and gold layers were no longer required.

 

          A single strip of titanium was applied directly to the silicon substrate and this proved easier to fabricate that the concentric circles because the geometry was simpler and the acceptable tolerances greater.

 

The only disadvantage of the TiUL contacts was the need to protect the metal if any harsh post-treatments that were applied to the diamond sample, such as acid dips. However, this drawback could be avoided by designing a cell to selectively expose only the diamond surface to the treatment. The PTFE electrochemistry cells, described in a later chapter, proved to be adequate for the treatments used in this study.

 

The fabrication process for the TiUL contacts is summarised in table 3.2 and the electrical characteristics are described in section 3.6.

 



Step

Description

1

abrasion of the silicon substrate

abrade with diamond powder to provide nucleation sites for subsequent diamond growth, leaving a strip of smooth silicon

2

cleaning substrate to remove excess diamond powder

(a)      wipe with IPA soaked cotton buds, working away from the smooth end

(b)      clamp the substrate in an ultrasonic bath so that the abraded area is dipped into IPA

(c)      rinse sample with IPA flowing away from smooth end

3

titanium deposition

(a)      load substrate into evaporator

(b)      align mask over the surface of the sample, leaving the smooth area and part of the abraded area exposed.

(c)      evacuate chamber

(d)      heat sample to remove physisorbed species

(e)      evaporate titanium on to diamond surface

(f)        cool sample to room temperature in vacuum

(g)      repeat of steps (c) to (f) with more titanium

4

diamond deposition

(a)      load substrate into chamber

(b)      evacuate chamber

(c)      pre-heat substrate

(d)      deposit diamond for several hours

(e)      initially cool in a hydrogen atmosphere

(f)        cool to room temperature in vacuum

5

attachment of wires

(a)       attach copper wire with silver dag and allow to dry

(b)       cover silver dag with epoxy resin to protect contact

 

Table 3.2 - Summary of the fabrication procedure for TiUL contacts

 


3.4 Characteristics of Silver Epoxy Resin Contacts

 

Silver dag contacts were considered adequate for highly doped films (those grown with a boron to carbon ratio of more than 3000 p.p.m. in the gas phase) when used over a narrow potential range.

 

Figure 3.3 shows a current-voltage plot for a diamond sample with a doping level of 3000 p.p.m. Two silver dag contacts were applied to the top of the as-grown sample and the scan was taken with a m-Autolab potentiostat (Eco Chemie B.V.) in a two electrode configuration with a scan rate of 50 mV/s. The plot was asymmetrical over the ten volt range of the scan. However, over a reduced range of - 0.5 V to 0.5 V, the plot was linear (see inset). The reciprocal of the gradient was 475 W.

 

Figure 3.4 shows a similar plot for a low doped sample. The diamond film was only doped by the residual boron that contaminated the chamber after a growth run with an active doping level of 3000 p.p.m. The graph is significantly less symmetrical than that for the more highly doped sample and the curve is non-linear even near the origin (see inset). Figure 3.5 shows the two graphs superimposed on the same axes. The residually doped film was, as expected, more electrically resistive than the actively doped film.

Figure 3.3 - Current-Voltage characteristics for an as-grown moderately doped diamond film with two silver dag contacts [sample B111]

Figure 3.4 - Current-Voltage characteristics for an as-grown low doped diamond film with two silver dag contacts [sample B112]


Figure 3.5 - Current-Voltage characteristics for diamond films with two silver dag contacts [sample B111 & B112]

 

3.5 Characteristics of Evaporated Gold Contacts

 

Figures 3.6 and 3.7 show current-voltage plots for two pairs of diamond samples. Each pair was grown in a single deposition run to minimise variation between the films. One sample from each pair was refluxed in chromic acid to oxidise the surface of the film; while the other sample from the pair was tested as grown. Two gold strips were evaporated onto each sample and current-voltage plots were measured. The gold layers had a thickness in the order of 100 nm.

 

Samples B128a and B128b were grown with a gas phase boron to carbon ratio of 50 p.p.m.

 

Samples B123a and B123b were only doped by the residual boron contamination in the CVD chamber.

 

The results showed asymmetrical non-linear behaviour. The actively doped samples (B128a and B128b) gave an approximately Ohmic response over a reduced potential range (-0.5 V - 0.5 V) while the residually doped samples (B123a and B123b) gave a non-linear response.

 

It was believed that the exposure to boiling chromic acid had successfully altered the hydrogen termination of the diamond surfaces. A simple test with ultrapure water showed the as grown samples to be hydrophobic and the oxidised samples to be hydrophilic.

 

Figure 3.7 shows an increased resistance for the oxidised film. This agrees with theory outlined in section 3.1 but the effect of surface termination on the contact properties could not conclusively determined due to the lack of reproducibility of these results.

 

Evaporated gold contacts did not provide a significant improvement in performance over the simpler silver dag contacts. The lack of adhesion between the gold and diamond may have been responsible for this poor performance.


Figure 3.6 - Current-Voltage characteristics for diamond films with two gold contacts [sample B128a & B128b]

Figure 3.7 - Current-Voltage characteristics for diamond films with two gold contacts [sample B123a & B123b]

 

3.6 Characteristics of Three Layer Metal Contacts

 

Figure 3.8 shows three current-voltage plots for a low doped (250 p.p.m.) diamond sample with 3LM contacts. Measurements were taken before and after a 500 C anneal. The pre-anneal measurement, shown in blue, was approximately linear but exhibited a high resistance. The two post-anneal plots, shown in orange and pink, showed the effect of reversing the connections to the 3LM contacts on the asymmetrical response of the device.

 

The anneal decreased the resistance of the device but the response did not become linear.

 

A series of four-point probe measurements were taken for the samples to gauge the sheet resistance of the material. This proved to be significantly less than the contact resistance of the 3LM contacts and so the contacts could not be considered to be Ohmic.

 

Ohmic 3LM contacts are suitable for many device applications. However, reliable fabrication of 3LM contacts could not achieved in the laboratory setting. Further development of the fabrication process would have required substantial investment in order to obtain the necessary levels of precision and purity. Therefore, development of the techniques required for the fabrication of 3LM contacts was stopped in favour of developing the new TiUL contacts.

Figure 3.8 - Current-Voltage characteristics for a diamond film with two 3LM contacts [sample B122]

 

3.7 Characteristics of Titanium Underlayer Contacts

 

Figure 3.9 shows the current-voltage plot for a low doped (50 p.p.m.) diamond sample with two TiUL contacts (sample B147). The plot is linear over a potential range of approximately ten volts (see insets). This represents a significantly better response than that obtained with any other of type of contact tested.

 

The TiUL contact was a novel design of Ohmic contact that performed well with low doped diamond film over a wide potential range from -5 V to 5 V. The range of the linear response exceeded the electrochemical potential of diamond in aqueous media.

 

While the TiUL contacts were adequate for aqueous electrochemistry, experiments in non-aqueous electrolytes may be more problematic as wider potential windows may be encountered.

Figure 3.9 - Current-Voltage characteristics for an as-grown diamond film with two TiUL contacts [sample B147a]

 

3.8 Four Point Probe Measurements

 

A series of four point probe measurements were taken to obtain values for the sheet resistance of the samples without measuring the contact resistance. A Solartron 1287 Electrochemical Interface potentiostat was used in four electrode mode. A test rig was designed with four electrodes built into a PTFE block. The four spring-loaded brass electrodes had flat circular contact pads (1 mm diameter) and were aligned in a straight line and separated by 2 mm gaps. The samples were raised up to the electrodes on an adjustable ramp.

 

Reference 113 provides an equation the calculation of the sheet resistivity which is reproduced below.

 

 

where r = sheet resistivity (in W cm)

dV/di = reciprocal of the gradient of the current-voltage plot

W = thickness of the film

CF = correction factor dependant on the probe separation and the size of the sample (approximately equal to 4)


3.9 Summary

 

          For highly doped diamond films used over a limited potential range, silver dag contacts gave a sufficiently linear response.

 

          Low doped films required more sophisticated contacts to avoid the formation of Schottky barriers.

 

          Evaporated gold contacts were found to be insufficient to provide Ohmic contacts.

 

          3LM contacts were found to be difficult to fabricate and it proved impossible to prepare an Ohmic contact in a laboratory environment.

 

          TiUL contacts provided a novel solution to the problem. They were relatively simple to fabricate and gave a linear response.



Care was required to release the excess gas pressure from the quartz cell during heating.