Use of Atomic Layer Deposition to Grow Platinum Films

By AZoNano Editors

Table of Contents

Introduction
ALD Process
Discussion of the Experimental Results
     Thermal ALD
     Remote Plasma ALD
     Comparison of the Two Methods
Conclusions
About Oxford Instruments Plasma Technology

Introduction

Ultra-thin platinum films deposited on oxide substrates find a number of applications in microelectronics, nanotechnology, etc., due to the fact that platinum exhibits very good electronic properties, is chemically stable and exhibits catalytic activity. The Atomic layer deposition (ALD) technique is a self- limiting technique capable of accurate and uniform deposition of thin films. Using this technique, ultra-thin metal layers having thickness at the nanoscale level with high aspect ratios can be deposited. There are basically two ALD methods, namely, thermal ALD and remote plasma ALD. In subsequent sections, the behaviour of platinum ALD growth deposited using both these techniques will be discussed.

ALD Process

The FlexAL-MK II Pt deposition system was connected to an inductively coupled plasma (ICP) source powered at 300 W and an ellipsometer. This arrangement can perform remote plasma as well as thermal ALD. Trimethyl (methylcyclopentadienyl) platinum(IV) (MeCpPtMe3) (SAFC, Sigma-Aldrich) was used as the platinum source (precursor), this compound was housed in a stainless steel bubbler and subjected to 70°C of heating. The resulting vapour was drawn into the chamber by the vapour draw method. In order to ensure maximum usage of the precursor, MeCpPtMe3 precursor was present during the first half-cycle without pumping and the holding time was 5 to 10 s. In the case of oxide samples, Si(100) substrates were coated with 10 to 20 nm thickness of ALD Al2O3, HfO2 and SiO2 before the ALD process began. Table 1 provides details of the four different substrates.

Table 1. The substrates used for ALD-Pt film deposition

Substrate ALD-oxide process ALD oxide film thickness (nm) ALD process temperature (oC) ALD precursors
Si(100)

/

/

/

/

SiO2/Si

Plasma-ALD

10

200

TRDMAS

Al2O3/Si

Plasma-ALD

18

200

TMA

HfO2/Si

Plasma-ALD

10

290

TEMAH

During the experiment, the chamber pressure was varied from 10 to 40 mT and parts like holder, chamber and the delivery line were subjected to heating to temperatures of 120 and 80°C, respectively. The deposited Pt film thickness was measured using J.A. Woollam M2000V spectroscopic ellipsometer and chemical composition was checked by Energy dispersive X-Ray Analysis (EDX) and Auger Electron Spectroscopy (AES). A four-point probe was used to test the electrical properties.

Discussion of the Experimental Results

Thermal ALD

A plot of growth rate against resistivity of the Pt films deposited by thermal-ALD method up to 600 cycles is shown in Figure 1.

Figure 1. Growth rate and resistivity of platinum films by thermal-ALD at 300°C vs precursor dose-time for 600 cycles

The growth rate (GR) plotted against resistivity data upto 2250 cycles is shown in Figure 2. From the figure, it is evident that there is a slight increase in GR for long Pt deposition. Also, the resistivity of the Pt layer exhibited a decrease when deposited on Si with increasing layer thickness.

Figure 2. Growth rate (GR) and resistivity of platinum films by thermal-ALD vs cycle number and it is found that a GR of Pt thermal-ALD is around 0.45-0.47Å/cycle and the resistivity range of 14.1 to 12.8μΩ-cm from 500 cycle to 2250 cycle

Figure 3 shows the nucleation delay in thermal ALD at 70 cycles.

 

Figure 3. Thickness of platinum films by thermal-ALD vs cycle number at 300°C and the nucleation delay of Pt thermal-ALD to be found around 70 cycles.

The nucleation characteristic study of Pt and Pd on different substrates revealed that when Pt is deposited concurrently on Si substrates it showed an increase in particle size and Pt film was continuous after 75 to 100 cycles.

Remote Plasma ALD

Figure 4 is the plot of the GR of Pt films by plasma ALD against the precursor dose-time at 300°C. The value of GR was 0.43-0.45 Å/cycle which is almost equal to that obtained by thermal ALD technique.

Figure 4. Growth rate of platinum films by plasma-ALD at 300°C vs precursor dose-time

Also Figure 5 shows the resistivity and thickness of the Pt film with the cycle number at 300°C. After 500 cycles the Pt film offers a resistivity of 14.5 μΩ.cm and the nucleation delay is around 20 cycles, which is much less than the value in thermal method and a uniform deposition of Pt was seen on various substrates.

Figure 5. Thickness and resistivity of platinum films by plasma-ALD vs cycle number at 300°C and the nucleation delay of Pt plasma-ALD is around 20 cycles. Comparing to the nucleation delay of Pt thermal-ALD of 70 cycles, it shows that plasma-ALD can reduce the nucleation delay of Pt.

Figure 6 shows the plot of the resistivity exhibited by the Pt film when deposited on oxide substrates by plasma method at 300°C against the precursor dose-time. It can be observed from the figure that resistivity of Pt showed decrease with increase in dose-time upto 1.5s with lowest resistivity on the HfO2 substrate.

Figure 6. Resistivity of platinum film on various oxides by plasma-ALD at 300oC vs precursor dose- time. It is clear that the order of resistivity of Pt film grown on oxides is Si/SiO2 > Si/Al2O3> Si/HfO2

The results of the AES profile scan and the EDX testing that were carried out on the pt film are shown in Figure 7.

Figure 7. AES of 30nm Pt film grown by plasma-ALD.

Comparison of the Two Methods

Figure 8 (a and b) provides the data obtained when Pt film was deposited by combining plasma and thermal ALD for 500 cycles at 300°C. The data revealed that particle size of Pt grown by plasma method was bigger than that grown by thermal method at the same cycle numbers.

Figure 8. SEM of Pt-ALD films (cross-section of thickness and particle-size measurement).

Table 2 provides the data of the particle sizes at various cycle numbers and data of Pt deposition on various substrates.

Table 2. The process data of Pt films on the surface of Si, SiO2, Al2O3 and HfO2 deposited at 300°C by thermal and remote plasma ALD using MeCpPtMe3 and O2 gas or O2 plasma (500 cycles)

Pt-sample runs

ALD process Substrate Particle size at 50 cycles Particle size at 100 cycles Growth rate (Å/cycle) Resistivity (μΩ-cm)
1

Thermal-ALD

Si/native SiO2(~1nm)

1.6 ±0.2

2.1 ±0.2

0.44 ±0.01

14.1 ±0.2

2

Plasma-ALD

Si/native SiO2(~1nm)

2.0 ±0.2

3.2 ±0.2

0.45 ±0.01

14.5 ±0.2

3

Thermal-ALD

Si/SiO2(10nm ALD)

2.2 ±0.2

2.6 ±0.2

0.43 ±0.01

15.1 ±0.2

4

Plasma-ALD

Si/SiO2(10nm ALD)

2.5 ±0.2

3.6 ±0.2

0.44 ±0.01

31.2 ±0.5

5

Thermal-ALD

Si/Al2O3(18nm ALD)

/

/

0.46 ±0.01

25.2 ±0.5

6 Plasma-ALD Si/Al2O3(18nm ALD) / / 0.47 ±0.02 18.3 ±0.3
7 Plasma-ALD Si/HfO2 (10nm ALD) 3.7 ±0.3 5.6 ±0.5 0.49 ±0.02 14.0 ±0.5

Figure 9 depicts the GR and resistivity of the Pt layers deposited on oxide substrates.

Figure 9. Growth rate and resistivity of Pt plasma-ALD layers on various oxides. HfO2 is shown the highest growth rate and the lowest resistivity of them. It is believed that surface functionalization by plasma-ALD and rich-absorbed oxygen radicals on HfO2 surface are the reasons.

Conclusions

To conclude, both the thermal and the plasma ALD methods deposit a high-quality, uniform Pt layer with low resistivity. Compared to the thermal method, the plasma ALD method shows lesser nucleation delay and the ALD Pt films showed lowest resistivity.

About Oxford Instruments Plasma Technology

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This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma technology.

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Date Added: May 17, 2011 | Updated: Sep 24, 2013
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