A Study on Influence of Deposition Process

Photonics and Microsystems 2013

A Study on Influence of Deposition Process Parameters on Optical Properties of Si3N4 Films Deposited by PECVD Method W. Kijaszek1), W. Oleszkiewicz1), A. Zakrzewski2), S. Patela2), M. Tłaczała1) 1) Division of Microelectronics and Nanotechnology, Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Janiszewskiego 11/17, 50-372 Wrocław, Poland 2) Division of Microsystems and Photonics, Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Janiszewskiego 11/17, 50-372 Wrocław, Poland [email protected] power of RF generator (PRF), table’s temperature (T), total pressure (p) in the working chamber during the deposition process on optical properties of the deposited film. Basing on the acquired measurements results the calibration and optimization of the deposition process was carried out.

Abstract: The Si3N4 films were deposited by Plasma Enhanced Chemical Vapour Depostion (PECVD) method on 10 mm × 10 mm p-type polished silicon (100) wafers (cut from 3” substrate). The films were deposited using Plasmalab 80 Plus system manufactured by Oxford Instruments Company. The plasma was capacitively coupled by RF signal (13.56 MHz). The reaction precursors were 2%SiH4/98%N2 and NH3 with set flow rates on 1000 sccm and 20 sccm respectively. The RF power was changed from 15 to 25 W, the table’s temperature was ranging from 325 to 375 °C and the pressure in processing chamber was varying from 600 to 700 mTorr. Taguchi method were applied in the experiment for optimization of the deposition technology. The calibration of the deposition process were performed for specified changes of values of the process parameters. The influence of PECVD process parameters values on deposition rate, refractive index (n) and extinction coefficient (k) were estimated from spectroscopic ellipsometry measurements. The optical coefficients were measured in spectral range from 350 nm to 1000 nm. Depending on deposition conditions, the Si 3N4 refractive index was ranging from 2.0318 to 2.0836 and the extinction coefficient was changing from 0.0007 to 0.0023.

II. EXPERIMENTAL METHOD A. Si3N4 film deposition The Si3N4 films were deposited using silane diluted in nitrogen (2% SiH4/98% N2) and ammonia (NH3) as gaseous precursors in a commercially available reactor, the Plasmalab 80 Plus system manufactured by Oxford Instruments. In each deposition process the 2% SiH4/98% N2 flow rate was set on 1000 sccm and the NH3 flow rate was set on 20 sccm. The films were deposited on p-type polished silicon wafers (10 mm x 10 mm) with a (100) surface orientation. Every silicon piece was cut from the same 3” silicon wafer. In presented research the design of experiment (DoE) approach was applied. The total pressure in the working chamber, power value of the RF signal (13.56 MHz) and table’s temperature were selected as input factors. The pressure were controlled by the APC valve and was ranging from 600 to 700 mTorr. The RF signal were supplied to the upper electrode for capacitive plasma coupling and the forward power of the signal was changed from 15 to 25 W. The table’s temperature was ranging from 325 to 375 °C. In these conditions the plasma discharge was stable and deposition of the film with expected properties occurs.

I. INTRODUCTION Silicon nitride (Si3N4) films have received major attention over the last decade due to their unique properties and potential applications [1-3]. These films are of intense interest for their optical properties, high anti-reflectivity, high mechanical hardness, high resistivity and passivation properties [4-6]. Additionally the deposited film’s properties can be precisely controlled by the film’s growth conditions. There is also a possibility of manufacturing thick Si3N4 stacked films with compensated stresses by the dual frequency Plasma Enhanced Chemical Vapour Deposition [7, 8]. The mentioned films’ characteristics make them interesting in the novel electronic applications such as photonic structures, photovoltaic cells, high quality MIM capacitors, transistors and MEMS structures [9-12]. In this work we have carried out a characterization of the Si3N4 films’ properties, prepared by radio frequency (RF) plasma enhanced chemical vapour deposition (PECVD) method. The investigation focused on the influence of deposition process’s parameters, such as applied forward

B. Measurements The optical coefficients (refractive index and extinction coefficients) of the Si3N4 films were calculated from measurements by the spectroscopic ellipsometry. The spectra were acquired for the wavelengths ranging from 350 nm to 1000 nm with resolution of 10 nm. The measurements were performed by the V-VASE ellipsometer with HS-190 scanning monochromator manufactured by J.A.Woollam Company. Additionally the thickness of the deposited films was estimated from the ellipsometric measurements. The values of the optical coefficients and the film’s thickness were calculated as an arithmetical mean from four measurements results. C. Design of the experiment The experiment’s program was based on the orthogonal matrix and involves elements of Taguchi’s optimization method. The selected orthogonal matrix for the experiment is called L934. These research plan is often used for the 42

In each fit the linear correlation coefficient (R2) was higher than 0.95. The calculated values confirmed that the deposition process is stable and the deposition rate during the process is constant in the investigated range of the process input parameters. The slopes of the fitted curves were adopted as the deposition rates of the Si3N4 film in specified conditions. The estimated calibration curves are showed in the Fig. 1. In order to investigate the influence of the deposition process conditions on the optical properties of the deposited Si3N4 films, the refractive index and extinction coefficient were measured by the spectroscopic ellipsometry. The measurements were carried out for spectral range from 350 nm to 1000 nm with resolution of 10 nm. The acquired results are consistent with the values found in the literature [17, 18] and are close to the values that characterize stoichiometric Si3N4. The results implies that the deposited films are good quality and have got similar Si to N ratio as stoichiometric Si3N4, but it is necessary to validate the assumption by further studies (e.g. SIMS investigation). Example measured values of the refractive index and extinction coefficient are showed in the Fig. 2.

optimization of technological processes [13, 14]. The approach using orthogonal matrix provides a possibility for acquiring the maximum amount of the information with minimum number of experimental runs and additionally simplifies the analysis and interpretation of the acquired results [15]. Beside mentioned earlier input parameters, the thickness of the dielectric film in the test capacitor structure was selected as an additional input factor. These structures are used for currently carried out study on electrical properties of the Si 3N4 films. The flow rates of the 2% SiH4/98% N2 and NH3 were selected as constant factors in all deposition processes. In L934 matrix, the input parameters have got three levels of value. According to the matrix the values of input factors are specified for each run of the experiment (see Table 1). In order to calculate the deposition rate of the Si3N4 film in specific conditions of the PECVD process, the calibration of the process was performed before analysis. For each set of the parameters four sample films with different thickness were deposited. The thickness of the films were regulated by the duration of the deposition process. In order to achieve the repeatability of the deposition process and ensure the same initial conditions, the appropriate reactor’s chamber cleaning procedure was applied between deposition processes.

Thickness of the deposited film, d (nm)

500

Tab. 1. Values of the input factors in programmed experiment and programmed experiment plan Level

Forward Power, PRF (W)

Temperature, T (°C)

Total pressure, p (mTorr)

Film’s thickness, d (nm)

1 2 3

15 20 25

325 350 375

600 650 700

50 100 150

Run no.

PRF (W)

T (°C)

p (mTorr)

d (nm)

1 2 3 4 5 6 7 8 9

(1) 15 (1) 15 (1) 15 (2) 20 (2) 20 (2) 20 (3) 25 (3) 25 (3) 25

(1) 325 (2) 350 (3) 375 (1) 325 (2) 350 (3) 375 (1) 325 (2) 350 (3) 375

(1) 600 (2) 650 (3) 700 (2) 650 (3) 700 (1) 600 (3) 700 (1) 600 (2) 650

(1) 50 (2) 100 (3) 150 (3) 150 (1) 50 (2) 100 (2) 100 (3) 150 (1) 50

Experiment 1 Experiment 2 Experiment 3 Experiment 4 Experiment 5 Experiment 6 Experiment 7 Experiment 8 Experiment 9

400

300

200

100

0 0

5

10

15

20

25

30

35

Duration of the deposition process, t (min)

Fig. 1. Calibration curves of the Si3N4 deposition process carried out for conditions specified in Table 1.

In the analysis of the acquired results were applied elements of the Taguchi’s method. Before the analysis the designed matrix was expanded and filled in with the calculated deposition rates and measured values of the refractive indexes and extinction coefficients (both for 550 nm). The summarized results are showed in Table 2. The differences (Δ) between maximal and minimal calculated level’s mean values, for each input-output factor pair, are then calculated. The bigger the difference is, the more influence have got the input factor on the output factor. For accurate interpretation the difference should also be compared to the measured values of the analyzed output factor. From the analysis it follows that the deposition rate is mainly dependant from applied RF power to the electrode (Δ = 5.4) and the total pressure (Δ = 1.96), while the table’s temperature affects the deposition rate only in a minor degree (Δ = 0.71). In case of the influence of the PECVD process parameters on the refractive index of the deposited Si3N4 films, the most influent parameter is the table’s temperature (Δ = 0.0348). The total pressure (Δ = 0.0183) and the applied RF power (Δ = 0.0134) are less influent factors that affects the refractive

III. RESULTS AND DISCUSSION In order to carry out the experiment as it was planned, the calibration of the deposition process was performed for input parameters specified in Table 1. Therefore it was possible to precisely estimate the deposition rate and control the thickness of the deposited dielectric films in the test structure. The calibration curves were estimated from four points for each experimental run. As mentioned the thickness of the test films were regulated by the duration of the deposition process. The duration of the following deposition processes were 2, 3, 5 and 10 minutes. For experiment 1 and 5 the duration of the processes were increased to 5, 10, 20 and 30 minutes in order to investigated the possibility of manufacturing thicker films that are adherent to the substrate and have got small internal stresses. The thicknesses of the deposited Si3N4 films measured by ellipsometric method were fitted by linear function [16]. The fitted curves were estimated using the least square method.

43

a)

b)

T = 325 °C p = 600 mTorr NH3 - 20 sccm 2% SiH4/98% N2 - 1000 sccm

2,1

d = 56 nm d = 113 nm d = 232 nm d = 356 nm

Experiment 1 PRF = 15 W

Extinction coefficient, k

Refractive index, n

d = 56 nm d = 113 nm d = 232 nm d = 356 nm

Experiment 1 PRF = 15 W

2,2

nav = 2,0318

2,0

T = 325 °C p = 600 mTorr NH3 - 20 sccm

0,04

2% SiH4/98% N2 - 1000 sccm kav = 0,0017

0,02

0,00

1,9 400

600

800

400

1000

600

800

1000

Wavelength (nm)

Wavelength (nm)

Fig. 2. Ellipsometric spectra of the deposited Si3N4 films: a) refractive index; b) extinction coefficient.

index of the deposited film. The acquired Δ values are relatively small comparing to the measured values of the refractive index, but are enough significant that it is possible to assume that the refractive index of the deposited film can be controlled by the parameters of the deposition process. The extinction coefficient of the deposited Si3N4 films was the most dependent on the RF signal’s power value (Δ = 0.0007). The table’s temperature (Δ = 0.00047) and the total pressure in the working chamber (Δ = 0.00043) are also significant factor, because these Δ values are in the same value’s range as the measured values of deposited films’ extinction coefficients. From the calculated mean values it is possible to create graphs that represent the mean influence of each input factor on the investigated output factors. Acquired graphs show the relationship between the input and the output factors. Additionally the graphs helps in the interpretation of the investigated phenomena. The influence of the subsequent input factors on the output values are presented in the grouped graphs (Fig. 3-5).

plasma’s density and therefore increase the intensity of the ion bombardment of the substrate. The decrease of the deposition rate with the the table’s temperature. The effect is caused by the increase of the adatoms’ mobility on the substrate’s surface and the growing film has got more ordered structure. A linear relationship between the deposition rate and the total pressure in the working chamber is due to the increasing amount of the reactants particles with the total pressure. B. Refractive index, n Observing the change in the refractive index value as a function of the RF discharge power, it can be observed that there are local maximum of the value of the coefficient. The maximum is probably due to the increase of the amount of the unbound silicon atoms in the growing film. For higher values of the RF discharge’s power the refractive index value decrease may be due to an increase in the probability of occurrence of a binding reaction between the “free” silicon atoms and the unbound nitrogen. The observed effect of the substrate temperature, at a specified RF discharge’s power, on the value of the refractive index is caused by the increasing organization degree of the film’s structure with the substrate’s temperature. The change in the refractive index with the increase of the total pressure in the reactor’s chamber is a result of increased amount of the molecules (from precursors gases, carrier gas and residual gases) participating in the film’s growth.

A. Deposition rate, Drate Increasing power value of the applied RF signal to the electrode results in the increase of the deposition rate. The relationship is connected with the increasing effectiveness of the reagents’ ionization process in the glow plasma discharge. Higher ionization effectiveness causes the increase of the

Tab. 2. Experiment Matrix With Calculated Deposition Rates And Measured Optical Coefficients Of The Deposited Si 3N4 films. Run no.

Forward Power, PRF (W)

Temperature, T (°C)


Deposition rate (nm/min)

Refractive index, n


1

(1) 15

(1) 325

(1) 600

11.75

2.0318

0.0017

2

(1) 15

(2) 350

(2) 650

12.52

2.0397

0.0012

3

(1) 15

(3) 375

(3) 700

12.54

2.0762

0.0009

4

(2) 20

(1) 325

(2) 650

15.17

2.0408

0.0010

5

(2) 20

(2) 350

(3) 700

15.97

2.0836

0.0020

6

(2) 20

(3) 375

(1) 600

14.20

2.0634

0.0007

7

(3) 25

(1) 325

(3) 700

19.43

2.0446

0.0023

8

(3) 25

(2) 350

(1) 600

16.11

2.0543

0.0015

9

(3) 25

(3) 375

(2) 650

17.48

2.0820

0.0020

44

a)

a)

18

17


17


18

16

15

14

13

16

15

14

13

12

12 15

20

320

25

340

RF power, PRF (W)

b)

b)

2,08

Refractive index, n

Refractive index, n

2,06

2,05

2,08

2,06

2,05

2,04

2,04

2,03

2,03 15

20

320

25

340

360

380

Temperature, T [°C]

RF power, PRF (W)

c)

0,0020

0,0018

0,0020

0,0018



380

2,07

2,07

c)

360

Temperature, T [°C]

0,0016

0,0014

0,0016

0,0014

0,0012

0,0012 15

20

25

RF power, PRF (W)

320

340

360

380

Temperature, T [°C] Fig. 3. The changes of: a) deposition rate; b) refractive index and c) extinction coefficient of the deposited Si3N4 film in a function of the applied RF power to the electrode.

45

Fig. 4. The changes of: a) deposition rate; b) refractive index and c) extinction coefficient of the deposited Si3N4 film in a function of the table’s temperature.

a)

A decrease in the value of the extinction coefficient with the temperature of the substrate is associated with increasing degree of order of the growing film’s structure. The effect can also be caused the stresses’ relaxation that occurs in higher temperature. The Debye’s length is reduced with increasing pressure, which results in the increase in the probability of accidental collisions of accelerated particles with other particles in the plasma discharge during the deposition process, thereby increasing the number of generated defects. The defects in the film’s structure is probably the cause of the increase of the extinction coefficients value.

18


17

16

15

14

13

IV. CONCLUSIONS The results and assumptions acquired from the carried out investigation and characterization of the deposited Si3N4 films can be summarized: - During the study the design of experiment approach was applied. The experiment plan was based on the orthogonal matrix and elements of Taguchi’s analysis methods. - The calibration procedure proofed that the PECVD deposition process is stable for the specified values of the input parameters and the repeatability of the process is ensured. - The deposition rate was calculated for each deposition procedure and was ranging from 11,75 nm/min to 19,43 nm/min depending on the conditions of the deposition process. - The optical properties of the deposited film (refractive index and extinction coefficient) were measured with the usage of the spectroscopic ellipsometry method. The characterization was carried out in spectral range from 350 nm to 1000 nm. - Depending on deposition conditions, the Si3N4 refractive index was ranging from 2.0318 to 2.0836 and the extinction coefficient was changing from 0.0007 to 0.0023. - The influence of the deposition process parameters (forward power applied to the electrode (PRF), table’s temperature (T) and total pressure (p) in the processing chamber) on deposition rate, refractive index and extinction coefficient of the deposited Si3N4 were analysed by Taguchi’s method. - The observed changes of the film’s optical properties give opportunity to interpret and correlate the changes with the changes of the film’s structure during the growth process in specified conditions. The presented hypothesis should be verified by the further studies, using other investigation methods (i.e. SIMS, impedance spectroscopy). - The results of the PECVD process optimization can be used for quick selection of the input parameters values in order to ensure the desired film’s properties in specified applications and structures.

12 600

620

640

660

680

700

680

700

680

700


b)

2,08

Refractive index, n

2,07

2,06

2,05

2,04

2,03 600

620

640

660


c)

0,0020


0,0018

0,0016

0,0014

0,0012

600

620

640

660

Total pressure (mTorr)

ACKNOWLEDGMENT This work was co-financed by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08-5), National Centre for Science under the grant no. N N515 495740, The National Centre for Research and Development through Applied Research Programme nr 178 782 and Statutory activities.

Fig. 5. The changes of: a) deposition rate; b) refractive index and c) extinction coefficient of the deposited Si3N4 film in a function of the total pressure in the processing chamber.

C. Extinction coefficient, k The increase in the extinction coefficient value with the power of the RF discharge is associated with the increase of the amount of defects in the deposited film’s structure caused by high energy ions.

46

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