Silicon nitride density estimation with FTIR, Ellipsometry and BHF -

Silicon nitride (SiNx) is a dielectric material that is widely used as passivation layers, electrical isolation layers, as well as etch and diffusion masks for semiconductor devices. The density of SiNx layer may influence its effectiveness in achieving these roles.

FTIR and Ellipsometry

Here, we will present a relatively simple method to determine the density of SiNx (and other related) layers. This is achieved from Fourier transform infra-red spectroscopy (FTIR) and ellipsometry measurements. These two metrology tools are commonly found in semiconductor foundries/factories with thin film processes.

First, we define the mass density $\rho (g/cm^3)$ of the SiNx film as [1]:

$\rho = [Si] . m_Si + [N] . m_N + [H] . m_H$

Where $m_i$ is the atomic mass of each element Si, N and H that are present in the film.

$[Si], [N], [H]$ can be approximated from bond densities as measured by FTIR. The bond density [A-B], can be determined by [4]:

$[A-B]=k_A_-_B\int\limits\dfrac{ \alpha(\omega)}{\omega} d\omega$

Where $\alpha(\omega)$ is the absorption coefficient at wavenumber $\omega$ , $k_A_-_B$ is the proportional constant for in cm^-2

For SiNx, $k_[_S_i_-_N_] = 2 \times 10^{19}$ , $k_[_S_i_-_H_] = 2 \times 10^{20}$ , $k_[_N_-_H_] = 1.2 \times 10^{20}$ at wavenumbers ~850, ~2220 and ~3340 cm⁻¹ respectively [4].

$[N], [H]$ can then be approximated as followed [1] [3]:

$[H] = [Si-H] + [N-H]$

$3[N]= [Si-N]+ [N-H]$

$[Si]$ can be determined from:

$[Si] = [N]/X$

$X$ can be obtained, for example, by using sophiscated tools such as Rutherford backscattering spectrometry. However, Blech et al have also shown that the refractive index of the film as measured from ellipsometry can also be used to obtain $X$ as followed [1]:

$n = 1.35 + 0.74 /X$

As a result, with just FTIR and ellipsometry, we may be able to quickly determine the density of SiNx layer without the need for sophisticated and high cost equipment.

HF and BHF etch rate

In addition, SiNx will also get etched in hydroflouric acid (HF) and buffered hydroflouric acid (BHF). Hence, HF and BHF could also be used to estimate the density of SiNx, where the denser layer will get etch slower in these solutions, as shown in figure as followed [6]:

Etch rates for SiNx layers of different density in HF and BHF(BOE) [6]

[1] M. Blech, et al., 24^thEUPVSEC2009
[2] B. Stannowski, et al., J. Appl. Phys., 93, 2618 (2003)
[3] E. Bustarret, et al., Phys. Rev. B 38, 8171 (1988)
[4] M. Cardona, physica status solidi (b) 118, 463 (1983)
[5] F. Giorgis, Philosophical Magazine Part B 77, 925 (1998)
[6] J. Han, et al., Mater. Res. Express 4, 096301 (2017)