Applied Surface Science 50 (1991) 73-78 73 North-Holland

Micro-Raman spectroscopy for characterization of semiconductor devices

G e r h a r d Abs t r e i t e r Walter Schottky Institut, Technische Universitiit Mi~nchen, W-8046 Garching, Germany

Received 8 March 1991; accepted for publication 12 March 1991

Selected examples of the usefulness of micro-Raman spectroscopy for the analysis of semiconductor devices are discussed. This includes the determination of local temperatures in devices under operational conditions, built-in strain in processed silicon, and local crystal orientation.

1. Introduction

Novel semiconductor devices and integrated circuits become more complex and smaller in size with each generation. Length scales of less than 1 /~m are common nowadays. This reduction in size leads to a strong increase of electric fields and local power consumption. As a consequence, hot- electron effects and rises in lattice temperatures become important. In addition, the combination of different materials and various steps in process technology, like oxidation and etching, cause local strain which may influence the device operation. New semiconductor systems based on heterostruc- tures, superlattices and quantum well structures offer the possibility for novel device applications. The relevant length scale is further reduced to below 0.1 /~m in such future devices. The fast developments in the field of microstructured semi- conductors as well as the use of different materials require new and powerful characterization meth- ods, especially techniques which provide good spa- tial resolution. In this contribution a few examples are described which demonstrate that Raman scattering can be applied to micro regions, and thus represents a versatile tool for the analysis of semiconductor devices.

Inelastic light scattering is a non-destructive optical technique which enables one to obtain

information on various properties of the investi- gated materials. The semiconductors of interest are highly absorbing in the visible spectral range. Therefore light scattering is usually performed in backscattering geometry. This is shown schemati- cally in fig. 1, together with the relevant informa- tion which can be extracted from such measure- ments. Here we concentrate on Raman scattering by optical phonons. Energy, intensity, polarization and lineshape of the scattered light depend on device-relevant pa ramete rs like composit ion, crystal orientation, strain and temperature. The basic scattering mechanisms are well understood by now [1]. Therefore, inelastic light scattering can be used indeed as a versatile and non-destructive characterization method. High spatial resolution is achieved by use of an optical microscope for focussing the incident light. Such applications be- came possible through the development of highly sensitive multichannel detector arrays which make light scattering experiments in semiconductors feasible with laser powers of less than 1 mW. Raman signals can be obtained from scattering volumes as small as 10 -3/ . tm 3. In the following we discuss selected examples of application of micro- Raman scattering for characterization of semicon- ductor devices under operation. This includes the measurement of the local mirror temperatures in G a A s - G R I N S C H quantum well lasers, the lattice

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74 G. Abstreiter / Micro-Raman spectroscopy for characterization of semiconductor deoices

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temperature of short channel Si-MOSFET's and the built-in strain and local crystal orientation in processed Si.

2. Device characterization - selected examples

2.1. Local mirror temperatures in GaAs lasers

A serious problem of high-power GaAs/ AlxGal_xAs laser diodes is the strong mirror heating which leads to surface degradation, catastrophic optical damage, and finally failure of the devices. Knowledge of the local operating tem- peratures allows an optimization of the device technology by employing mirror coatings, non-ab- sorbing mirrors or separate contacts in order to

achieve current blocking layers. An easy and non- destructive way to perform a temperature map- ping is micro-Raman spectroscopy [2-5]. In GaAs or AlxGaa_xAs the local temperature can be ex- tracted from the Stokes/anti-Stokes intensity ratio of Raman scattering by optical phonons using the following expression [6]:

Is/IAs = CK [(~L - 0~p)/(~L + ~p)]3

× exp( ho~p/kr ).

The constants C and K take into account the difference in the optical properties of the material and of the experimental set-up at the energies (0~ L + ~p) where ~L is the exciting laser frequency and O~p the optical phonon frequency. The temper- ature dependence of the Raman scattering cross-

G. Abstreiter / Micro-Raman spectroscopy for characterization of semiconductor devices 75

section is neglected. This method has been applied recently to study the spatial localization of hot regions in GaAs/AlxGal_xAs quantum well lasers. The temperature profiles exhibit "hot spots" which image the near-field pattern of the laser. An example is shown in fig. 2 where the temperature profile of a metal-clad ridge wave- guide multi-quantum well laser with a 4/~m wide ridge and cleaved mirrors is displayed for two different directions [5]. The inset in fig. 2a gives the geometry. The semiconductor laser was oper- ated with 4 mW output power. The probe laser spot was slightly defocussed to avoid damage of the cleaved surface by the high power density. The measured temperature profile normal to the active layer (fig. 2a) is a convolution of the actual tem- perature distribution and the spot profile. The hot spot is localized clearly within less than 1 /~m perpendicular to the layers. The scan parallel to the quantum wells exhibits not only a temperature increase at the emitting zone but two additional

hot spots outside due to heat dissipation at the edges of the ridges. This is probably caused by the local strain in these regions.

It has been also demonstrated that the tempera- ture rise with increasing laser power depends criti- cally on the mirror properties. The lowest temper- ature increase was observed with antireflection mirror coatings which also yielded the highest power before degradation occurred [3,4]. After degradation additional phonon lines appear in the Raman spectra either due to disorder-activated scattering processes or due to local precipitation of crystalline arsenic. A careful analysis of the various aspects of the Raman spectra leads to detailed information on the reasons for degrada- tion. Recently, Herrmann et al. [7] demonstrated that it is possible to reduce the mirror temperature drastically by applying a suitable potential to sep- arated mirror contacts. The lowest temperature was achieved at a mirror potential of 0 V which is easily obtained by just grounding the contact. This

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76 G. Abstreiter / Micro-Raman spectroscopy for characterization of semiconductor devices

may lead to an improvement in maximum output power and laser lifetime in cases where the mirror temperature is responsible for degradation.

2.2. Local lattice temperature in Si-MOSFET's

The local lattice temperature in semiconductor devices under operational conditions can also be measured by the temperature-dependent shift of the optical phonon energy. This is especially suita- ble for Si-based devices where the temperature shift of the Si phonon is ZI~ /AT=-0 .021 c m - ~ / K from room temperature up to about 100 °C [8,9]. A high-resolution Raman spectrome- ter allows the determination of the Si optical phonon energy with a relative accuracy of at least +0.04 cm -~ which corresponds to A T = +2 K. Ostermeir et al. [9] have used this method recently to study the local temperature distribution in short-channel Si-MOSFET's. The experimental set-up is shown schematically in fig. 3. The sam- ples are mounted on a high-precision x / y transla- tion stage. The laser is focussed onto the poly-Si gates of the transistors through a high-resolution microscope. Exact positioning is achieved by mea- suring the laser-induced photocurrent in the source and drain windows of the MOSFET's. This posi-

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tioning technique was first applied in connection with the analysis of mirror temperature of GaAs lasers [5]. The temperature distribution in MOS- FET's was analyzed by scanning from source to drain over the polycrystaUine gate. Beyond pinch- off conditions the temperature profiles are asym- metric even for a gate length of only 0.6 /~m. An

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Fig. 4. Temperature rises at source and drain side of a MOSFET with 2.5/tm wide gate electrode versus drain voltage. The transistor was operated at a gate voltage of 6 V (from ref. [9]).

G. A bstreiter / Micro-Raman spectroscopy for characterization of semiconductor devices 77

example is shown in fig. 4 where AT is plotted versus drain voltage for a 2 .5 / tm wide transistor. The measuring points on the transistor are indi- cated schematically by arrows in the inset. Above 2.5 V the temperature on the drain side is con- sistently higher as compared to the source side. Although the temperatures are measured mainly within the polycrystalline gate electrode, they agree well with the calculated channel temperatures be- cause of the good thermal coupling to the channel. Peak temperatures at the pinch-off position are, however, not completely resolved due to the broadening of the temperature profile by heat diffusion but also due to the finite spatial resolu- tion of the Raman set-up. At 5 V operating condi- tions the maximum temperature rise is about 20 K for an effective channel length of 0.3 /~m. In addition to the direct temperature measurements it is also possible to determine the thermal time constant by transient heating under pulsed oper- ation and analyzing the inhomogeneous broad- ening of the Raman line. Time constants of about 200 ns were obtained [9].

2.3. Bui#-in strain in Si-microstructures

Device fabrication processes often cause local stress which may influence device operation and may lead to faster degradation. The corresponding strain lowers the crystal symmetry which results in a splitting and shift of the optical phonons. The detailed Raman studies which have been pub- lished for defined uniaxial stress conditions [10- 12] enable us nowadays to apply this method to the stress analysis in microstructured devices. Built-in stress has been studied for example in Si on sapphire [13], underneath or in the vicinity of local oxides on Si [14,15] and in laser-crystallized Si [16,17]. In backscattering from [100] oriented Si the correlation between stress and phonon shift is Ao~ = - 0 . 4 0 kbar -1 cm -1, where a is the mecha- nical stress in kbar. As an example, the optical phonon frequency shift near an oxide stripe (LOCOS) of 4 /~m width is shown in fig. 5 [15]. There is a rather abrupt change from compressive to tensile stress at the edge of the oxide. The compression of 1.5 kbar close to the oxide stripe relaxes to zero at a distance of about 3 /xm.

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Fig. 5. Scan of the local optical phonon shift in Si near a 4 # m wide SiO 2 stripe. The typical information depth is shown in the

inset (from ref. [15]).

-Z0 _

Similar measurements have been performed in dif- ferent kinds of oxidized Si structures in various scattering geometries. A detailed analysis of the Raman results allows the determination of the complete strain distribution in microstructured Si devices.

Apart from the strain analysis it is also possible to obtain information on the local crystal orienta- tion from micro-Raman spectroscopy. Such an analysis is based on the Raman selection rules which lead to a strongly varying phonon intensity with respect to the light polarization and the di- rection of the crystal axis. Recently, this method has been used to study the orientation of laser- crystallized Si on insulators [17-19].

3 . C o n c l u s i o n s

The few selected examples discussed in this contribution demonstrate the usefulness of micro- Raman spectroscopy for analysis of microstruc- tured semiconductors and semiconductor devices even under operational conditions. There are many other aspects which make Raman scattering an interesting and extremely versatile, non-destruc-

78 G. Abstreiter / Micro-Raman spectroscopy for characterization of semiconductor devices

tive technique to study various properties of semi- conductors. For more details I refer the interested reader to the excellent review articles published in ref. [1] and to the original references theroin.

References

[1] See, for example, the review articles published in the series Topics in Applied Physics, Vols. 8, 50, 51, 54, 66, Eds. M. Cardona and G. Giintherodt (Springer, Berlin). This series is focussed on the subject "Light Scattering in Solids".

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[5] S. Beeck, F.U. Herrmann, G. Abstreiter, C. Hanke and L. Korte, in: Proc. Int. Symp. on GaAs and Related Com- pounds, Jersey, 1990, Inst. Phys. Conf. Ser. No. 112 (Hilger, IOP Publishing, 1990) p. 561.

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[9] R. Ostermeir, K. Brunner, G. Abstreiter and W. Weber, in: Proc. 20th ESSDERC, Nottingham, 1990, in press; also: IEEE, submitted for publication.

[10] E. Anastassakis, A. Pinczuk, E. Burstein, F.H. Pollack and M. Cardona, Solid State Commun. 8 (1970) 133.

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[13] Th. Englert, G. Abstreiter and J. Pontcharra, Solid State Electron. 23 (1980) 31.

[14] K. Kobayaski, Y. Inoue, T. Nishimura, T. Nishioka, H. Arima, M. Hirayama and T. Matsukawa, in: Proc. 19th Conf. on Solid State Devices Miter., Tokyo (1987) p. 323.

[15] K. Brunner, G. Abstreiter, B.O. Kolbesen and H.W. Meul, Appl. Surf. Sci. 39 (1989) 116.

[16] S.A. Lyon, R.J. Nemanich, N.M. Johnson and D.K. Bie- gelsen, Appl. Phys. Lett. 40 (1982) 316.

[17] G. Kolb, Th. Salbert and G. Abstreiter, Appl. Phys. A, in press.

[18] J.B. Hopkins, L.A. Farrow and G.J. Fisanick, Appl. Phys. Lett. 44 (1984) 535.

[19] S. Nakashima and H. Hangyo, IEEE J. Quantum Elec- tron. 25 (1989) 965.