New Platform for Multimodal Spectroscopic Characterization of Semiconductors

Semiconductor devices are some of the most complex engineered devices in today’s world. Their manufacturing often requires an even more complex sequence of steps, first to understand and tailor the material properties so that desired electrical, optical or mechanical properties are possible, followed by very intricate and controlled fabrication processes so that these devices can be made reliably and reproducibly and at a reasonable cost. The above sequence of events demands that the practitioner is able to measure the material and device properties at every step of the design and production process to ensure that yields are optimal and to remove defective material or components as early as possible in the production process as every step add cost and complexity. A vast number of techniques have been developed for these types of material, product and process characterization. Amongst these, optical or spectroscopic techniques are usually preferred. The preference for optical spectroscopy in semiconductor material and process characterization derives from several factors. Optical spectroscopic (OS) techniques are often non-contact, hence reducing the chances of contamination. Secondly,manyOS techniques are comparatively fast and require little to no sample preparation.

Given the large number of material and process parameters that can affect the behavior of the final semiconductor product, it follows that control of the material design and fabrication process is necessarily a multi-modal effort. Meaning that many measurement techniques are required to measure and control all the parameters that lead to desired device behavior. Instrument vendors have accordingly developed a wide variety of specialized optical spectroscopy instruments often specialized on each of the required techniques. In addition, as device features go from micro to nano, many of these instruments are based on a microscope or other submicron to nanometer measurement platforms. For example, it is commonin a semiconductor research or fabrication facility to have one micro-Raman spectroscopy instrument used to characterize crystallinity of epitaxial deposition or stress and a separate Photoluminescence instrument to measure wafer homogeneity, etc. Beyond the cost burden of having multiple instruments to perform these necessary measurements, the task itself, in going from instrument to instrument has become quite challenging in recent time as the features of interest become smaller and approaching the micro to nanoscale.

In this article, we describe a novel approach to achieve multi-modality on one microspectroscopy platform (fig 1), enabling the practitioner to characterize various semiconductor samples using different spectroscopies with the benefit of sample co-location. In this approach, various complementary measurements can be performed at the same micro location and in so doing obtain deeper insights into sample or process. In addition, when there is a necessity to perform micro or nano measurements across different instruments, we present a new coordinate transformation technology (nano-GPS) that enables fast and accurate location of nanostructures across different measurement instruments.

Example 1: LEDs

One of the most important considerations of LEDs is the emission wavelength. For example, most common Lidar sources have semiconductor materials designed to havelight emission at 905nm or 1550nm. The primary material property that controls this parameter is the optical bandgap, which is readily measured using micro photoluminescence (fig 1a). After the material properties are understood, a fabrication process engineer could be interested in the uniformity of the epitaxial deposition on a large wafer, so that a Lidar laser die cut from one part of the wafer can be expected to perform and behave as one from any other part of the wafer. Once again, this property is readily characterized using a photoluminescence (PL) map across the wafer (figure 1b).

If for some reason some of the dies from this wafer do not show optimal luminescence compared to others, the process engineer might be interested in understanding the nature of the defects causing this suboptimal luminescence efficiency and could use time-resolved PL to undertake these studies (figure1c). Alternatively, and as part of a QA process the engineer might want to measurement the performance of the device under conditions similar to actual use by measuring the electroluminescence of the device before it is packaged (figure 1d).

“Speed, cost minimization and reproducibility are persistent drivers for decision making when it comes to instrumentation choices for semiconductor research and fabrication”

Fig. 1a Photoluminescence spectra for various semiconductor materials with different bandgaps. (b) Photoluminescence (PL) map of two inch Indium Phosphide water showing distribution of various parameters – an indication of homogeneity. (c) measurement of time-resolved PL at three points on an LED die. (d) Spatially resolved electroluminescence of an LED

Example 2: Photovoltaics

Development and fabrication of Photovoltaicsshow a similar need for multimodal characterization. For example, at the material stage it is important that the material bandgap be engineered to optimally absorb the solar spectrum. Once again, PL is a good technique for determining that property (figure 1a). The photovoltaic effect relies on the efficient movement of charge carriers either to the electrical load for use or to a battery for energy storage. Time-resolved PL is often used to characterize carrier dynamics (figure 1c) or Raman spectra to determine micro crystallinity (which in turn affects carrier dynamics (2a). Finally, in the QA process of the solar cell device one might be interested in measuring the overall device efficiency by measuring the spectral photocurrent response (figure 2b).

                                                (a)                                            (b)

Fig. 2(a) Raman map and spectra (insert) of a possible contaminant flake on the surface of a semiconductor sample. (b) Photocurrent map of a sheet of Silicon PV material showing hot spots (possible defects). The red strip is a piece of the conducting electrode.

Although contrived, the above measurements are typical occurrences in the design and manufacture of LED and PV devices and it is also common to see that all these measurements are typically made on different instruments. The novelty in this paper is showing that they can be made on one instrument (the SMS system from HORIBA), resulting in cost savings and adding convenience to the process.

Correlative spectroscopy on semiconductor materials

Simple, fast and non-contact microspectroscopy techniques such as the ones described above are usually preferred in semiconductor material characterization, but it is sometimes necessary to use other more complex techniques that are not easy to combine with the above. For example, defect characterization sometime require high spatial resolution (nanometers) that is only available on instruments such as electron (SEM) or AFM microscopes (figure 3a). In those instances, and due to the cost and complexity involved in using a specialized instrument such as an SEM, it is desirable to establish a correlative optical spectroscopy so that that such a defect can be identified in future using a simpler spectroscopic technique rather than doing the measurement on an SEM repeatedly, which can be costly and slow. To achieve this correlation, it becomes necessary to identify a nanoscale feature in the SEM and to also be able to identify the same feature under an optical microspectrometer which can be a laborious and time consuming without some type of coordinate system matching between the two instruments. To facilitate this process automated nanoscale coordinate transformation technologies such as navYX® from HORIBA have been developed to enable fast, accurate and repeatable localization of nanoscale objects between different measurement system (fig 3b)

(a)

Fig. 3(a) High resolution cross-sectional view of semiconductor device measured with an SEM with an overlay of Cathodoluminescent centers and wavelengths on the device. A PL measurement of same device could be made a microspectrometer using the NanoGPS tag (blue) shown in figure (b) to quickly navigate to the sample location of interest

(b)

In conclusion, speed, cost minimization and reproducibility are persistent drivers for decision making when it comes to instrumentation choices for semiconductor research and fabrication. In this paper, we introduce a novel modular and multimodal platform that enables the efficient combination of several complementary spectroscopic techniques relevant for semiconductor characterization on one platform. Furthermore, and for when it is necessary to measure across different platforms, we introduce a new coordinate transformation technology that enables the fast and accurate localization of nanoscale features across different measurement and metrology platforms. For further information about the different spectroscopies available on such a platform, please visit www.microscpectroscopy.com and download the application handbook.