Nano Scale Materials and Device Characterization via Scanning Microwave Microscope Hassan Tanbakuchi, Matt Richter, Ferry Kienberger, Han-Peter Huber All of Agilent Technologies
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Abstract-A new Scanning Microwave Microscope (SMM) that uses half a wavelength resonator in conjunction with a diplexer connected to a Vector Network Analyzer (VNA), which can perform a very sensitive capacitance measurement at the tip of a conductive Atomic Force Microscope is the subject of this talk. This is achieved through transformation of the high impedance (very small capacitance between tip and sample to the ground) to 50 Ohm (the measurement system’s characteristic impedance) through a half a wavelength resonator and diplexer. The VNA measurement architecture as exists today has the capability of measuring impedances close to the characteristic impedance of the analyzer (i.e. 50 Ohms) to a good precision and up to 100 GHz stimulus frequency .The measurement precision and resolution of the DUT (Device under Test) impedance markedly drops as it deviates from 50 ohms by a couple of order of magnitudes. This is exactly the case of the impedance between the AFM tip and sample to the ground. We are proposing a solution which remedies the lack of precision and resolution for large and small impedances (small capacitance) when measured by existing VNA.
of both the surface and material properties such as the profiling the of dopant of silicon substrates. II. Measurement tools in the nano technology space Nanotechnology is divided into three distinct interrelated domains figure 1. They are Nanostructred Materials, Nanodevices and Nanotools. It is evident that one of the reasons behind the slow technological advancement of nano materials and devices has been the lack of advance and comprehensive nano measurement tools (Nanotools), specifically nano measurement equipment. Agilent due to its diverse technological (calibrated electronic measurement and Life Science) capabilities, is in a unique position to develop advanced easy to use Nano measurement tools to help accelerate the advancement of the Nano materials and devices.
I. BACKGROUND INFORMATION Emerging nanotechnology and biotechnology are in direct need of metrology tools with better than 10 –nm spatial resolution that are capable of imaging surfaces and nano-embedded structures. Scanning local probe microscopy (SLPM) techniques have advanced our knowledge of surface and materials at atomic scales. These tools include the scanning tunneling microscope (STM), atomic force microscope (AFM), scanning capacitance microscope (SCM), magnetic force microscope (MFM), scanning thermal microscope (SThM), and scanning near field optical microscope (NSOM). In the Scanning Capacitance Microscopy (SCM) space, commercially available SCM probes operate far below 1 GHz, which reduces the sensitivity of the existing SCM. This paper describes a new approach for SCM. We have selected the measurement operating frequency between 1 t0 16 GHz in order to increase the sensitivity of measurement system. Also the SMM works as both nanoscale impedance analyzer and AFM, enabling the characterization
Figure 1 III. HIGH IMPEDANCE MEASUREMENT CHALLENGE The electric field at and the vicinity of the conductive AFM tip can be described by a Qausi –Static electric field. Thus tip will produce a capacitive load (10 fF capacitive impedance) that is connected through a low loss transmission line a Vector Network Analyzer (VNA). Also
classical nano device impedance can be express as multiples of the quantum resistance given by R0-=h/2e^2=12.96 K Ohms. The modern VNA is based on 50 Ohms characteristic impedance, thus the measurement accuracy and resolution suffers markedly, when load impedances are substantially greater or smaller than 50 Ohms. Figure2 shows a simplified VNA block diagram as a reflectometer. It is evident by close examination of the graph showing S11 versus resistive impedances, that the sensitivity of the changes in refection coefficient (S11) drops markedly as the load impedance deviates from characteristics impedance (50 Ohms) of the measurement system. Very small capacitor High SNR Low Resolution
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design was the speed and ease of conductive tip replacement. We developed a new Nosecone assembly that not only allows a person to replace a conductive SMM tip with speed and ease, but also gives a repeatable and low loss Microwave connection to the tip. Figure 3 shows the mechanical detail of the SMM nose assembly and the placement of the tip.
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Highly resistive load High SNR Low Resolution
Load close to 50 Ohms Low SNR High Resolution
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Figure 1: reflection coefficient vs.. impedance
Low resistive load High SNR Low Resolution
Figure2 IV. SOLUTIONS TO THE MEASUREMENT CHALLENGE: A novel impedance analyzer, MW interferometer or a 50 Ohm shunt at the end of half a wavelength solve the said measurement problem with varying degrees. The 50 Ohm shunt will be discussed here only. Figure 3 shows the half a wavelength transmission line in conjunction with the 50 Ohm load solution.
IV. DOPANT PROFILE MEASUREMENT MODULE (DPMM). The SMM design philosophy was base on the premise to employ the accuracy and flexibility of the VNA measurement instrument. In order to achieve dopant profile measurement with accuracy and speed we have designed a Dopant Profile Measurement Module (DPMM) that can be attached to Agilent VNA instruments. Figure 5 Shows the DPMM internal layout and DPMM VNA analyzer setup, DPMM
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Figure 3 V. DELIVERING MICROWAVE SIGNAL TO CONDUCTIVE AFM TIP. Microwave connection at nanoscale presents a great deal of mechanical challenges and opportunities. We have solved the difficulties by developing new machining and assembly processes. A critical parameter that droved the
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DPMM as an add measurement module to
Figure 5 Figure 6 shows the simplified block diagram of DPMM and VNA as a dopant profile measurement system. DPMM transfers the nonlinear characteristics of tip/ sample interaction from the low RF frequencies to MW frequencies. The majority of the samples/ specimens (Polymers, semiconductors, biological samples) exhibit low conductivity (high capacitive impedance), and if they exhibit nonlinear
behavior when subject to low frequency RF signal, causing the change in the real and imaginary part of the capacitive tip/ sample impedance. The change of the said sample impedance (capacitive impedance) is measured by SMM (Scanning Microwave Microscope). The said change is magnified by the difference between the PNA incident MW and RF stimulus frequencies. In other word if the change of the capacitance driven by the low frequency RF is ΔC the change in the impedance is 1/ (2πjf ΔC) where f is the measurement frequency. It is clear from the equation there is significant gain in sensitivity when the changes in capacitance is measured with a MW stimulus verses the RF. Therefore it is clears from the equation that the sensitivity of the measurement (changes in the impedance) is multiplied by the stimulus frequency. Therefore measuring the changes of the sample impedance induced by low frequency RF signal at MW frequencies increases the sensitivity markedly. The block diagram of the DPMM is shown in Figure 6. For clarity the relevant components of the PNA are shown here. The blue shaded area represents the DPMM and the green shaded area represents relevant components from the VNA. The MW signal from PNA enters the DPMM from the Source Out of the PNA. This signal is divided into two portions inside the DPMM. The first potion is amplified and used as the local oscillator signal (LO) for the DPMM mixer. The second portion is amplified and sent through the main arm of the coupler to the AFM probe tip. A second RF signal is also applied to the tip from an external source (the MACIII box on the Agilent Picoscope AFM). Therefore the tip /sample interface has two signals: the first is a low frequency RF signal (i.e.
