L.H. Huang, Peking U. and Keithley Instruments, Beijing, China; Y.G. Zhao, Keithley Instruments, Cleveland, OH USA; N. Sa, B. Zhao, Keithley Instruments, Beijing, China; J.Q. Yang, J. F. Kang, Peking U., Beijing, China
In MOSFETs used for analog and RF circuits, 1/f noise is an important figure of merit. The test equipment configuration and methodology allows 1/f noise measurements well below 100Hz, which is a limitation of earlier methods. A low pass filter and shielded metal box are used to remove noise artifacts >0.5Hz, which greatly improves the accuracy of the 1/f noise measurements. This paper describes a new method for measuring the 1/f noise in these devices at the wafer-level. Results of using the new methodology are presented, demonstrating that 1/f noise characteristics of nMOS and pMOS devices with different sizes, under different bias conditions, can be evaluated with higher accuracy and at lower frequencies than ever before.
MOSFETs have been widely used in RF and analog circuits. However, low frequency noise in MOSFETs, especially for the higher frequency 1/f noise, is a major concern for the application in analog and RF circuit application. Furthermore, 1/f noise will significantly increase when the device size shrinks . Therefore, it is necessary to set up a reliable, repeatable, and precise measurement method and system to measure 1/f noise.
Several measurement solutions and setups of 1/f noise have been developed over the years. In the first measurement setup, demonstrated in 1990 , SMUs together with two low pass filters supply bias voltage for the drain and gate. The noise in the drain is detected by a preamplifier and a dynamical signal analyzer. The second setup was similar to the first one but using battery-biased. In the third solution , several key elements such as a low noise amplifier (LNA), a Cascade probe, and a filter were adopted. However, the developed setups were only applied to measure noise at frequencies >100Hz.
A few theories have been proposed to state the origins of 1/f noise, such as the carrier number fluctuation theory proposed by Whorter , the mobility fluctuation theory based on Hooge’s empirical results , and the unified noise model . In 2003, Wong published a review paper on the development and recent progress of 1/f noise .
Although Hooge’s results sometimes incorporated the module, Whorter’s theory is commonly applied to simulate the 1/f noise of MOSFETs. For example, the noise module in HSPICE is based on Whorter’s theory. Table 1 lists the noise module in HSPICE.
Table 1: HSPICE expressions of 1/f noise in MOSFET.
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Generally, the drain current noise spectrum can be described using the following equation:
From the equation, a log linear equation at constant frequency can be deduced as follows:
The goal of the new measurement method and setup is to extract the parameters of AF and KF. These two parameters can be extracted from the power spectrum scaled by frequency.
The noise measurement setup was constructed of the Keithley series of instruments including the Semiconductor Characterization System KI4200-SCS, programmed low current amplifier KI428-PROG, and a low pass filter, as well as Keithley’s ACS (Automation Characterization Suite) software. Special caution is needed to minimize the pickup noise when the setup was constructed.
The schematic diagram of the setup is shown in Figure 1, where the dotted lines indicate the ACS control flow and the solid lines indicate the data flow.
Figure 1: The schematic diagram of the new developed measurement setup.
The KI Semiconductor Characterization System with the installed software and the embed dual channel scope can apply the input voltage, conduct the measurement of current-voltage, operate the noise signal measurement, control the current amplifier, and analyze the test results.
A KI 4200 SMU and a 0.5Hz filter are used to apply the input bias. As the low pass filter removes any noise above 0.5Hz, the 1/f noise measurement accuracy is greatly improved. The filter is shielded with a metal box so that the input bias is less likely to pick up noise.
A probe station is used to measure the 1/f noise at the wafer level. The probe station, DUT (device under test) and filter are provided with an electrically shielded metal box to eliminate and reduce interference from external noise.
The programmed low current amplifier plays an important part in 1/f noise measurement. It is battery powered and used to amplify the current noise from the DUT; it can also supply the bias voltage to the output of the DUT. The output terminal of the DUT is directly connected to the input port of the amplifier. The amplifier can supply an output voltage ranging from -5V to 5V with 2.5mV resolution. Therefore, the DUT can be biased at the desired voltage and prevented from noise of AC lines. The gain of the amplifier can be adjusted from 103 to 1011. Since the amplifier is equipped with GPIB ports, it can be programmed by the ACS over the IEEE-488 bus. The amplifier combined with the different bias voltage, makes the device work in different regions.
The scope is connected to the output of the current amplifier and is a dual-channel digital storage oscilloscope with an embedded digital signal processor. Under software control, it can monitor, capture, and analyze the output signals.
The ACS software platform supports semiconductor characterization at the cassette, wafer, and device levels using multiple test instruments, and automatic parametric test with semi-automatic and automatic probe stations. It controls hardware on the Semiconductor Characterization System or on external instrumentation through a general-purpose interface bus (GPIB) interface. As the amplifier has a GPIB control, it is used to automate the noise measurement system.
All test procedures, including the Semiconductor Characterization System to supply the bias voltage and measure noise, control amplifier to amplify the noise current, and the scope to get the drain noise current, are coded as a unified test module of ACS. Under the ACS test environment, this module can be executed and copied. After setting a series of test modules under different conditions, ACS provides several different test modules. Using modules belonging to the same device, they can be tested at the device level.
Validation and discussion
To verify the setup, the 1/f noise characteristics of nMOS and pMOS devices with different sizes under different bias conditions are evaluated and compared with the simulation. Figure 2 shows the results of drain current noise measurements for a p-type MOSFET. Figure 2a shows the noise current signal captured by a KI 4200-SCP2 over 20 average number measurement circles, which averages the calculated power spectra to obtain a smooth spectrum under the control of the ACS software. Figure 2b is obtained by performing a fast Fourier transform on these measured data and shows that the drain current noise spectrum clearly has the 1/f dependence on frequency.
Figure 2: The measured drain current noise of a pMOS.
As discussed above, the goal of measurement is to extract the parameters AF and KF. To extract the AF and KF the current noise under different bias condition was measured. Figure 3 shows the measured results of a pMOS applied different bias.
Figure 3: The measured noise data under different gate bias.
To inspect the oxide capacitance dependence or other further research, 1/f noise of different oxide thicknesses is also measured; Figure 4 shows the test results. The 1/f noise parameters can then be evaluated and different module simulations can be generated. Figure 5 shows the drain current noise power measured in a strong inversion region for a p-channel MOSFET.
Figure 4: The measured data of 1/f noise of pMOS devices with different oxide thickness.
Figure 5: Gate bias dependence of drain current 1/f noise.
In this paper, a wafer level measurement method and setup are developed to evaluate the 1/f noise of MOSFETs. The measurement can be automatically performed on the wafer. Because the setup can measure the low frequency noise components lower than 100Hz, the setup can effectively extract the 1/f noise in MOSFETs.
Jinfeng Kang received his BS degree in physics from Dalian U. of Technology, Masters and PhD degrees in microelectronics from Peking U., and is a professor at the Institute of Microelectronics, Peking U., Beijing, 100871 China; email [email protected].
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Lihua Huang received her Bachelor’s degree in electronic science and technology from Xi’an Electronic and Technology U. in 2004, and Master’s degree in microelectronics from Peking U. in 2008. She is an application development engineer at Keithley Instruments, Inc. Beijing, Rm. A1301, Chengjian Plaza, Beitaipingzhuanglu, Haidian, Beijing, 100088 China.
Yuegang Zhao received his MBA from Case Western Reserve U., an MS in semiconductor physics from the U. of Wisconsin, Madison, and his BS in physics from Peking U., Beijing, China. He is a marketing director at Keithley Instruments, Inc., Cleveland, OH USA.
Ning Sa received a Bachelor’s degree in microelectronics from Sichuan U. in 2003, Master’s degree in microelectronics from Peking U. in 2006, and has been working as an application development engineer at Keithley Instruments, Inc., Beijing, China.
Bin Zhao received his Bachelor’s degree in Electronics Engineering from Nanjing University of Aeronautics and Astronautics and is working as a project manager at Keithley Instruments, Inc., Beijing, China.
Jiaqi Yang received her Bachelor’s degree in electronic science and technology from Dalian U. of Technology, Dalian, China and is working toward a Ph.D degree at the Institute of Microelectronics, Peking University, Beijing, China.