From: Dennis Doering Date: Wed, 18 Dec 2013 20:43:40 +0000 (+0100) Subject: Iworid final submission X-Git-Url: https://jspc29.x-matter.uni-frankfurt.de/git/?a=commitdiff_plain;h=3d21e145bf1b9c8bbd6682dc63b6dc6440575993;p=radhard.git Iworid final submission --- diff --git a/IWORID2013.zip b/IWORID2013.zip new file mode 100644 index 0000000..ae9e1b9 Binary files /dev/null and b/IWORID2013.zip differ diff --git a/IWORID2013/10MradNoiseGain.pdf b/IWORID2013/10MradNoiseGain.pdf index 12b2f25..521cd46 100644 Binary files a/IWORID2013/10MradNoiseGain.pdf and b/IWORID2013/10MradNoiseGain.pdf differ diff --git a/IWORID2013/10MradStoN.pdf b/IWORID2013/10MradStoN.pdf index cc7e9b0..d6e2781 100644 Binary files a/IWORID2013/10MradStoN.pdf and b/IWORID2013/10MradStoN.pdf differ diff --git a/IWORID2013/DiodeSurface.pdf b/IWORID2013/DiodeSurface.pdf index bdb7028..392bdb3 100644 Binary files a/IWORID2013/DiodeSurface.pdf and b/IWORID2013/DiodeSurface.pdf differ diff --git a/IWORID2013/IWORID.pdf b/IWORID2013/IWORID.pdf index 184844e..1b18374 100644 Binary files a/IWORID2013/IWORID.pdf and b/IWORID2013/IWORID.pdf differ diff --git a/IWORID2013/IWORID.tex b/IWORID2013/IWORID.tex index a6fd719..be84e59 100644 --- a/IWORID2013/IWORID.tex +++ b/IWORID2013/IWORID.tex @@ -12,7 +12,7 @@ \pdfminorversion=6 \title{Noise performance and ionizing radiation tolerance of CMOS Monolithic Active Pixel Sensors using the $0.18\mum$ CMOS process} -\author{Dennis Doering$^a$\thanks{doering@physik.uni-frankfurt.de; Phone: +49 69 798-47118}, Jerome Baudot$^b$, Michael Deveaux$^a$, Benjamin Linnik$^a$, Mathieu Goffe$^b$, Serhiy Senyukov$^b$, Stefan Strohauer$^a$, Joachim Stroth$^a$ and Marc Winter$^b$\\ +\author{Dennis Doering$^a$\thanks{doering@physik.uni-frankfurt.de; Phone: +49 69 798-47118}, Jerome Baudot$^b$, Michael Deveaux$^a$, Benjamin Linnik$^a$, Mathieu Goffe$^b$, Serhiy Senyukov$^b$, Stefan Strohauer$^a$, Joachim Stroth$^a$ and Marc Winter$^b$ for the CBM-MVD collaboration\\ \llap{$^a$} Institut für Kernphysik, Goethe University Frankfurt, Germany\\ \llap{$^b$}IPHC Strasbourg, France\\ E-mail: \email{doering@physik.uni-frankfurt.de}} @@ -30,7 +30,7 @@ For the vertex detectors of CBM and ALICE, we are aiming at developping large sc CMOS Monolithic Active Pixel Sensors (MAPS) found numerous applications in the field of heavy ion physics and particle physics. They are being installed in the STAR Heavy Flavor Tracker \cite{RHIC} and will be used in the Micro Vertex Detector of the future Compressed Baryonic Matter (CBM) experiment \cite{Vertex08}. Moreover, their use is considered for the vertex detectors of the International Linear Collider (ILC) \cite{ILC} and the upgrade of the ALICE-ITS \cite{Musa}. The expected integrated radiation doses in these applications range from several $10^{10}\neqcm$ and few $100~\rm krad$ (ILC) to \mbox{$\gtrsim 10^{13} \neqcm $} and $\gtrsim 1\Mrad$ (CBM). -MAPS integrate their sensitive volume and the pixel readout electronic on one chip, which is produced with commercially available CMOS processes. As shown in figure \ref{fig:mapssensor}, the sensitive epitaxial layer is surrounded by two layers made from P++ doped silicon. N doped implantations in one P++ doped layer form a P$_{\rm Epi~ Layer}$/N$_{\rm Well}$-diode. Signal electrons, generated by impinging particles, travel in the epitaxial layer and are reflected back at its interfaces to the surrounding layers until they are collected by a diode. The details of the charge collection process depend significantly on the doping of the epitaxial layer. In case this volume is made from the moderately doped ($\sim 10~\rm \Omega \cdot cm$) silicon as found in standard CMOS processes, the charge collection is dominated by thermal diffusion. Significant improvements can be reached by exploiting the lower doping ($\sim 1~\rm k\Omega \cdot cm$) to CMOS-processes dedicated to commercial optical imaging devices. In this case, the sensitive volume is partially depleted, which accelerates the charge collection and hence improves substantially the tolerance of MAPS to non-ionizing radiation damage. After this improvement, the tolerance of MAPS to bulk damage was extended to $\gtrsim 10^{14}~\rm n_{eq}/cm^2$ \cite{RESMDD2012,Mi25}. Consequently, their tolerance to non-ionizing radiation dose currently outshine the ionizing radiation tolerance. Therefore, the life-time of MAPS in vertex detectors is limited by the effects of surface damage.\newline +MAPS integrate their sensitive volume and the pixel readout electronic on one chip, which is produced with commercially available CMOS processes. As shown in figure \ref{fig:mapssensor}, the sensitive epitaxial layer is surrounded by two layers made from P++ doped silicon. N doped implantations penetrating one of the P++ doped layers form a P$_{\rm Epi~ Layer}$/N$_{\rm Well}$-diode. Signal electrons, generated by impinging particles, travel in the epitaxial layer and are reflected back at its interfaces to the surrounding layers until they are collected by a diode. The details of the charge collection process depend significantly on the doping of the epitaxial layer. In case this volume is made from the moderately doped ($\sim 10~\rm \Omega \cdot cm$) silicon as found in standard CMOS processes, the charge collection is dominated by thermal diffusion. Significant improvements can be reached by exploiting the lower doping ($\sim 1~\rm k\Omega \cdot cm$) of CMOS-processes dedicated to commercial optical imaging devices. In this case, the sensitive volume is partially depleted, which accelerates the charge collection and hence improves substantially the tolerance of MAPS to non-ionizing radiation damage. After this improvement, the tolerance of MAPS to bulk damage was extended to $\gtrsim 10^{14}~\rm n_{eq}/cm^2$ \cite{RESMDD2012,Mi25}. Consequently, their tolerance to non-ionizing radiation dose currently outshine the ionizing radiation tolerance. Therefore, the life-time of MAPS in vertex detectors is limited by the effects of surface damage.\newline \begin{figure} \begin{minipage}{8cm} @@ -45,23 +45,25 @@ MAPS integrate their sensitive volume and the pixel readout electronic on one ch \end{figure} Since recently, a dedicated imaging process with $0.18~\rm \upmu m$ feature size became available for particle detectors. This CMOS-process provides the high-resistivity epitaxial layer discussed above. Moreover, it features deep P- and N-wells, which allows conceptually for using full CMOS also in the pixel area\footnote{Without this feature, the N-well implantation required for building PMOS-transistors would act as parasitic collection diode and therefore destroy the sensing abilities of the pixel.}, which is helpful for improving the readout speed of the device. Finally, CMOS-processes with a smaller feature size are known for providing an improved tolerance to ionizing radiation. In order to exploit those features, we aim at migrating our successful sensor designs to this new CMOS-process. +In the following, we will present some first devices manufactured in the novel process. Hereafter, we will show our findings concerning the noise of those devices and discuss the results of different strategies to reduce the noise. Finally, we will present first results on the tolerance of the devices to ionizing radiation. + \section{Sensor design} -Some exploratory devices were designed to study the properties of sensing nodes integrated in the novel CMOS process. The chips were named MIMOSA-32, MIMOSA-32ter and MIMOSA-34. Each of those chips host matrices with 32 different pixels types, which vary in terms of pixel pitch, as well as in the sensing node and preamplifier layouts. The 1024 pixels of each pixel matrix are arranged in 16 columns with 64 pixels per column. The columns are readout in parallel and their signal is sent to 16 external ADCs\footnote{Only 8 out of 16 columns were read out due to the limited number of ADCs available in the used readout system. This restriction remains without impact on the conclusions of our study.}. The readout time is $32~ \rm \upmu s$, which represents the design goal of the future sensors for CBM and ALICE. +Some exploratory devices were designed to study the properties of sensing nodes integrated in the novel CMOS process. The chips were named MIMOSA-32, MIMOSA-32ter and MIMOSA-34. Each of those chips host matrices with 32 different pixels types, which vary in terms of pixel pitch, as well as in the sensing node and preamplifier layouts. The 1024 pixels of each pixel matrix are arranged in 16 columns with 64 pixels per column. The columns are readout in parallel and their signal is sent to 16 external ADCs\footnote{Only 8 out of 16 columns were read out due to limitations of our readout system. This restriction has no impact on the conclusions of this study.}. The readout time is $32~ \rm \upmu s$, which represents the design goal of the future sensors for CBM and ALICE. In the pixels discussed in the following, the charge collected by the P$_{\rm Epi~ Layer}$/N$_{\rm Well}$-diode is stored in the parasitic capacitance C of the sensing node and hereafter buffered by means of a source follower (see figure \ref{fig:preamplifier}). The signal charge as well as the accumulated charge generated by the leakage current of the collection diode is cleared by means of a continuous bias, which is realized with a permanently opened reset switch based on a NMOS transistor (labeled reset-transistor in figure \ref{fig:preamplifier}). In this particular mode of operation, the switch should act as a high resistivity forward biased diode and the pixel should be equivalent to the self-bias pixels discussed in \cite{Deveaux2010428}. The charge-to-voltage amplification gain of CMOS-pixels depends predominantly on the size of the parasitic capacitance of the sensing node. Contributors to this parasitic capacitance are in particular the P$_{\rm Epi~ Layer}$/N$_{\rm Well}$-junction, the drain of the reset transistor and the gate of the source follower transistor. The smaller feature size of the $0.18~\rm \upmu m$-process allows for reducing the size and such the capacitance of those structures, which turns into a sizable potential for improving the amplification gain of the pixel. However, reducing the diode size may come with drawbacks in terms of charge collection efficiency and reducing the size of the transistor gates was reported to cause significant 1/f- and RTS-noise in MAPS used for optical imaging \cite{RTS0.18,RTS0.182}. \begin{figure} -\begin{minipage}[t]{8cm} -\includegraphics[width=8cm]{schaltplanpreampf3T.pdf} +\begin{minipage}[t]{0.49\textwidth} +\includegraphics[width=\textwidth]{schaltplanpreampf3T.pdf} \caption{A 3T preamplifier with continuous bias} \label{fig:preamplifier} \end{minipage} -\hspace{0.2cm} -\begin{minipage}[t]{8cm} -\includegraphics[width=8cm]{Mi18Mi32Noisevergleich.pdf} -\caption{Noise distribution. Mind the different choosen bin size.} +\hspace{0.02\textwidth} +\begin{minipage}[t]{0.49\textwidth} +\includegraphics[width=\textwidth]{Mi18Mi32Noisevergleich.pdf} +\caption{Noise distribution of Pixel A and R. Mind the different bin sizes.} \label{fig:Mi18Mi32Mi34Noisevergleich} \end{minipage} \end{figure} @@ -89,53 +91,55 @@ Difference A$\rightarrow$C & & & & +15\% \label{tab:Mi32-1-f-noise-table} \end{table} -According to our measurement standard, we defined the noise as the standard deviation of the dark signal of the individual pixel after performing correlated double sampling, pedestal correction and common mode correction. Details on the related measurement procedure were discussed in \cite{Dev07}. The noise of a pixel matrix is defined as the median of the noise of all individual pixels of this matrix and was measured at a temperature of $T=+20\rm^{\circ}C$. Typically the number of $\lesssim 1\%$ "noisy" pixels could be tolerated. Therefore, in the following, we evaluate not only the median value of the pixel noise distribution but also the noise limit so that $\gtrsim99\%$ of the pixels have a lower noise. We translate this value to the signal to noise to receive a lower limit also for the signal to noise. $\gtrsim99\%$ of the pixels have a better signal to noise ratio, which should exceed 15 for a reliable MIP-detection. \newline +According to our measurement standard, we defined the noise as the standard deviation of the dark signal of the individual pixel after performing correlated double sampling, pedestal correction and common mode correction. Details on the related measurement procedure were discussed \mbox{in \cite{Dev07}.} The noise of a pixel matrix is defined as the median of the noise of all individual pixels of this matrix and was measured at a temperature of $T=+20\rm^{\circ}C$. Typically, masking $\lesssim 1\%$ "noisy" pixels can be tolerated. Therefore, besides the median of the pixel noise distribution, we evaluated a noise limit defined by the requirement that $99\%$ of the pixels should have a lower noise. An analog concept is applied for the signal to noise: We consider a sensor as good, if the S/N of $99\%$ of all pixels exceeds the value of 15, which is known to allow for save MIP-detection. +%$\gtrsim99\%$ of the pixels have a better signal to noise ratio, which should exceed 15 for a reliable MIP-detection. \newline +%Therefore, in the following, we evaluate not only the median value of the pixel noise distribution but also the noise limit so that $\gtrsim99\%$ of the pixels have a lower noise. We translate this value to the signal to noise to receive a lower limit also for the signal to noise. $\gtrsim99\%$ of the pixels have a better signal to noise ratio, which should exceed 15 for a reliable MIP-detection. \newline \subsection{Impact of the transistor layout on the noise} -Figure \ref{fig:Mi18Mi32Mi34Noisevergleich} compares the performances of the \mbox{Pixel A} ($0.18\rm ~\upmu m$, biggest source follower transistor gate) with the one of the established \mbox{Pixel R} \mbox{($0.35\rm ~\upmu m$ feature} size). \mbox{Pixel R} shows a small distribution with a median noise of $10.7 \rm \e$, and $\gtrsim 99\%$ of all pixels indicate a noise below $18 \rm \e$. The noise of \mbox{Pixel A} follows a broad distribution with a median of $19.8\rm \e$ and $\gtrsim 99\%$ of all pixels remain below a noise of $41\rm \e$. Based on the results for Pixel A-C, one may state that reducing the surface of the gate of the source follower transistor increases slightly the gain of the pixel. However, the median noise does also slightly increase, mostly because more pixels with very high individual noise are observed in the presence of a small gate: Once the width of the gate is reduced from $1.5 \mum$ to $0.5 \mum$, the ``99\%-noise'' raises from 41 to 63 electrons. - -This unintuitive finding can be understood by studying the detailed properties of noise pixels and comparing the results with the observations reported in \cite{RTS0.18,RTS0.182}. As shown in \mbox{figure \ref{fig:Mi32-1-f-noise-CDS-Signal}} and in \mbox{figure \ref{fig:Mi32-1-f-noise-Distribution}}, one observes that the dark signal after CDS of a representative noisy pixel varies between three well defined levels. This signature is the origin of the high noise. The observation is compatible with the presence a Random Telegraph Signal in the source follower transistor. Random Telegraph Signal is most likely caused by individual defects in the silicon, which may absorb or emit an individual electron. The field of this electron adds to the field applied to the gate of the FET. Therefore, the current passing the FET is modulated to two well separated levels depending on the charge state of the defect. As we apply correlated double sampling, which means subtracting the values of two consecutive frames, we observe three levels representing a stable state, the absorption, and the emission of an electron in the defect during the integration time. +Figure \ref{fig:Mi18Mi32Mi34Noisevergleich} compares the performances of the \mbox{Pixel A} ($0.18\rm ~\upmu m$, biggest source follower transistor gate) with the one of the established \mbox{Pixel R} \mbox{($0.35\rm ~\upmu m$ feature} size). \mbox{Pixel R} shows a narrow distribution with a median noise of $10.7 \rm \e$, and $\gtrsim 99\%$ of all pixels indicate a noise below $18 \rm \e$. The noise of \mbox{Pixel A} follows a broad distribution with a median of $19.8\rm \e$ and $\gtrsim 99\%$ of all pixels remain below a noise of $41\rm \e$. Based on the results for Pixel A-C, one may state that reducing the surface of the gate of the source follower transistor increases slightly the gain of the pixel. However, the median noise does also slightly increase, mostly because more pixels with very high individual noise are observed in the presence of a small gate: Once the width of the gate is reduced from $1.5 \mum$ to $0.5 \mum$, the ``99\%-noise'' raises from 41 to 63 electrons. +This unintuitive finding can be understood by studying the detailed properties of noise pixels and comparing the results with the observations reported in \cite{RTS0.18,RTS0.182}. In \mbox{figure \ref{fig:Mi32-1-f-noise-CDS-Signal}}, we display the distribution of the dark signal (after correlated double sampling, CDS) of a selected pixel C with low noise. In \mbox{figure \ref{fig:Mi32-1-f-noise-Distribution}}, the dark signal of the neighboring, noisy pixel is shown. While the first pixel shows the expected, mostly gaussian noise distribution, the dark signal of the latter pixel varies between three well defined levels. The observation is compatible with the presence a Random Telegraph Signal (RTS) in the source follower transistor of this pixel. According to standard literature, RTS is caused by individual defects in the silicon, which may absorb or emit an individual electron. The field of this electron adds to the field applied to the gate of the FET. Therefore, the current passing the FET is modulated to two well separated levels depending on the charge state of the defect. As we apply CDS, which means subtracting the values of two consecutive frames, we observe three levels representing a stable state, the absorption, and the emission of an electron in the defect during the integration time. \begin{figure} \begin{minipage}[t]{0.49 \textwidth} \includegraphics[width=\textwidth]{Mi32-1-f-noise-CDS-Signal.pdf} -\caption{CDS-Signal of a selected "noisy" pixel} +\caption{Dark signal of a regular pixel. See text.} \label{fig:Mi32-1-f-noise-CDS-Signal} \end{minipage} \hspace{0.02 \textwidth} \begin{minipage}[t]{0.49 \textwidth} \includegraphics[width=\textwidth]{Mi32-1-f-noise-Distribution.pdf} -\caption{Distribution of the CDS-Signal of the selected "noisy" pixel} +\caption{Dark signal of a noisy pixel. See text.} \label{fig:Mi32-1-f-noise-Distribution} \end{minipage} -\newline -\hspace{0.02 \textwidth} + +%\hspace{0.02 \textwidth} \begin{minipage}[t]{0.49 \textwidth} -\includegraphics[width=\textwidth]{Diodesurface.pdf} -\caption{CCE, noise and gain of Pixel D as function of the diode surface. The error bars indicate the ``99\%-noise''.} +\includegraphics[width=\textwidth]{DiodeSurface.pdf} +\caption{CCE, noise and gain of Pixel D as function of the diode surface. The noise of 99\% of the pixels remain below the line indicating ``Noise (99\% pixel)''. See text.} \label{fig:Diodesurface} \end{minipage} \hspace {0.02 \textwidth} \begin{minipage}[t]{0.49 \textwidth} \includegraphics[width=\textwidth]{StoNDiodeSize.pdf} -\caption{Most probable signal, median noise and S/N as function of diode size. The error bars indicate the impact of the ``99\%-noise'' on the S/N.} +\caption{Most probable signal, median noise and S/N as function of diode size. The S/N of 99\% of all pixels remains above the line indicating ``Signal to Noise (99\% pixel)''. See text.} \label{fig:StoNDiodeSize} \end{minipage} \end{figure} -This RTS dominates the usual pixel noise, which determines the width of the individual peaks. Increasing the transistor gate surface seems to reduce the relative impact of the RTS and is therefore found to be beneficial. This holds also for the gate of the reset transistor, which was enlarged in \mbox{Pixel D}. After this modification, the median noise was reduced from \mbox{$19.8~\rm e$} \mbox{(Pixel A)} to \mbox{$16.2~\rm e$} \mbox{(Pixel D)}. -Note that, while enlarging the transistor size reduces the RTS, cooling seems not to show a positive impact. This is in contrast to our observations on RTS-noise originating from the pixel \mbox{diodes \cite{RTS}}. +Obviously, the RTS dominates the other sources of pixel noise, which determine the width of the individual peaks. + +Increasing the surface of the transistor gate seems to reduce the relative impact of the RTS and is therefore found to be beneficial. This holds also for the gate of the reset transistor, which was enlarged in \mbox{Pixel D}. After this modification, the median noise was reduced from \mbox{$19.8~\rm e$} \mbox{(Pixel A)} to \mbox{$16.2~\rm e$} \mbox{(Pixel D)}. Note that, while enlarging the transistor size reduces the RTS, cooling seems not to show a positive impact. This is in contrast to our observations on RTS-noise originating from the pixel \mbox{diodes \cite{RTS}}. \subsection{Impact of the transistor layout on the sensor performance} -The relation between the charge collection efficiency (CCE), the gain and the median noise of the pixels was measured with MIMOSA-34. All pixels were derived from pixel D and the diode size was varied. Figure \ref{fig:Diodesurface} shows the noise and the gain of the different pixels. Moreover, the CCE representing the most probable fraction of charge collected by the seed pixel of a pixel cluster is shown. For the CCE, only the matrices with the same pitch of $22\mum\times 33\mum$ are comparable. The CCE was measured by means of a $^{55}$Fe-source. +The relation between the charge collection efficiency (CCE), the gain and the median noise of the pixels was measured with MIMOSA-34. All were derived from pixel D and the diode size was varied. Their noise and gain is displayed in Figure \ref{fig:Diodesurface}, which indicates also the CCE (for the seed pixel of a cluster as measured with a $^{55}$Fe-source). The CCE shown is restricted to the subset of pixels with $22\mum\times 33\mum$ pitch. This choice was done to obtain comparable results. It reflects the fact that the gain and the noise of non-irradiated pixels depends in first order on the properties of the diodes only, while the CCE is strongly pitch dependent. -One observes that the gain is strongly reduced with increasing diode size and the capacitive noise of the pixels raises accordingly. However, this effect comes with an increase in terms of charge collection efficiency, which raises the signal. This is shown in figure \ref{fig:StoNDiodeSize}, which shows the noise, the most probable signal and the S/N as recorded once the sensor was illuminated by $\upbeta$-rays from a $^{90}$Sr-source. One observes that the S/N, which is defined as the most probable signal in the seed pixel divided by the median of the noise distribution, is in the order of 50. Once propagating the ``99\%-noise'' to the S/N, one finds that 99\% of all pixels exceeds 22 and no significant impact of the diode pitch is observed. Note that this S/N is sufficient for reliable MIP-detection and remains fairly above the average S/N of the early successful prototypes like MIMOSA-2. +One observes that the gain is strongly reduced with increasing diode size and the capacitive noise of the pixels raises accordingly. However, this effect comes with an increase in terms of charge collection efficiency, which raises the signal. This is shown in figure \ref{fig:StoNDiodeSize}, which shows the noise, the most probable signal and the S/N as recorded once the sensor was illuminated by $\upbeta$-rays from a $^{90}$Sr-source. One observes that the S/N (MPV), which is defined as the most probable signal in the seed pixel divided by the median of the noise distribution, is about 50. Once propagating the ``99\%-noise'' to the S/N, one finds that 99\% of all pixels exceeds 22 and no significant impact of the diode surface is observed. Note that this S/N is sufficient for reliable MIP-detection and remains fairly above the average S/N of the early successful prototypes like MIMOSA-2. \section{Radiation tolerance} -In the next step, we studied the performances of irradiated sensors. In a first exploratory study, we irradiated some MIMOSA-32 sensors at CERN with X-rays of a dose of up to $10\Mrad$ and tested them hereafter. The dosimetry was carried out by the staff of the facility and is considered to have a $10\%$ absolute precision and the sensors were powered during irradiation. The sensors were operated at $T=+20\C$. The tests were carried out with a number of different pixel designs showing similar behavior. The preliminary results of one pixel type are shown in the following. +In the next step, we studied the performances of irradiated sensors. In a first exploratory study, we irradiated some MIMOSA-32 sensors at CERN with X-rays of a dose of up to $10\Mrad$ and tested them hereafter. The dosimetry is considered to have a $10\%$ absolute precision and the sensors were powered during irradiation. The sensors were operated at $T=+20\C$. The tests were carried out with a number of different pixel designs showing similar behavior. The preliminary results of one pixel type are discussed in the following. -After an irradiation dose of up to $3\Mrad$, we do not observe significant changes in the pixel performance. After a dose of $10\Mrad$, the gain of the sensor dropped by a factor of two (figure \ref{fig:10MradNoiseGain}). The origin of this effect is under investigation. Despite the drop, the sensor remained operational and the ENC increased only moderately from $\sim 20\rm \e$ to $\sim 25\rm \e$. -Evaluating the signal to noise ratio of the pixels by means of $\upbeta$-rays of a $^{90}$Sr-source, one observes a good signal to noise ratio (median) of 42, which drops to 33 due to the noise increase (figure \ref{fig:10MradStoN}). Accounting for the width of the distribution, the radiation effect becomes small: 99\% of all pixels show a satisfactory S/N above 20 before and above 17 after irradiation. Again, the noise appears to be dominated by RTS-noise and a solid quantitative understanding of the radiation effects on the noise might require sensors, which are optimized for low RTS. However, the results provide an encouraging first evidence that the manufacturing process and our sensor concept is suited to reach the ambitioned tolerance to some $\rm Mrad$. +After an irradiation dose of up to $3\Mrad$, we do not observe significant changes in the pixel performance. After a dose of $10\Mrad$, the gain of the sensor drops by a factor of two (figure \ref{fig:10MradNoiseGain}). The origin of this effect is under investigation. Despite the drop, the sensor remains operational and the median noise increases only moderately from $\sim 20\rm \e$ to $\sim 25\rm \e$. +Evaluating the signal to noise ratio of the pixels by means of $\upbeta$-rays of a $^{90}$Sr-source, one observes a good signal to noise ratio (MPV) of 42, which drops to 33 due to the noise increase (figure \ref{fig:10MradStoN}). Accounting for the width of the distribution, the radiation effect becomes small: 99\% of all pixels show a satisfactory S/N above 20 before and above 17 after irradiation. Again, the noise appears to be dominated by RTS-noise and a solid quantitative understanding of the radiation effects on the noise might require sensors, which are optimized for low RTS. However, the results provide an encouraging first evidence that the manufacturing process and our sensor concept is suited to reach the ambitioned tolerance to some $\rm Mrad$. \begin{figure} \begin{minipage}{0.49\textwidth} \includegraphics[width=\textwidth]{10MradNoiseGain.pdf} @@ -150,13 +154,14 @@ Evaluating the signal to noise ratio of the pixels by means of $\upbeta$-rays of \end{minipage} \end{figure} \section{Summary and conclusion} -Aiming at applications like the vertex detectors of CBM and ALICE, we are evaluating radiation tolerant large scale sensors with an integration time of $\lesssim 30\mus$. A $0.18\mum$ CMOS process providing a high-resistivity epitaxial layer, deep P- and N-wells and potentially a high tolerance to ionizing radiation is considered as a well suited technology for manufacturing those sensors. The process was explored by means of sensor prototypes hosting numerous pixels, which were varied in different key parameters. +Aiming at applications like the vertex detectors of CBM and ALICE, we are evaluating radiation tolerant large scale sensors with an integration time of $\lesssim 30\mus$. An $0.18\mum$ CMOS process providing a high-resistivity epitaxial layer, deep P- and N-wells and potentially a high tolerance to ionizing radiation is considered as a well suited technology for manufacturing those sensors. The process was explored by means of sensor prototypes hosting numerous pixel types, which were varied in different key parameters. -Guided by observations made previously in the field of optical imaging, we studied the relation between sensor capacitance and the RTS - 1/f noise. We find that the use of sensor gates with a length close to the minimum feature size introduces significant RTS-noise into some of the pixels. As the moderate amount of noisy pixels determines the threshold settings on future particle sensor, the advantages of the small gates in terms of reduced capacitance and therefore the improved gain cannot be exploited. Concerning the optimal width of the sensing diode, we find that the increase of noise and of the CCE, which are caused by an increasing diode, do mostly cancel each other out and a very good S/N is reached with diode surfaces scaling from $2 \mum^2$ to $11 \mum^2$. %The use of bigger diodes appears slightly preferable. +Guided by observations made previously in the field of optical imaging, we studied the relation between sensor capacitance and the RTS - 1/f noise. We find that the use of sensor gates with a length close to the minimum feature size introduces significant RTS-noise into some of the pixels. As even a moderate amount of noisy pixels determines the threshold settings on future particle sensor, the advantages of the small gates in terms of reduced capacitance and therefore the improved gain cannot be exploited. We observe, however, that a very good S/N is reached with diode surfaces scaling from $2 \mum^2$ to $11 \mum^2$. The increase of the diode size increases both, the noise and the CCE by about the same amount. Therefore, no reliable conclusion concerning the optimal diode size can be drawn. This holds in particular, as the optimum may still change once further progresses in reducing the RTS - 1/f noise are made. +%The use of bigger diodes appears slightly preferable. -Concerning the radiation tolerance, we observe that the devices tolerate a dose of $3\Mrad$ without significant losses in the signal to noise ratio, measured with $\rm \beta$-rays while for a dose of $10\Mrad$ a tolerable drop of the gain of the pixels was observed. The origin of this finding is currently under investigation. In any case, the S/N of the device remains satisfactory (above 17 for 99\% of all pixels for the pixel type discussed in this work), which is considered as sufficient for a reliable sensor operation. +Concerning the tolerance to ionizing radiation, we observe that the devices stand a dose of $3\Mrad$ without significant losses in the signal to noise ratio for $\rm \beta$-rays. For a dose of $10\Mrad$, a tolerable drop of the gain of the pixels was observed. The origin of this finding is currently under investigation. In any case, the S/N of the device remains satisfactory (above 17 for 99\% of all pixels for the pixel type discussed in this work), which is considered as sufficient for a reliable sensor operation. -Over all, we conclude that one cannot straight forwardly exploit the lower feature sizes of the $0.18 \mum$-process studied in order to reduce the capacitive noise. This is as the so far dominating capacitive noise is exceeded by the so far insignificant RTS-noise. However, the source of noise is clearly defined and significant noise improvements should be feasible with moderate modifications of the few relevant transistors. Apart from this complication, the $0.18 \mum$-process studied appears well suited for building CMOS sensors for particle detection. Despite the RTS-noise, most pixel designs show a quite satisfactory S/N. This holds in even after irradiating the sensor to a dose of $10 \Mrad$, which extends the tolerance of MAPS of the MIMOSA-family by one order of magnitude and allows for matching the requirements of the vertex detectors of both, ALICE and CBM in this field. +Over all, we conclude that the lower feature size of the $0.18 \mum$-process under study cannot be fully exploited to reduce the capacitive noise of the pixels. This is as the smaller transistors show an RTS-1/f noise, which dominates the capacitive noise. Knowing this effect, it should be possible to reduce the noise of the above presented sensors by means of optimizing the layout of the few relevant transistors. Apart from this issue, the $0.18 \mum$-process studied appears well suited for building CMOS sensors for particle detection: already the non-optimized pixels show a quite satisfactory S/N. This holds in even after irradiating the sensor to a dose of $10 \Mrad$, which extends the tolerance of MAPS of the MIMOSA-family by one order of magnitude and allows for matching the requirements of the vertex detectors of both, ALICE and CBM in this field. \acknowledgments @@ -167,11 +172,10 @@ This work has been supported by BMBF (05P12RFFC7), HIC for FAIR and GSI. \begin{thebibliography}{15} \bibitem{RESMDD2012} -D. Doering et al., \emph{Pitch dependence of the tolerance of CMOS monolithic active pixel sensors to non-ionizing radiation, NIM-A} 730 111-114 (2013) \href{http://dx.doi.org/10.1016/j.nima.2013.04.038} +D. Doering et al., \emph{Pitch dependence of the tolerance of CMOS monolithic active pixel sensors to non-ionizing radiation, NIM-A} 730 111-114 (2013) \bibitem{RHIC} - L. Greiner et al. \emph{Sensor development and readout prototyping for the STAR Pixel detector,JINST 4 P03008} (2009) - + L. Greiner et al. \emph{Sensor development and readout prototyping for the STAR Pixel detector, JINST 4 P03008} (2009) \bibitem{Vertex08} M. Deveaux et al.,\emph{Design considerations for the Micro Vertex Detector of the Compressed Baryonic Matter experiment, POS(VERTEX2008)028} (2008) @@ -187,25 +191,25 @@ A.Dorokhov et al. \emph{Improved radiation tolerance of MAPS using a depleted ep \bibitem{Deveaux2010428} M. Deveaux et al.,\emph{Radiation tolerance of CMOS monolithic active pixel sensors with self-biased pixels, NIM-A} - 624 2 428-431 (2010) \href{10.1016/j.nima.2010.04.045} + 624 2 428-431 (2010) \bibitem{Dev07} M. Deveaux, \emph{Development of fast and radiation hard Monolithic Active Pixel Sensors (MAPS) optimized for $D^0$ detection with the CBM - vertex detector, IKF Frankfurt} (2007) \bibitem{DevXray} -M. Deveaux et al. \emph{Charge collection properties of X-ray irradiated monolithic active pixel sensors, NIM-A}, 552 0168-9002 (2005) \href{ http://dx.doi.org/10.1016/j.nima.2005.06.020} +M. Deveaux et al. \emph{Charge collection properties of X-ray irradiated monolithic active pixel sensors, NIM-A}, 552 0168-9002 (2005) \bibitem{RTS} -M. Deveaux et al. \emph{Random Telegraph Signal in Monolithic Active Pixel Sensors, Nuclear Science Symposium Conference Record}, 3098-3105 (2008) \href{ 10.1109/NSSMIC.2008.4775010} +M. Deveaux et al. \emph{Random Telegraph Signal in Monolithic Active Pixel Sensors, Nuclear Science Symposium Conference Record}, 3098-3105 (2008) \bibitem{RTS0.18} Martin-Gonthier, Philippe, Magnan, Pierre. \emph{RTS Noise Impact in CMOS Image Sensors Readout Circuit, 16th IEEE International Conference on Electronics, Circuits, and Systems}, IEEE, , 928-931 (2010) ISBN 978-1-4244-5090-9 \bibitem{RTS0.182} -Xinyang Wang et al. \emph{Random Telegraph Signal in CMOS Image Sensor Pixels, Electron Devices Meeting}, IEDM '06. International 1,4, 11-13 Dec. (2006) doi: 10.1109/IEDM.2006.346973 +Xinyang Wang et al. \emph{Random Telegraph Signal in CMOS Image Sensor Pixels, Electron Devices Meeting}, IEDM '06. International 1,4, 11-13 Dec. (2006) \bibitem{Senyukov} -S. Senyukov et al. \emph{Charged particle detection performances of CMOS pixel sensors produced in a $0.18~\rm \mu m$ process with a high resistivity epitaxial layer, NIM-A} 730 0 115-118 (2013) \href{http://dx.doi.org/10.1016/j.nima.2013.03.017} +S. Senyukov et al. \emph{Charged particle detection performances of CMOS pixel sensors produced in a $0.18~\rm \mu m$ process with a high resistivity epitaxial layer, NIM-A} 730 0 115-118 (2013) \end{thebibliography} \end{document} diff --git a/IWORID2013/IWORID.zip b/IWORID2013/IWORID.zip new file mode 100644 index 0000000..1b70115 Binary files /dev/null and b/IWORID2013/IWORID.zip differ diff --git a/IWORID2013/IWORID_FinalSubmission.pdf b/IWORID2013/IWORID_FinalSubmission.pdf new file mode 100644 index 0000000..6741077 Binary files /dev/null and b/IWORID2013/IWORID_FinalSubmission.pdf differ diff --git a/IWORID2013/Mi18Mi32Noisevergleich.pdf b/IWORID2013/Mi18Mi32Noisevergleich.pdf index 7abeae9..8678dbd 100644 Binary files a/IWORID2013/Mi18Mi32Noisevergleich.pdf and b/IWORID2013/Mi18Mi32Noisevergleich.pdf differ diff --git a/IWORID2013/Mi32-1-f-noise-CDS-Signal.pdf b/IWORID2013/Mi32-1-f-noise-CDS-Signal.pdf index 1730556..6ff6485 100644 Binary files a/IWORID2013/Mi32-1-f-noise-CDS-Signal.pdf and b/IWORID2013/Mi32-1-f-noise-CDS-Signal.pdf differ diff --git a/IWORID2013/Mi32-1-f-noise-Distribution.pdf b/IWORID2013/Mi32-1-f-noise-Distribution.pdf index d19459c..413fc59 100644 Binary files a/IWORID2013/Mi32-1-f-noise-Distribution.pdf and b/IWORID2013/Mi32-1-f-noise-Distribution.pdf differ diff --git a/IWORID2013/Mi32-1-f-noise-Distribution.png b/IWORID2013/Mi32-1-f-noise-Distribution.png deleted file mode 100644 index a284097..0000000 Binary files a/IWORID2013/Mi32-1-f-noise-Distribution.png and /dev/null differ diff --git a/IWORID2013/StoNDiodeSize.pdf b/IWORID2013/StoNDiodeSize.pdf index 7081dc0..5b02cc3 100644 Binary files a/IWORID2013/StoNDiodeSize.pdf and b/IWORID2013/StoNDiodeSize.pdf differ