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| | Passive devices such as resistors, capacitors, and inductors, are commonly used in biasing circuits, feedback networks, and signal or energy storage blocks. However, these passive devices, when built on fabrication processes that are optimized for transistors, may have characteristics different from their ideal or discrete counterparts. In this module, we examine the behavior of passive devices built alongside CMOS transistors. | | Passive devices such as resistors, capacitors, and inductors, are commonly used in biasing circuits, feedback networks, and signal or energy storage blocks. However, these passive devices, when built on fabrication processes that are optimized for transistors, may have characteristics different from their ideal or discrete counterparts. In this module, we examine the behavior of passive devices built alongside CMOS transistors. |
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| − | == Resistors ==
| + | We will divide this module into the following sections: |
| − | In standard digital CMOS processes, there is usually no provision for high resistance layers, since resistances are typically deemed bad for digital circuits. But in analog design, we often need well-controlled resistors, with relatively large resistance values. In order to evaluate if a particular layer could be used as to build a resistor, we look to their sheet resistance. Recall that for a resistor:
| + | * [[Integrated Resistors]] |
| | + | * [[Integrated Capacitors]] |
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| − | {{NumBlk|::|<math>R = \frac{\rho \cdot \ell}{A} = \frac{\rho}{t}\frac{\ell}{w}= R_\mathrm{sh} \frac{\ell}{w}</math>|{{EquationRef|1}}}}
| + | Read the two modules above, the do the activity below. |
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| − | Where <math>R_\mathrm{sh}</math> is the ''sheet resistance'' of the layer, <math>\rho</math> is the resistivity of the material, <math>\ell</math> is the length along the direction of the current, and <math>A = w\cdot t</math>, is the cross sectional area normal to the current flow, which is equal to the product of the layer thickness, <math>t</math> and <math>w</math> is the width of the layer perpendicular to the current flow. The table below shows indicative values of the common conductive layers in a CMOS process:
| + | {{Note|[[220-A2.1 | '''Activity A2.1''' Integrated Resistors and Capacitors]] -- This activity walks you through the analysis and simulation of integrated RC circuits.|reminder}} |
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| − | {| class="wikitable" style="text-align: center;" | |
| − | ! Layer
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| − | ! Sheet Resistance
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| − | |-
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| − | | Metal
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| − | | <math>50-70\,\mathrm{m\Omega/\square} \pm 30\mathrm{%}</math>
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| − | |-
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| − | | Polysilicon
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| − | | <math>5\,\mathrm{m\Omega/\square}</math>
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| − | |-
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| − | | <math>n^+</math> or <math>p^+</math> Diffusion
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| − | | <math>5\,\mathrm{m\Omega/\square}</math>
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| − | |-
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| − | | <math>n</math>-well
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| − | | <math>1-5\,\mathrm{k\Omega/\square} \pm 40\mathrm{%}</math>
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| − | |-
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| − | |}
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| − | | |
| − | An illustrative cross-section of resistors using these layers are shown in Figs. 1 and 2.
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| − | | |
| − | {|
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| − | | [[File:Si resistors.png|thumb|520px|Figure 1: Resistors built using semiconductor layers<ref name="Allen2016">Phillip Allen's [https://aicdesign.org/wp-content/uploads/2018/08/lecture03-151116.pdf slides] on Deep Submicron CMOS Technologies</ref>.]]
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| − | | [[File:Metal resistor.png|thumb|450px|Figure 2: Resistors built using metal layers<ref name="Allen2016"/>.]]
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| − | |-
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| − | |}
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| − | | |
| − | [[File:Silicide cmos.png|thumb|300px|Figure 3: Silicide layers on diffusion and polysilicon layers to reduce the effective sheet resistance<ref>Krishna Saraswat's [https://web.stanford.edu/class/ee311/NOTES/Silicides%20&%20Metal%20gate%20Slides.pdf slides] on Polycides, Salicides andMetals Gates.</ref>.]]
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| − | In a CMOS process optimized for digital circuits, the low sheet resistances of the semiconductor layers are due to a low-resistance silicide layer deposited on top of the semiconductor layer, as shown in Fig. 3. In analog CMOS processes, we are usually given the option to place a silicide block, i.e. to prevent the silicide layer from being deposited on specific locations on the die. The table below contains illustrative values of the sheet resistance of the semiconductor layers with the silicide block option.
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| − | | |
| − | {| class="wikitable" style="text-align: center;"
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| − | ! Layer
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| − | ! Sheet Resistance
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| − | |-
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| − | | <math>n^+</math>-polysilicon
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| − | | <math>100\,\mathrm{\Omega/\square}</math>
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| − | |-
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| − | | <math>p^+</math>-polysilicon
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| − | | <math>180\,\mathrm{\Omega/\square}</math> | |
| − | |-
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| − | | <math>n^+</math>-diffusion
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| − | | <math>50\,\mathrm{\Omega/\square}</math>
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| − | |-
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| − | | <math>p^+</math>-diffusion
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| − | | <math>100\,\mathrm{\Omega/\square}</math>
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| − | |-
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| − | | <math>n</math>-well
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| − | | <math>1000\,\mathrm{\Omega/\square}</math>
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| − | |-
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| − | |}
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| − | | |
| − | Note that the non-silicided layers have significantly larger sheet resistances.
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| − | | |
| − | == Capacitors ==
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| − | | |
| − | == Inductors ==
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| − | | |
| − | == References ==
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| − | <references />
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Passive devices such as resistors, capacitors, and inductors, are commonly used in biasing circuits, feedback networks, and signal or energy storage blocks. However, these passive devices, when built on fabrication processes that are optimized for transistors, may have characteristics different from their ideal or discrete counterparts. In this module, we examine the behavior of passive devices built alongside CMOS transistors.
We will divide this module into the following sections:
Read the two modules above, the do the activity below.
