96 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 22, NO. 1, FEBRUARY 2009 Comb Capacitor Structures for On-Chip Physical Uncloneable Function Deepu Roy, Johan H. Klootwijk, Senior Member, IEEE, Nynke A. M. Verhaegh, Harold H. A. J. Roosen, and Rob A. M. Wolters Abstract—Planar inter-digitated comb capacitor structures are an excellent tool for on-chip capacitance measurement and evalua- tion of properties of coating layers with varying composition. These comb structures are easily fabricated in a single step in the last metallization layer of a standard IC process. Capacitive coupling of these structures with a coating layer is modelled based on the electric field distribution to have a detailed understanding of con- tributing capacitance components. The coating composition is op- timized to provide maximum spread in capacitance values of comb capacitor structures. This spread in measured capacitance values can be used to implement a physical uncloneable function (PUF). A PUF is a random function which can be evaluated only with the help of a physical system. We present an on-chip capacitive PUF for chip security and data storage in which the unlock key algo- rithm is generated from capacitors which are physically linked to the chip in an inseparable way. The strength of this key increases with the spread in capacitance values and measurement accuracy. Index Terms—Comb capacitors, physical uncloneable function (PUF), postprocessing, security coating. I. INTRODUCTION T O meet the increasing demands and performance require- ments microelectronic industries have to deliver complex technological solutions which require the input and or storage of personal and confidential information. For example, a smart card is a simple plastic card with an integrated chip equipped with a microprocessor and a memory to save and process in- formation. Secure storage of information in a smart card is a necessity for the user and at the same time it is a challenge for the manufacturer. Different types of cryptographic encryption algorithms are implemented depending on the security level of the chip. Any physical attack on the smart card chip can be pre- vented by the use of an optically opaque and chemically inert se- curity coating layer over the chip. This coating acts as a dielec- tric layer for the comb capacitor structures in which the mea- sured value of capacitance depends on the presence and com- position of the coating layer. Comb capacitor structures are for instance studied for applications like humidity sensors or gas detectors [1], [2] where the principle employed is the change in Manuscript received July 01, 2008; revised October 22, 2008. Current version published February 04, 2009. D. Roy, N. A. M. Verhaegh, and R. A. M. Wolters are with NXP Semiconduc- tors Research, 5656 AE, Eindhoven, The Netherlands (e-mail: deepu.roy@nxp. com; nynke.verhaegh@nxp.com; Rob.wolters@nxp.com). J. H. Klootwijk and H. H. A. J. Roosen are with AE, Philips Research, 5656 AE, Eindhoven, The Netherlands (e-mail: johan.klootwijk@philips.com; harold.h.roosen@philips.com). Color versions of one or more of the figures in this paper are available online at http://www.ieeexplore.ieee.org. Digital Object Identifier 10.1109/TSM.2008.2010738 capacitance value with dielectric constant of the layer applied on top of these structures. Adding functionalities on top of met- allization layers of a finished chip through extra layers is called postprocessing. For sensing applications the change in proper- ties of the postprocessed layer (which may or may not be CMOS process based) with the measured parameter needs to be sensed and converted to measurable parameters. In this paper, an alu- minum phosphate-based coating is applied as the postprocessed layer on top of the chip. This coating layer is hard, opaque, and relatively chemically inert and it is used as the first security mea- sure to prevent physical attacks to the content of the chip. The advantages of this type of system are the relative simplicity of the measurement and the easy processing steps. At the first in- stance, the presence of the coating can be sensed by the comb ca- pacitor structures. The composition of this postprocessed layer is varied by changing the composition of TiO particles (con- tributing to the effective dielectric constant of the layer) and con- ducting TiN particles (as floating electrodes in the layer). This results in increased capacitance values with a large spread. This spread in capacitance values can be utilized as an additional se- curity measure for on-chip data storage. This is implemented by a physical uncloneable function (PUF) function from the mea- sured capacitance values to generate for example an access key to the chip. This type of PUF is called capacitive PUF or coating PUF (c-PUF), of which the schematic is shown in Fig. 1. PUF is a function that is realized by a physical system, such that the function is easy to evaluate but hard to characterize, model, and reproduce. PUF’s were first introduced by Pappu et al. [3] and different variations in PUFs were presented, depending on the type of the key generation system used [4], [5]. To have larger information density the range of measured values of capacitance should be maximized. To achieve this, the comb structures have been modelled based on the electric field distribution between the combs and through the coating layer. The uncloneable nature of the security coating together with the comb structure will be elucidated. The properties and function of the different particles in the aluminum phosphate based coating matrix are evaluated. Based on these optimizations a physical model is proposed for increase and spread in the capacitance values for comb struc- tures. II. TEST STRUCTURE FABRICATION The comb capacitor structures fabricated on the chip are as shown in Fig. 2(a). These structures have two interlinked finger- like metallic combs of equal height and width. Across the two combs a capacitance is established, the variation of which is used to analyze the coating layer. The advantages of using the comb structures are as follows: 0894-6507/$25.00 © 2009 IEEE Authorized licensed use limited to: UNIVERSITEIT TWENTE. Downloaded on February 4, 2009 at 06:01 from IEEE Xplore. Restrictions apply.