Journal of the Korean Physical Society, Vol. 40, No. 1, January 2002, pp. 56∼59 Abnormal ID –VD Curves Due to Contact Structures in ULSI Devices Yong Hae Kim∗ Advanced Micro-Electronics Research Lab., Electronics and Telecommunications Research Institute, Taejon 305-350 Jong Oh Kim, Kuk Seung Yang, Sang Hee Lee and Young Hoon Kim TG–1/P1, Memory Research and Development Division, Hynix Semiconductor, Ichon 467-701 (Received 11 February 2001) We have analyzed the transistor characteristics for various contact structures in ULSI devices. Abnormal ID –VD curves are observed with contact structure variations even though the contact resistance is ohmic. The saturation current, the contact resistance, and the number of drain contacts have a major effect on the abnormal behavior. We derive the variable, IDSAT RD /ND , over which the abnormal ID –VD curves are observed. PACS numbers: 85.30.De reported layout-dependent device characteristics. The driving-current reduction, which depends on the diffusion overlap of the gate, was explained by the mobility degradation induced by the trench stress. In this paper we analyze the transistor characteristics for various contact structures. I. INTRODUCTION As DRAM technology enters into the deep submicron era, the effects of the contact resistance and the contact layout on the transistor characteristics are becoming more important [1,2]. Models have been developed for the uniform sheet resistance or the uniform contact resistance, which decreases as the transistor width is increased [3–6]. Moon et al.  derived an analytical model for pMOS based on the unified charge control model. The parasitic resistance is included by using VDS = Vds −IDS (RS +RD ). Chen et al.  reported the semi-empirical saturation drain current model for lightly doped drain (LDD) nMOS including velocity saturation, the mobility degradation due to increased vertical effective field, and the source/drain series resistance of LDD structure. Chyau and Jang  presented a physics-based and analytic I–V model for submicron buried-channel pMOS including the short channel effects such as drain induced barrier lowering, channel length modulation, velocity saturation, mobility degradation due to the transverse electric field, and uniform parasitic source/drain resistance. Recently, the effects of nonuniform parasitic resistance, such as the contact resistance and the contact layout details, have been a topic of considerable interest [7, 8]. Lee et al.  analyzed the contact structuredependent hot-carrier effect. Hot-carrier degradaton increases rapidly as the number of contact holes is increased while the current-driving capability improves moderately. They developed a HSPICE circuit model to simulate various contact structures. Scott et al.  ∗ E-mail: II. EXPERIMENTAL The devices were fabricated using 0.13µm DRAM technology with a 65 Å gate oxide, a polymetal (W/WNx/Poly) gate, and a W direct bitline contact both at the nMOS and the pMOS. We designed the following bitline contact structures shown in Fig. 1: N/N contact (N = 12 for W = 10 µm, N = 5 for W = 4 µm, and N = 2 for W = 2 µm), N/1 contact, 3/1 contact, and 1/1 conact. Also, the bitline contact to wordline spacing was varied. The contact size was 0.2 µm × 0.2 µm. Fig. 1. Schemetic diagram of various contact sturctures: (a) N/N contact (N = 12 for W = 10 µm, N = 5 for W = 4 µm, N = 2 for W = 2 µm), (b) N/1 contact, (c) 3/1 contact, and (d) 1/1 contact. [email protected] -56- Abnormal ID –VD Curves Due to Contact Structures in ULSI Devices – Yong Hae Kim et al. III. RESULTS AND DISCUSSTION Figure 2 shows the ID –VD characteristics of the (a) source/drain (S/D) = N/N contact and the (b) S/D = 1/1 contact. Abnormal ID –VD curves (a current damping in the linear region), which are more exaggerated as the gate voltage (VG ) is increased, are observed in the S/D = 1/1 contact structures for both the nMOS and the pMOS. To extract the relationship between the contact structures and the abnormal curves, we measure the ID –VD curves of the (a) nMOS and the (b) pMOS for VDSAT various contact structures [Fig. 3]. As the number of source contact (NS ) is decreased from N to 1, the saturation current (IDSAT ) is decreased by 40% independently of the number of drain contact (ND = N, 3, 1). However, when the ND is decreased from N to 1, the saturation voltage (VT SAT ) is increased about 0.5 V independently of the number of source contact. That behavior can be explained with Hu’s model by separating the source and the drain contact resistances : 2 1 − RS + · · · VGS − VT VGS − VT + ESAT L 2 1 = VDSAT 0 + IDSAT 0 · RD + IDSAT 0 2 − (R∗ − RD )RS + · · · VGS − VT VGS − VT + ESAT L IDSAT = IDSAT 0 − IDSAT 0 2 -57- (1) (2) From the above equation, the saturation current mainly depends on the source contact resistance (RS ) while the saturation voltage is affected by the drain contact resistance (RD ). However, the abnormal ID –VD curves which occur at S/D = N/1, 3/1, 1/1 contact structures in the nMOS and at S/D = N/1, 3/1, 1/1, 1/3 contact structures in the pMOS with VG = 2.5 V cannot be explained. The fact that when the number of drain contacts is reduced the abnormal ID –VD curves are more easily observed implies that the drain contact resistance has an important role. To confirm the contact resistance effects, we split the contact resistance, 2kΩ/240Ω(pMOS/nMOS) of base Fig. 2. ID –VD curves with VG = 1 V∼4 V of nMOS (Lg = 0.24 µm) and pMOS (Lg = 0.29 µm) in the (a) source/drain (S/D) = N/N contact and (b) S/D = 1/1 contact structures. Fig. 3. ID –VD curves at VG = 2.5 V with various source and drain contact structures of (a) nMOS (Lg = 0.24 µm) and (b) pMOS (Lg = 0.29 µm). -58- Journal of the Korean Physical Society, Vol. 40, No. 1, January 2002 Fig. 6. Useful variable, IDSAT RD /ND , as a function of the saturation current. The abnormal behavior is observed when IDSAT RD /ND is above 500 mV in pMOS and above 1000mV in nMOS. Fig. 4. Changes of (a) contact resistance in the contact chain pattern and (b) ID –VD characteristics of nMOS and pMOS between the base process and the process A. Fig. 5. ID –VD curves at VG = 2 V for various transistor widths and contact numbers in nMOS (Lg = 0.24 µm) and pMOS (Lg = 0.29 µm). process versus 10.5kΩ/330Ω(pMOS/nMOS) of process A in which the contact resistance is also ohmic as shown in Fig. 4(a). When the contact resistance is increased, the ID –VD curves change from normal to abnormal curves in the pMOS, but there is little change in the nMOS [Fig. 4(b)]. We also varied the transistor width [Fig. 5]. While the ID –VD curves did not change with the transistor width in the S/D = N/N contact structure, the ID –VD curves in the S/D = 1/1 contact structure changed from abnormal behavior to normal behavior when the transistor width was decreased. The variation of ID –VD curves with the transistor width variation implies there is a limitation on the current per unit contact. We additionally measured the ID –VD curves for various contact-to-wordline spacings, but we could not observe any change in the transistor characteristics. To extract the experimental variable that signals the abnormal the ID –VD curves, we combined the saturation current, the contact resistance, and the number of drain contact. We found that IDSAT RD /ND is parameter for determing where the ID –VD curves show abnormal behavior. Abnormal ID –VD curves are seen when the value of IDSAT RD /ND is about 1000 mV in the nMOS and 500 mV in the pMOS [Fig. 6]. This empirical variable tells us that the abnormal ID – VD curves occur when the number of drain contacts is too small or the contact resistance is too high, as well as when the contact is non-ohmic. The above case can occur in the core region of a DRAM cell. Although we have tried a DAVINCI simulation for various contact structures, we failed to obtain abnormal transistor behavior. IV. CONCLUSION Abnormal ID –VD curves are observed when the contact structure is varied, even though the contact resistance is ohmic. The saturation current, the contact resistance, and the number of drain contacts have a major effect on the abnormal behavior. We derive the variable IDSAT RD /ND , above which abnormal ID –VD curves are observed. 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