Control of Ion Energy in a Capacitively Coupled Reactive Ion Etcher H. M. Park , C. Garvin, D. S. Grimard and J. W. Grizzle Electronics Manufacturing and Control Systems Lab oratory, Dept. of Electrical Engineering and Computer Science, UniversityofMichigan, Ann Arb or, MI 48109-212 2, USA ABSTRACT The energy of ions b ombarding the wafer is prop ortional to the p o- tential di erence b etween the plasma and the p owered electro de in Reactive Ion Etching (RIE) systems. This work seeks to control the ion energy without altering the applied RF p ower or the chamb er pres- sure since these variables are closely tied to other imp ortantquanti- ties, such as reactivechemical sp ecies concentrations in the plasma and wafer etch uniformity.Avariable resistor placed in parallel with the blo cking capacitor allows the plasma self bias voltage (V )tobear- bias bitrarily varied b etween its nominal value and zero. Optical emission sp ectroscopy for a CF plasma reveals that the nominal plasma chem- 4 ical concentrations do not change under this control metho d. The use of a Langmuir prob e to measure the plasma p otential shows that the ion energy changes by approximately one half of the change in V . bias The p otential uses of this ion energy control technique to plasma self bias voltage regulation, etch selectivity and plasma cleaning of cham- ber walls are demonstrated. A p otential drawback, namely decreased plasma stability, is also indicated. Author to whom corresp ondence should b e addressed. email: [email protected] 1 1 Intro duction and Background Reactive Ion Etching (RIE) is the dominant etching pro cess for the transfer of ne features from masks to wafers. It is well known that ion b ombardment plays an imp ortant role in the creation of anisotropic pro les, etch rate enhancement and surface damage [1{3] and hence signi cant research has b een dedicated to the control of the ion b ombardment energy in RIE systems [4{8]. In standard capacitively coupled RIE systems, the conventional approach to ion energy control has b een through chamb er pressure or applied RF p ower. The drawbacks are that changing power also alters the chemical concentrations in the plasma and changing pressure will a ect the uniformity of the etching pro cess across the wafer [9, 10]. In part, to overcome these drawbacks, new typ es of plasma sources have b een develop ed so that ion energy can b e controlled indep endently of the chemical concentration and chamb er pressure. These include Electron Cyclotron Resonance (ECR), helicon, helical resonator, Transformer Coupled Plasma (TCP), and Inductive Plasma Source (IPS), which op erate in low pressure and high plasma density range [11{14]. This pap er is exclusively concerned with ion energy control in classical capac- itively coupled RIE systems. Figure 1 is a schematic of a typical parallel plate RIE system along with a corresp onding electrical p otential from ano de to catho de. In an asymmetric RIE system (smaller p owered electro de area than grounded electro de area), the presence of a blo cking capacitor leads to a large negative plasma self bias voltage (V ) b eing induced on the p owered electro de. The time-averaged plasma bias p otential (V ) can b e estimated as [15] p V + V rf bias V = ; (1) p 2 2 where V is the p eak amplitude of the applied voltage of the RF generator p ower. rf Ions b ombard the wafer surface due to the p otential di erence b etween the plasma and the p owered electro de, and this p otential di erence de nes the average ion energy (E ) in a collisionless sheath via ion E = q (V V ); (2) ion p bias where q is the charge of an ion [16]. The plasma p otential (V ) is an equip otential p across the bulk plasma. In comparison to V , V is rather small in magnitude, and bias p therefore, V is traditionally used to represent ion energy. bias Several di erenttechniques can b e found in the literature for the control of ion energy to increase etch rate [4, 5], or selectivity [6, 7], to pro duce a p ositively tap ered etch pro le [8] or to minimize radiation damage [17]. The most common approach for the control of ion energy has b een DC-biasing of the p owered electro de. Nagy [4] increased the ion energy by adding a negative DC bias to the p owered electro de, resulting in increased etch rate of p olysilicon and SiO . In his exp eriment, the 2 plasma p otential was slightly decreased with the addition of the negative DC bias; however, the ion energy, V V , increased prop ortionally to the applied DC bias p bias voltage after the applied DC voltage exceeds a few tens of volts. Tai et al. used a zener dio de tied to the blo cking capacitor to control the ion energy [6]. Tai achieved a 50% selectivity improvementofSiO to HPR-206 photoresist by increasing (i.e. 2 decreasing its magnitude) of the V from -470 V to -250 V in CHF plasma. bias 3 The metho d used in this pap er to control ion b ombardment energy is related to [6]. As shown in Fig. 1, in a conventional RIE system, the net Direct Current (DC) through the p owered electro de must b e zero due to the DC blo cking capacitor. 3 However, if a variable resistor is connected to the p owered electro de as shown in Fig. 2, then the net DC is no longer forced to b e zero. Weobserved that the V bias develop ed at the p owered electro de increases (i.e. decreases in magnitude) as the DC settles to a nonzero value. The result is that the V can b e controlled by bias regulating the amountofDCandthus can b e manipulated byavariable resistor. From the de nition of ion energy (2), an increase of V by the use of a variable bias resistor will decrease the ion energy if the V is not varied during the control of the p variable resistor. To b etter understand the e ects of this variable resistor on ion energy,itis helpful to analyze two extreme cases illustrated in Fig. 3: a) If the variable DC resistance is set to 1, then the system reduces to a conventional RIE system with nom nom nom V = V and E = q (V V ); b) If the DC resistance is zero, then the bias ion bias p bias system turns into a DC coupled RIE system with V = 0 and E = q V . In the bias ion p latter case, the higher mobility of the electrons leads to electron current dominating the DC through the electro des. The higher eux of electrons than ions from the plasma will lead to an increase in the plasma p otential. This is illustrated in Fig. 3, where the plasma p otential in a DC coupled RIE system is much higher than in a capacitively coupled system, and its lowest value do es not approach zero [15, 18]. Therefore, the increase in the plasma p otential acts as a limiting factor in ion energy control by this metho d. The amountofchange in ion energy that can b e obtained by this metho d is investigated in Section 4.1. Two applications of ion energy control by the use of a variable resistor are explored in this pap er. In the rst one, the selectivity of p olysilicon with resp ect to SiO is investigated. It is known that the etch rate of p olysilicon dep ends more on 2 the concentration of the reactivechemical sp ecies (chemical etching) than the ion 4 energy (physical etching); in contrast to this, the etch rate of SiO dep ends more 2 strongly on the ion energy than the chemical reaction [16, 19, 20]. Therefore, in order to increase the selectivity of p olysilicon to SiO , the ion energy should b e decreased 2 while the reacting chemical concentration is either increased or kept constant. This work is presented in Section 4.2. Section 4.3 investigates an application of the ion energy control technique to an ion enhanced cleaning pro cess. Polymer buildup on the etching chamber wall is regarded as a signi cant disturbance to the etching pro cess and has b een correlated to reduced device p erformance and yield [21{23]. By increasing the plasma p otential, the ion b ombardment energy toward the grounded wall will b e increased, p ossibly enhancing a plasma cleaning pro cess. 2 Exp erimental Setup The exp eriment to measure the plasma p otential with a Langmuir prob e and the selectivity enhancement exp erimentwere p erformed on a Gaseous Electronics Conference (GEC) reference cell that is describ ed in detail in [24, 25]. The schematic of the exp erimental apparatus is shown in Fig. 4. The GEC is an RIE system having parallel, four-inch diameter, one-inch spaced, water-co oled stainless steel electro des, housed in a stainless steel chamb er. Vacuum is established bya mechanical pump and a turb o pump connected to the b ottom of the chamb er and pressure is controlled through a butter y throttle valve. Pressure is measured byanMKStyp e 127A baratron capacitive manometer. RF p ower at 13.56 MHz is supplied by an ENI ACG-5 p ower supply to the b ottom electro de through an ENI MW-5 matching network.