ÿþ<HEAD><TITLE>July, 1998: Atmosph&#228;risches Plasma mit dem Austrag durch Strom &#8211; Behandlung der Verd&#252;nnung der 300 mm gro&#223;en Scheibe, halbglobaler Bericht f&#252;r 300 mm</TITLE> <META content="Based in Silicon Valley, California, Tru-Si Technologies is the world leader in manufacturing its proprietary Atmospheric Downstream Plasma (ADP) equipment for semiconductor wafer processing applications." name=description> <META content="Processing Equipment, flat panel display, cleaning equipment, dry etch systems, photoresist application, stripping systems," name=keywords> <META content="Tru-Si Technologies" name=Name> <META content="text/html; charset=windows-1251" http-equiv=Content-Type><LINK href="trusi.css" rel=stylesheet title=style type=text/css></HEAD> <BODY><BR> <H2>Atmospheric Downstream Plasma-An Approach to 300-mm Wafer Thinning, Semi Global 300mm Report</H2> <CENTER> <TABLE border=0 width="95%"> <TBODY> <TR> <TD> <H3>Publication: Semi Global 300mm Report</H3>Electronics applications such as hand-held devices, portable communications, and smart cards are driving IC manufacturing in the direction of thinner chips and packages. According to DataQuest, by the year 2000, 21.5% of all IC chips will be thinned varieties. At the same time, and in apparent conflict, the IC industry's focus on productivity improvement is the basis for the transition to large, but thick (e.g., 775 µm) 300-mm silicon substrates. How can these diverging trends be resolved? At present, the answer resides in the industry's growing use of backgrinding and other wafer thinning technology. <BR><BR> <P><IMG align=right height=247 src="articlesep2.gif" width=252>Backgrinding uses abrasive slurries and traditional substrate fabrication equipment to produce gross removal of material from the obverse face of processed IC wafers. The front wafer surface containing the processed ICs must be protected during the backgrinding process. Grinding produces crystalline stress and damage that propagates into the bulk of the wafer. In thick wafers, back surface damage is far removed from the front surface semiconductor device region and neither the integrity of the wafer nor the electronic function of the ICs are compromised. But when wafers are thinned to the 100 to 200 µm thickness range by backgrinding, crystalline stresses can lead to increased wafer breakage and a sharp drop in manufacturing yield. Moreover, crystalline damage can extend close to the active device regions in thin wafers that survive grinding, and can adversely affect circuit performance. <BR><BR>Wet etching is often used to remove backside grinding damage and stress. However, aqueous etching has significant limitations with respect to substrate uniformity, throughput, and cost of ownership. <BR><BR>A new dry plasma etching process has been developed which operates at ambient pressure and achieves a silicon etch rate great enough to make it viable for silicon wafer thickness reduction in actual manufacturing operations. Plasma etching for wafer thinning offers the advantages of: <BR><BR> <UL> <LI>damage-free crystals that allow higher device frequency response and decreased noise <LI>uniform stress fields allowing better matching of across-wafer device characteristics <LI>improved thickness uniformity and better control of capacitance and inductance between circuitry and package ground planes <LI>smooth back surface appearance that reduces metallization problems, reduces morphology-induced resistivity variations between circuitry and package, and meets visual criteria for flip-chip packaging. </LI></UL>This new plasma technology is the development of Tru-Si Technologies, a Sunnyvale, Calif.-based equipment startup. Tru-Si has named it Atmospheric Downstream Plasma"! etching (ADP). To implement it, the company has introduced a fully-automated wafer etching system that invokes some novel principles. Among them, TruSi's Tru-Etch 3000 system operates at ambient pressure, thereby eliminating the need for vacuum pumps and vacuum chambers. The system clamps wafers in a special downward facing platen by employing a Bernoulli-based, differential pressure effect that Tru-Si calls No-touch"! wafer holding. <BR><BR> <H4>Atmospheric Plasma Etching Principles</H4>Plasmas created at atmospheric pressure have the requisite energy density and reactive species flux to achieve high rate silicon etching. While atmospheric plasma sources have been used previously for industrial powder spray coating and plasma cutting, they have not been used in microelectronic processing for fear of damaging and contaminating the silicon substrates. Among the concerns: possible overheating due to high power density, contamination from chemical attack and erosion of electrodes, and non-uniform etching. <BR><BR> <P><IMG align=right height=218 src="articlesep3.gif" width=253>ADP etching obviates these concerns by utilizing a precise, magnetically controlled, high density, inert gas, dc arc-plasma discharge. Silicon etching reactions are invoked by injecting reactant gases directly into the atmospheric-pressure plasma. At a temperature of 10,000 K, the plasma decomposes 100% of all reactants and, at ambient pressure, charged species quickly recombine downstream from the plasma thereby eliminating their potential for causing ion damage. The ADP source is shown in Fig. 1. Each unit consists of an electrode placed inside a chamber with a water-cooled orifice. The orifice and electrode are located along the chamber axis so that the mainstream plasma gas (usually argon), when injected into the chamber, will exit through the orifice. The chambers are electrically isolated from the electrodes. <BR><BR>The plasma jet velocity is about 10m/sec. High energy, activated species are created in the plasma region, but at atmospheric pressure the plasma is thermal due to the small mean free path (&lt;1 µm) of the plasma species. The kinetic energy of the ions, electrons and neutrals is about equal and is very low ( &lt; 1 eV) (Table 1). Particles in the chemical reaction zone surrounding the source (Fig. 2) thus also have low kinetic energy. These characteristics, combined with the low floating potential of the wafers in the process chamber explain why, unlike vacuum plasmas, the Tru-Si ADP process is not accompanied by high energy electron-ion bombardment and electrostatic charging of dielectric surfaces. <BR><BR>The particle density in an atmospheric pressure plasma having a temperature of 6,000-10,000 K is about 10<SUP>18</SUP>/cm<SUP>3</SUP>. The corresponding particle flux of about 102°cm<SUP>-2</SUP>sec<SUP>-1</SUP> causes etch rates at least two orders of magnitude greater than those available with conventional vacuum plasma systems. <BR><BR>A big advantage of the ADP source is that by operating at atmospheric pressure, complicated vacuum systems are avoided. Many potential applications thus require no more than a clean air mini environment. Also, because the high temperature plasma totally decomposes all reactants, the need for complex hazardous waste disposal equipment for fluorocarbons is avoided and remaining gaseous reaction products are easily removed with a water-based effluent scrubber. <BR><BR> <H4>Applications</H4> <H4>While applications for isotropic ADP etching include: </H4> <UL> <LI>removal of lapping damage during wafer manufacturing <LI>removal of backside deposition <LI>wafer thinning and removal of wafer backside damage after backgrinding <LI>isotropic etching <LI>photoresist removal <LI>flat panel etching </LI></UL>the initial focus has been on wafer thinning and on the removal of backside damage and stress caused by backgrinding. The objective is to produce strong thin (&lt;200 µm) wafers. <BR><BR> <P><A href="file:///D:/Inetpub/wwwroot/trusi2/admin/img/articlesep4b.gif" target=CHILD><IMG align=right border=1 height=112 src="articlesep4s.gif" width=180></A>Etching uniformity and reproducibility are achieved by precisely controlling a) the location and power density of the plasma region, and b) the overlapped relative motion between the substrates in process and the plasma chemical reaction zone. In a demonstration sequence that removed 20 µm of silicon at a throughput of 60 wph, the etching process added a total thickness variation (TTY) of &lt;2% on 8-inch wafers. For silicon, the ADP etch rate is about 4 µm/min across the wafer platen. This value is determined by the motion of the wafers relative to the plasma, by the thermal characteristics of the process/equipment interaction, and by the requirement to maintain the wafer temperature at less than 150°C. <BR><BR>The Tru-Etch wafer platen accommodates 13 100-mm wafers (1021 cm<SUP>2</SUP>); 8 150-mm wafers (1414 cm<SUP>2</SUP>), 5 200-mm wafers (1571 cm<SUP>2</SUP>), or 3 300-mm wafers (2121 cm<SUP>2</SUP>). This shows some of the productivity enhancement achievable with 300-mm substrates. <BR><BR>Fig. 3 illustrates the significance of the wafer-thinning application. In processing 300-mm wafers, the very high rate of material removal achievable with backgrinding can accomplish substrate thinning from the initial thickness of 775 µm down to about 300 µm Further thinning with backgrinding <BR><BR>becomes a problem due to crystal damage. To achieve the 100 to 200 µm target thickness indicated in Fig. 3, the balance of the thinning operation may be transferred to an ADP tool. On the wafer, plasma thinning removes backgrinding damage, relieves stress, and smoothes the back surface to enable further processing prior to device separation. <BR><BR>Special tooling developed by Tru-Si allows practical handling of the highly flexible thinned wafers. Interestingly, early work suggests that on thin wafers, die separation may be better accomplished by a scribe and break operation rather than by wafer sawing. <BR><BR>In some manufacturing operations, it may be preferable to thin individual as-sawn dies, rather than thin the wafer and then continue with die separation. Tru-Si has developed tooling that can load processed dies from a chip tray carrier to a special Tru-Etch system platen for subsequent plasma thinning. Each die on the platen is held by differential pressure in an individual No-touch well. ADP thinning then proceeds normally. <BR><BR>Contact:: Sergey Savastiouk,Tru-Si Technologies, Inc. 657 N. Pastoria Ave., Sunnyvale, CA 94085; phone 408-720-3333; fax 408720-3334, info@trusi.com. <BR><BR></P></TD></TR></TBODY></TABLE></CENTER> <HR SIZE=2 width="80%"> <CENTER><B>Tru-Si Technologies</B><BR>657 N. Pastoria Ave. <BR>Sunnyvale, CA 94085 <BR>Phone: (408) 720-3333<BR>Fax: (408) 720-3334<BR>Email: <A href="mailto:inform@trusi.com">inform@trusi.com</A><BR><A href="http://www.trusi.com/" target=_top>Tru-Si Technologies Home Page</A><BR></CENTER><BR><BR></BODY>