ch02.2.htm

介绍asci设计的一本书

</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214887">
 </A>
	m1/2/3</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214889">
 </A>
	60 &#177; 6</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214891">
 </A>
m<SPAN CLASS="Symbol">
W</SPAN>
 / square</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214893">
 </A>
	m3</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214896">
 </A>
	30&#177; 3</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214898">
 </A>
m<SPAN CLASS="Symbol">
W</SPAN>
 / square</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214900">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214902">
 </A>
	metal4</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214904">
 </A>
	30 &#177; 3</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=214906">
 </A>
m<SPAN CLASS="Symbol">
W</SPAN>
 / square</P>
</TD>
</TR>
</TABLE>
<P CLASS="Body">
<A NAME="pgfId=122904">
 </A>
The <SPAN CLASS="Definition">
diffusion</SPAN>
 layers, <SPAN CLASS="EmphasisPrefix">
n</SPAN>
-diffusion and <SPAN CLASS="EmphasisPrefix">
p</SPAN>
-diffusion, both have a high resistivity&#8212;typically from 1&#8211;100 <SPAN CLASS="Symbol">
W</SPAN>
/square. We measure resistance in <SPAN CLASS="Symbol">
W</SPAN>
 <SPAN CLASS="Definition">
/</SPAN>
 <SPAN CLASS="Definition">
square</SPAN>
 (ohms per square) because for a fixed thickness of material it does not matter what the size of a square is&#8212;the resistance is the same. Thus the resistance of a rectangular shape of a sheet of material may be calculated from the number of squares it contains times the <SPAN CLASS="Definition">
sheet resistance</SPAN>
 in&nbsp;<SPAN CLASS="Symbol">
W</SPAN>
 / square. We can use diffusion for very short connections inside a logic cell, but not for interconnect between logic cells. Poly has the next highest resistance to diffusion. Most submicron CMOS processes use a <SPAN CLASS="Definition">
silicide</SPAN>
 material (a metallic compound of silicon) that has much lower resistivity (at several&nbsp;<SPAN CLASS="Symbol">
W</SPAN>
/square) than the poly or diffusion layers alone. Examples are tantalum silicide, TaSi; tungsten silicide, WSi; or titanium silicide, TiSi. The <SPAN CLASS="Definition">
stoichiometry</SPAN>
 of these deposited silicides varies. For example, for tungsten silicide W:Si <SPAN CLASS="Symbol">
&#170;</SPAN>
 1:2.6. </P>
<P CLASS="Body">
<A NAME="pgfId=150585">
 </A>
There are two types of silicide process. In a silicide process only the gate is silicided. This reduces the poly sheet resistance, but not that of the source&#8211;drain. In a self-aligned silicide (<SPAN CLASS="Definition">
salicide</SPAN>
) process, both the gate and the source&#8211;drain regions are silicided. In some processes silicide can be used to connect adjacent poly and diffusion (we call this feature <SPAN CLASS="Definition">
LI</SPAN>
, white metal, local interconnect, metal0, or m0). LI is useful to reduce the area of ASIC RAM cells, for example.</P>
<P CLASS="Body">
<A NAME="pgfId=201044">
 </A>
Interconnect uses metal layers with resistivities of tens of&nbsp;m<SPAN CLASS="Symbol">
W</SPAN>
/square, several orders of magnitude less than the other layers. There are usually several layers of metal in a CMOS ASIC process, each separated by an insulating layer. The metal layer above the poly gate layer is the first-level metal (<SPAN CLASS="Definition">
m1</SPAN>
 or metal1), the next is the second-level metal (<SPAN CLASS="Definition">
m2</SPAN>
 or metal2), and so on. We can make connections from m1 to diffusion using <SPAN CLASS="Definition">
diffusion contacts</SPAN>
 or to the poly using <SPAN CLASS="Definition">
polysilicon contacts</SPAN>
. </P>
<P CLASS="Body">
<A NAME="pgfId=162592">
 </A>
After we etch the contact holes a thin <SPAN CLASS="Definition">
barrier metal</SPAN>
 (typically platinum) is deposited over the silicon and poly. Next we form <SPAN CLASS="Definition">
contact plugs</SPAN>
 (<SPAN CLASS="Definition">
via plugs</SPAN>
 for connections between metal layers) to reduce contact resistance and the likelihood of breaks in the contacts. Tungsten is commonly used for these plugs. Following this we form the metal layers as sandwiches. The middle of the sandwich is a layer (usually from 3000 &Aring; to 10,000 &Aring;) of aluminum and copper. The top and bottom layers are normally titanium&#8211;tungsten (TiW, pronounced &#8220;tie-tungsten&#8221;). Submicron processes use <SPAN CLASS="Definition">
chemical&#8211;mechanical polishing</SPAN>
 (<SPAN CLASS="Definition">
CMP</SPAN>
) to smooth the wafers flat before each metal deposition step to help with step coverage.</P>
<P CLASS="Body">
<A NAME="pgfId=165037">
 </A>
An insulating glass, often sputtered quartz (SiO<SUB CLASS="Subscript">
2</SUB>
), though other materials are also used, is deposited between metal layers to help create a smooth surface for the deposition of the metal. Design rules may refer to this insulator as an <SPAN CLASS="Definition">
intermetal oxide</SPAN>
 (<SPAN CLASS="Definition">
IMO</SPAN>
) whether they are in fact oxides or not, or <SPAN CLASS="Definition">
interlevel dielectric</SPAN>
 (<SPAN CLASS="Definition">
ILD</SPAN>
). The IMO may be a spin-on polymer; boron-doped phosphosilicate glass (BPSG); Si<SUB CLASS="Subscript">
3</SUB>
N<SUB CLASS="Subscript">
4</SUB>
; or sandwiches of these materials (oxynitrides, for example).</P>
<P CLASS="Body">
<A NAME="pgfId=177748">
 </A>
We make the connections between m1 and m2 using <SPAN CLASS="Definition">
metal vias</SPAN>
, <SPAN CLASS="Definition">
cuts</SPAN>
, or just <SPAN CLASS="Definition">
vias</SPAN>
. We cannot connect m2 directly to diffusion or poly; instead we must make these connections through m1 using a via. Most processes allow contacts and vias to be placed directly above each other without restriction, arrangements known as <SPAN CLASS="Definition">
stacked vias</SPAN>
 and <SPAN CLASS="Definition">
stacked contacts</SPAN>
. We call a process with m1 and m2 a <SPAN CLASS="Definition">
two-level metal</SPAN>
 (<SPAN CLASS="Definition">
2LM</SPAN>
) technology. A <SPAN CLASS="Definition">
3LM</SPAN>
 process includes a third-level metal layer (<SPAN CLASS="Definition">
m3</SPAN>
 or metal3), and some processes include more metal layers. In this case a connection between m1 and m2 will use an m1/m2 via, or <SPAN CLASS="Definition">
via1</SPAN>
; a connection between m2 and m3 will use an m2/m3 via, or <SPAN CLASS="Definition">
via2</SPAN>
, and so on.</P>
<P CLASS="Body">
<A NAME="pgfId=5159">
 </A>
The minimum spacing of interconnects, the <SPAN CLASS="Definition">
metal pitch</SPAN>
, may increase with successive metal layers. The minimum metal pitch is the minimum spacing between the centers of adjacent interconnects and is equal to the minimum metal width plus the minimum metal spacing.</P>
<P CLASS="Body">
<A NAME="pgfId=5168">
 </A>
Aluminum interconnect tends to break when carrying a high current density. Collisions between high-energy electrons and atoms move the metal atoms over a long period of time in a process known as <SPAN CLASS="Definition">
electromigration</SPAN>
. Copper is added to the aluminum to help reduce the problem. The other solution is to reduce the current density by using wider than minimum-width metal lines.</P>
<P CLASS="Body">
<A NAME="pgfId=20998">
 </A>
Tables 2.5 and 2.6 show maximum specified <SPAN CLASS="Definition">
contact resistance</SPAN>
 and <SPAN CLASS="Definition">
via resistance</SPAN>
 for two generations of CMOS processes. Notice that a m1 contact in either process is equal in resistance to several hundred squares of metal.</P>
<TABLE>
<TR>
<TD ROWSPAN="1" COLSPAN="2">
<P CLASS="TableTitle">
<A NAME="pgfId=106399">
 </A>
TABLE&nbsp;2.5&nbsp;Contact resistance (1 <SPAN CLASS="Symbol">
m</SPAN>
m CMOS).</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128811">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="2">
<P CLASS="TableTitle">
<A NAME="pgfId=128854">
 </A>
TABLE&nbsp;2.6&nbsp;Contact resistance (0.35 <SPAN CLASS="Symbol">
m</SPAN>
m CMOS).</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=106403">
 </A>
<SPAN CLASS="TableHeads">
Contact/via type</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=106405">
 </A>
<SPAN CLASS="TableHeads">
Resistance (maximum)</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=128817">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=128858">
 </A>
<SPAN CLASS="TableHeads">
Contact/via type</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="TableFirst">
<A NAME="pgfId=128860">
 </A>
<SPAN CLASS="TableHeads">
Resistance (maximum)</SPAN>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106407">
 </A>
	m2/m3 via (via2)</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106409">
 </A>
 5 <SPAN CLASS="Symbol">
W</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128823">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128862">
 </A>
	m2/m3 via (via2)</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128864">
 </A>
 6 <SPAN CLASS="Symbol">
W</SPAN>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106411">
 </A>
	m1/m2 via (via1)</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106413">
 </A>
 2 <SPAN CLASS="Symbol">
W</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128829">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128866">
 </A>
	m1/m2 via (via1)</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128868">
 </A>
 6 <SPAN CLASS="Symbol">
W</SPAN>
</P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106415">
 </A>
	m1/<SPAN CLASS="EmphasisPrefix">
p</SPAN>
-diffusion contact</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106417">
 </A>
20 <SPAN CLASS="Symbol">
W</SPAN>
 </P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128835">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128870">
 </A>
	m1/<SPAN CLASS="EmphasisPrefix">
p</SPAN>
-diffusion contact</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128872">
 </A>
20 <SPAN CLASS="Symbol">
W</SPAN>
 </P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106419">
 </A>
	m1/<SPAN CLASS="EmphasisPrefix">
n</SPAN>
-diffusion contact</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106421">
 </A>
20 <SPAN CLASS="Symbol">
W</SPAN>
 </P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128841">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128874">
 </A>
	m1/<SPAN CLASS="EmphasisPrefix">
n</SPAN>
-diffusion contact</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128876">
 </A>
20 <SPAN CLASS="Symbol">
W</SPAN>
 </P>
</TD>
</TR>
<TR>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106423">
 </A>
	m1/poly contact</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=106425">
 </A>
20 <SPAN CLASS="Symbol">
W</SPAN>
</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128847">
 </A>
&nbsp;</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128878">
 </A>
	m1/poly contact</P>
</TD>
<TD ROWSPAN="1" COLSPAN="1">
<P CLASS="Table">
<A NAME="pgfId=128880">
 </A>
20 <SPAN CLASS="Symbol">
W</SPAN>
</P>
</TD>
</TR>
</TABLE>
</DIV>
<HR>
<DIV CLASS="footnotes">
<DIV CLASS="footnote">
<P CLASS="TableFootnote">
<SPAN CLASS="footnoteNumber">
1.</SPAN>
<A NAME="pgfId=195508">
 </A>
If only one well layer is drawn, the other mask may be derived from the drawn layer. For example, <SPAN CLASS="EmphasisPrefix">
p</SPAN>
-well (mask) = not (nwell (drawn)). A single-well process requires only one well mask.</P>
</DIV>
<DIV CLASS="footnote">
<P CLASS="TableFootnote">
<SPAN CLASS="footnoteNumber">
2.</SPAN>
<A NAME="pgfId=195560">
 </A>
The implant masks may be derived or drawn. </P>
</DIV>
<DIV CLASS="footnote">
<P CLASS="TableFootLast">
<SPAN CLASS="footnoteNumber">
3.</SPAN>
<A NAME="pgfId=195605">
 </A>
Largely for historical reasons the contacts to poly and contacts to active have different layer names. In the past this allowed a different sizing or process bias to be applied to each contact type when the mask was made.</P>
</DIV>
</DIV>
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