In past 50 years, semiconductor industry follows “Moore’s law” doubling the number of transistors every two years in a dense integrated circuit which is achieved by scaling the minimum feature size of a transistor with a factor 0.7 every two years ^[[1],[2]]. Self-aligned multiple patterning (SAMP) is widely used in advanced technology nodes due to lithography resolution limitation ^[[3]-[5]].

Self-aligned multiple patterning (SAMP) also called Sidewall Image Transfer (SIT) which the main concept is to deposit a conformal liner spacer on both sides of a mandrel material so that the pitch in between spacers reduced to half of the pitch in between mandrels. After pulling out mandrel material selective to spacer and hard mask (HM) underneath, final pattern on substrate is defined by spacer pattern with necessary line cut per design. Self-aligned double patterning (SADP) is the base of the self-aligned multiple patterning. Self-aligned quadruple patterning (SAQP) can be realized by repeating one SADP with spacer in first SADP as the mandrel material for second SADP. Figure 1 and Figure 2 are typical SADP and SAQP process flow.

One of the challenges of SAMP is pitch walking. Zero pitch walking means all CDs in between lines defined by self-aligned multiple patterning are equal. No pitch walking in SADP means mandrel CD (MCD) = non-mandrel CD (NMCD) while in SAQP means CD α = β = γ. Pitch walking could be caused by different reasons such as mandrel CD, spacer thickness, etch bias etc., CDSEM or OCD are widely used for pitch walking monitor ^[[6],[7]]. Another challenge of SAMP is line cut ^[[8]]. SAMP method is good at patterning 1D dense lines which need to do line cut after SAMP to cut the unnecessary lines per design. Line cut is quite challenging with pitch scaling to sub-30nm region due to edge placement error (EPE). Depends on the final pattern on substrate after SAMP follow the self-aligned spacer pattern or space in between self-aligned spacers, there are two types of SAMP: positive tone self-aligned patterning and negative tone self-aligned patterning. For positive tone self-aligned patterning, final pattern on substrate follow self-aligned spacer pattern which line cut can be performed either on spacer /HM underneath spacer (cut first) or final pattern (cut last). FIN/Gate in logic and word line/bit line in memory all can be patterned with positive tone self-aligned patterning. For Cu interconnect in logic BEOL, negative tone self-aligned patterning has to be used due to damascene process which means final pattern in BEOL inter-metal dielectric (IMD) is defined by space in between self-aligned spacers. To cut the line for negative tone self-aligned patterning, we have to either cut the mandrel or block the space in between spacers. Figure 3 and 4 are simplified scheme of positive and negative tone self-aligned patterning.

In this paper we focus on innovations to improve the line cut process margin and novel line cut methods.

2.1. Line Cut on Positive Tone Self-aligned Patterning

Final pattern follows the spacer pattern in positive tone self-aligned patterning, so line cut can be performed either on spacer /HM underneath spacer (cut first) or final pattern (cut last). Benefit of cut last is good CD uniformity due to better etch loading effect since there is only dense structure during final pattern etch, ISO or semi-ISO structures only formed after line cut. While etch process for HM cut at cut first is relatively easier since the HM height is much lower than final pattern, also we can play with HM CD at cut first for higher EPE margin. As show in Figure 5, EPE margin refers to distance between cut mask edge and nearest line edge which should not be cut. EPE margin equals line space/2. EPE margin increased if line CD reduced at a fix pitch.

2.1.1. One idea as Figure 6 below is to start with thinner spacer thickness so that smaller HM CD after SAMP, then do the cut first on this smaller HM CD for higher EPE margin. One extra spacer deposited on HM after line cut to target the final CD per design. EPE margin improvement equals (HM CD with extra spacer - HM CD)/2 ^[[9]].

2.1.2. Another idea as shown in Figure 7 is similar with idea above which is also start with thinner spacer thickness and cut first for higher EPE margin. Since the HM in this case is made with amorphous silicon, oxidization is applied on the HM to fully or partially convert amorphous silicon to silicon dioxide, 2.17 unit thickness of oxide will grow for every unit thickness of silicon consumed. Engineering work required for the oxidization process condition to meet the CD target for final HM etch ^[[10]].

2.2. Line Cut on Negative Tone Self-aligned Patterning

Final pattern is defined by space in between self-aligned spacers in negative tone self-aligned patterning. So, there is two way to do line cut: one way is to cut the mandrel which is called mandrel cut; the other way is to block the space in between spacers during final pattern etch which is called non-mandrel cut.

Typical non-mandrel cut as Figure 8 is to fill the space in between spacers with a different material, then remove the fill material at area no need line cut so that non-mandrel line cut by the block HM of the fill material ^{[[11] ]}.

Compare to non-mandrel cut, mandrel cut is more complicated since mandrel material at line cut region need to be replaced with a different material so that which works as HM during final pattern etch. Next I will focus on ideas to make the mandrel cut.

2.2.1. Mandrel Cut after Spacer Deposition ^{[[12], [13]]}

After spacer deposition, a typical mandrel cut as Figure 9 is to fill the space in between spacers with a different material after spacer etch to expose mandrel followed by planarization, then remove mandrel at mandrel cut area and filled with other material followed by recess which will work as HM for mandrel cut.

Optimized version of above idea as Figure 10 is to fill the space in between spacer with spin on carbon based optical planarization layer (OPL) material followed by a low temperature silicon oxide capping so that can do pattern on top of it without affecting the OPL underneath. The following steps for mandrel cut are similar as above idea.

In this paper we discussed some recent novel ideas of line cut on both positive and negative tone self-aligned patterning. Line cut with 193nm immersion lithography will face more and more challenges as the scaling continues. I see the trend in future for line cut of SAMP will be self-aligned or with EUV lithography.