Effect of ionic surfactants on shallow trench isolation for chemical mechanical polishing using ceria-based slurries

Abstract As the development of technology nodes proceeds to 7 nm node, chemical mechanical polishing (CMP) slurries for shallow trench isolation (STI) cannot fully meet the technical requirements. Higher goals are put forward for the polished surface qualities and the removal selectivity control. The polishing liquid exhibits issues such as easy agglomeration, removal rate of Si3N4 exceeding 50 Å/min, removal selectivity ratio of SiO2/Si3N4 below 20, increased surface scratches and roughness of SiO2 and Si3N4 after polishing exceeding 1 nm. Here, attention is given to studying the STI CMP process by introducing various ionic surfactants in ceria slurries, aiming to control removal rates, selectivity, as well as surface qualities. The findings of ball milling and settling tests were used as a starting point for choosing the effective surfactants, searching the minimal delamination phenomenon. Then the impact of surfactants on removal rates, selectivity and surface characteristics were next investigated in polishing trials at various pH levels. Depth of scratches on polished wafers and corresponding surface roughness, as well as morphology of ceria abrasive particles were characterized. Action mechanisms of selected ionic surfactants in ceria slurries have been revealed by solid-liquid interface adsorption characterization, thermogravimetric analysis and zeta potential tests. Through research finding the addition of piperazine and 2-methylpiperazine surfactant can reduce the number and depth of scratches on the surfaces of SiO2 and Si3N4 after polishing, and it exhibits better dispersibility in alkaline environment compared to acidic environment.


Introduction
Shallow trench isolation (STI) has been proposed as the isolation structure in integrated circuit manufacturing (Gan & Smith, 2000;Srinivasan et al., 2015;Steigerwald et al., 1997), where the isolation effect is expected to be executed as intended; otherwise, it will directly lead to severe leakage between connected transistors, ultimately resulting in the failure of the entire circuit.Typically, the manufacturing process of the STI structure involves depositing silicon nitride (Si 3 N 4 ) on a silicon wafer, etching isolation trenches, filling the trenches with high-density plasma silicon dioxide (HDP SiO 2 ) through bottom-to-top chemical vapor deposition, and then removing the SiO 2 and stopping at the Si 3 N 4 layer, exposing the active and isolation regions, respectively.Chemical mechanical polishing (CMP) is considered an indispensable method for achieving global and local planarization of the STI structure.As the technology node advances to the 7-nanometer node, the polishing slurry and related supporting technical parameters for STI cannot fully meet the technical requirements.Higher goals are set for controlling the polishing surface quality and the selectivity of SiO 2 removal over Si 3 N 4 .
Several aspects and consequent severe obstacles during the STI CMP process demand to be thoroughly discussed, where the first primary issue is the agglomeration phenomenon of polishing slurry particles.Scratches and defects will be easily caused on wafer surfaces if the agglomeration of abrasive particles cannot be adequately resolved (Lei et al., 2008).Typically, ceria (CeO 2 ) particles which have the advantages of low hardness and high chemical activity, have been used as the abrasive particles for the STI CMP process.G. B. Basim et al. (Basim et al., 2000) found that the wafer surface damages are positively related to particle sizes and the proportion of coarser abrasive particles in polishing slurries.Meanwhile, due to the capillary force in the ambient, CeO 2 abrasive particles usually stick together to form clumps, making it difficult to achieve direct dispersion (Pakarinen et al., 2005).Therefore, it is crucial to uniformly disperse CeO 2 abrasive particles during the preparation and modification of polishing slurries.Furthermore, with regard to the STI CMP process, since Si 3 N 4 layer acts as the protection layer of silicon substrates, the technical index of Si 3 N 4 material removal rate (MRR) is relatively low, generally less than ~ 50 Å/ min.On the other hand, rapid removal of SiO 2 at a MRR of roughly 1500 ~ 2000 Å/min is supposed to be realized, combined with a removal selection ratio of larger than ~ 20 between SiO 2 and Si 3 N 4 .Moreover, a higher standard for surface roughness of less than ~1 nm after polishing should be met as device dimensions continue to reduce down to 7 nm node (Shi et al., 2017(Shi et al., , 2020)).In a nutshell, the STI CMP process must address the high demands for surface qualities and defect control, as well as the difficulties of low Si 3 N 4 MRR and appropriate MRR selectivity between heterogeneous materials.
According to published researches, adding additives in CeO 2 slurries can inhibit Si 3 N 4 MRR and lessen the number and depth of scratches on polished SiO 2 and Si 3 N 4 surfaces.Amino acid as a representative of ionic surfactants has received the most attention (America & Babu, 2004;Granqvist et al., 2007;Penta et al., 2013;Zhuravlev, 2000).P. R. Dandu Veera et al. (Dandu Veera et al., 2009) revealed that piperazine hydrochloride, piperazine and imidazole could both form a strong bond with Si 3 N 4 at pH 4 and 5, thereby forming a dense adsorption film to inhibit the hydrolysis action and then reduce removal rates.By contrast, such bond does not exist when SiO 2 is being polished (Boumahraz et al., 1982).In addition, pH values of slurries have a substantial impact on the removal of SiO 2 and Si 3 N 4 wafers by affecting types and intensity of chemical reactions, as well as surfaces charge properties of CeO 2 abrasive particles and SiO 2 /Si 3 N 4 films.An attraction force (Suphantharida & Osseo-Asare, 2004) will be generated between CeO 2 abrasive particles and wafer surfaces when their surface charges are opposing, which will directly speed up the adsorption and interaction between the particles and surfaces.For instance, K. H. Bu et al. (Bu & Moudgil, 2007, 2004) discovered that the adsorption of sodium dodecyl sulfate on Si 3 N 4 was much greater than that of SiO 2 in acidic environment, thereby significantly inhibiting Si 3 N 4 MRR.
Here, attention is given to studying the STI CMP process by introducing various ionic surfactants in CeO 2 slurries, aiming to control MRRs, removal selectivity, as well as surface qualities after CMP process.The findings of ball milling and settling tests were used as a starting point for choosing the effective ionic surfactants, searching the minimal delamination phenomenon with the optimal dispersibility and stability by experimental comparison.The impact of chosen surfactants on MRRs, removal selectivity between SiO 2 and Si 3 N 4 , and surface characteristics were next investigated in polishing trials at various pH levels.Depth of scratches on polished wafers and corresponding surface roughness, as well as morphology of CeO 2 abrasive particles were characterized utilizing atomic force microscope (AFM) and scanning electron microscope (SEM), respectively.Furthermore, action mechanisms of selected ionic surfactants in CeO 2 slurries have been revealed by solid-liquid interface adsorption characterization, thermogravimetric analysis and zeta potential tests.

Experimental details
Prepare CeO 2 polishing slurry by mixing zirconia grinding balls (1 mm), CeO 2 bonded abrasive, surfactant, and deionized water (DIW) in a mass ratio of 5:1:1:32.Conduct sedimentation experiments on the polishing slurry containing different surfactants for different durations and select the surfactant with the best dispersibility and stability.Dilute the above CeO 2 polishing slurry 100 times as the material for CMP polishing.
Using the Universal 150 Plus Polisher (Hwatsing Technology) with a polishing head, perform CMP treatment on 4-inch SiO 2 and Si 3 N 4 wafers separately.The polishing time is 60 seconds, the applied slurry flow rate is 150 mL/min, and the rotation speeds of the wafer/polishing head are 93/87 rpm.Before each polishing experiment, use a rigid polishing pad (DH3002-T80D30-S20M3S1) for at least 180 seconds of conditioning.After polishing, rinse all wafers in DIW for 120 seconds and then dry in nitrogen for 60 seconds.Calculate the material removal rate by measuring the difference in film thickness before and after each CMP experiment.For oxide wafers, the film thickness is measured using an optical interferometer (F50, Filmetrics) at 81 evenly distributed positions across the entire wafer surface.The specific parameters of the used F50 are as follows: the measurement range is from 20 nm to 70 μm; the wavelength range is from 380 nm to 1050 nm; the spot size is standard at 1.5 millimeters, and can be as small as 150 micrometers.The reported material removal rate is the average of at least three wafers, and the standard deviation is calculated based on the relevant data.
Characterize the CeO 2 abrasive particles modified with the selected surfactants using a scanning electron microscope (SU8220, Hitachi) at a voltage of 5 kV.Measure the zeta potential and particle size of the modified slurry using a laser particle size analyzer (Nano-ZS90, Malvern Instrument), where transparent samples are prepared by diluting (3.125 wt.%), subjecting to 5 minutes of ultrasonic treatment (KUDOS), and oscillating at 1600 r/min for 5 minutes in a thermal vortex mixer.For the zeta potential on the wafer surface, use a solid surface zeta analyzer (SurPASSTM 3, Anton Paar).Additionally, perform thermal gravimetric analysis (TGA) of the modified CeO 2 slurry using a platinum crucible and nitrogen gas in the temperature range of 0-750 °C.Measure the rheological properties of the modified CeO 2 slurry using a rotational rheometer (Anton Paar Physica MCR302).Characterize and analyze the solid-liquid interfacial adsorption properties between the modified CeO 2 slurry and SiO 2 /Si 3 N 4 using a quartz crystal microbalance (Q-sense, Sweden Biolin Scientific).Furthermore, map the surface topography of the polished SiO 2 and Si 3 N 4 wafers using an atomic force microscope (Bruker's Dimension ICON), where AFM images are obtained by scanning all wafers at a rate of 1 Hz over a surface area of 5 × 5 μm.

Preparation and selection of CeO 2 slurries with various ionic surfactants
To improve the polishing slurry for STI CMP, various ionic surfactants with different functional groups were introduced in the preparation process of CeO 2 slurry.These surfactants include sodium dodecyl sulfate (C 12 H 25 SO 4 Na, sample A), piperazine (C 4 H 10 N 2 , sample B), piperazine-2-carboxylic acid dihydrochloride (C 5 H 12 C l2 N 2 O 2 , sample C), sodium hexametaphosphate (Na 6 P 6 O 18 , sample D), hydrochloride glycine (C 2 H 5 NO 2 •HCl, sample E), and 2-methylpiperazine (C 5 H 12 N 2 , sample F).The chemical structures of these surfactants are listed in Table 1. Figure 1 shows the sedimentation test of CeO 2 abrasive particles mixed with these ionic surfactants after 0, 1, 3, and 7 days of ball milling.Clearly, the sedimentation rate of CeO 2 abrasive particles is highest for samples C, A, E, D, B, and F in descending order.After 7 days of settling, CeO 2 suspensions containing surfactant B (piperazine) or F (2-methylpiperazine) exhibit the smallest water layer thickness and the best stability compared to suspensions containing other additives.
Figure 2 describes the SEM morphology of CeO 2 abrasive particles in the presence of piperazine and 2-methylpiperazine at pH of 10, without the addition of any pH adjuster.Clearly, without the use of any surfactant, the particle size of CeO 2 abrasive is quite large.When piperazine or 2-methylpiperazine is added to the CeO 2 suspension, similar improvements in particle morphology and size are observed, with corresponding particle sizes of 30-90 nm and 50-120 nm, respectively.At the same time, the distance and gap between each abrasive particle are larger than when no surfactant is used, indicating that piperazine and 2-methylpiperazine can disperse CeO 2 abrasive particles to some extent.

Stability of CeO 2 slurries with piperazine/2-methylpiperazine at different pH values
To further understand the stability of CeO 2 suspension in the presence of piperazine or 2-methylpiperazine, experiments were conducted to investigate the particle size distribution of CeO 2 particles at different pH values, as shown in Figure 3.Both the addition of piperazine and 2-methylpiperazine exhibited similar trends, with the peak width of the CeO 2 particle size distribution being significantly larger in acidic and neutral environments compared to alkaline environments, indicating that the particle state is more stable and uniform at higher pH values.Additionally, the addition of piperazine and 2-methylpiperazine resulted in a bimodal distribution, with the bimodal intensity being higher under neutral conditions.
Figure 4 displays the specific particle sizes of CeO 2 abrasives at different pH values after the addition of piperazine or 2-methylpiperazine.The research results indicate that these two surfactants have similar effects on the particle size of CeO 2 abrasives, with larger particle sizes typically observed under acidic and neutral conditions compared to pH 10 and 12.After the addition of piperazine and 2-methylpiperazine, the maximum particle sizes of CeO 2 abrasives were found to be 1979 nm and 1856 nm at pH 6, respectively, while their minimum particle sizes were 267 nm and 368 nm at pH 10.The variation in suspension stability and particle size at different pH values can be attributed to the diverse dissociation of the applied ionic surfactants.For example, piperazine can dissociate into different species under different pH conditions in an aqueous solution, as shown in Figure 5. Dehydrogenation of piperazine will occur when the pH value exceeds 9.83, resulting in the formation of protonated species.Additionally, piperazine is more prone to deprotonation in alkaline environments.When piperazine is adsorbed onto CeO 2 abrasive particles with negative charges, better dispersion and smaller particle sizes can be achieved.On the other hand, as the pH value of the mixed suspension of piperazine and CeO 2 abrasives starts to decrease with the addition of sulfuric acid,  piperazine reacts with H + and gradually consumes the negative charges on piperazine until reaching pH 6.With further increase in H + levels, it will combine with piperazine and generate additional repulsion due to excessive positive charges, thereby improving the properties of the abrasive particles.achieved (~15 Å/min), which is 80% lower than Si 3 N 4 without surfactant addition.It is worth noting that although the MRR of Si 3 N 4 is satisfactory at several pH values, the addition of piperazine or 2-methylpiperazine inhibits the removal of SiO 2 at all pH conditions, with the MRR of SiO 2 in CeO 2 slurry with piperazine and 2-methylpiperazine being less than 800 and 500 Å/min, respectively.Similar to the removal of Si 3 N 4 , 2-methylpiperazine exhibits a stronger inhibitory effect on the MRR of SiO 2 compared to piperazine.

Effect of CeO 2 slurries with piperazine/2-methylpiperazine on CMP process
Figure 7 illustrates the effect of adding piperazine or 2-methylpiperazine in CeO 2 -based slurry on the selectivity of removal between SiO 2 and Si 3 N 4 at different pH values.Due to the stronger inhibition effect of 2-methylpiperazine, the maximum selectivity ratio between SiO 2 and Si 3 N 4 is achieved when 2-methylpiperazine is added at pH 4, 10, and 12, reaching 16.Although the improvement in the polishing surface quality by these two ionic surfactants seems minimal, they both meet high requirements (Ra <1 nm), with the surface roughness of SiO 2 polished with piperazine and 2-methylpiperazine ranging from 0.22 to 0.32 nm, and that of Si 3 N 4 ranging from 0.11 to 0.19 nm.
The polishing defects of SiO 2 and Si 3 N 4 can be determined by calculating the number and maximum depth of surface scratches.Figure 9 describes the number of scratches on the polished surfaces of SiO 2 and Si 3 N 4 within a 5 × 5 (μm 2 ) area using these two surfactants.Under the shown pH values, the number of scratches on the polished SiO 2 surface using CeO 2 slurry with added piperazine is 13, 7, 10, 6, and 5, respectively, while the number of scratches on the polished SiO 2 surface using slurry with added 2-methylpiperazine is 13, 7, 11, 7, and 8, all of which are smaller than or equal to the number of scratches without any surfactant addition.Furthermore, the results for the polished Si 3 N 4 surface using these two surfactants are also the same, with the minimum number of scratches (~1) achieved when piperazine is added at pH 6 or 2-methylpiperazine is introduced at pH 12.It can be concluded that these two ionic surfactants reduce the number of scratches on the polished SiO 2 and Si 3 N 4 surfaces, and the overall performance of SiO 2 and Si 3 N 4 in alkaline environments is superior to that in acidic environments.
Figures 10 and 11 show the surface morphology of polished SiO 2 using these two solutions.The detailed surface roughness values can be found in Table 2.After polishing with piperazine, the maximum scratch depth on the SiO 2 surface occurs under neutral conditions (900 pm), followed by acidic conditions (700 pm), and then alkaline conditions (500 pm).After polishing with 2-methylpiperazine, the maximum scratch depth on the SiO 2 surface is highest in the acidic region (800 pm), followed by the neutral region (700 pm), and then the alkaline region (450 pm).It is evident that the scratches on the SiO 2 surface produced by polishing in alkaline environments are shallower compared to those polished in neutral or acidic environments.Similarly, Figures 12 and 13 show the surface morphology of polished Si 3 N 4 using these two slurries, with the minimum scratch depth on the Si 3 N 4 surface achieved after polishing in alkaline environments, regardless of the surfactant used.The detailed surface roughness values can be found in Table 3.

Adsorption behavior of piperazine and 2-methylpiperazine on SiO 2 and Si 3 N 4 surfaces
The adsorption behavior of piperazine and 2-methylpiperazine on the surfaces of SiO 2 and Si3N4 was characterized using a quartz crystal microbalance.Figures 14 and 15 show the effects of adding piperazine and 2-methylpiperazine to the CeO 2 polishing slurry on the vibration frequency and surface  where C is a constant related to the properties of quartz crystal (17.7 ng•Hz −1 •cm −2 ), n is the overtone of oscillation (3 for SiO 2 and Si 3 N 4 wafers with 150 mm diameter).The calculated adsorption masses (Δm) of piperazine and 2-methylpiperazine on the SiO 2 surface are 26 ng and 52 ng, respectively.
The adsorption behavior of piperazine and 2-methylpiperazine on Si 3 N 4 wafers was studied, as shown in Figures 16 and 17.It can be concluded that both piperazine and 2-methylpiperazine form rigid and low-viscoelasticity adsorption films on the Si 3 N 4 surface, with adsorption masses of approximately 7 and 9 nanograms, respectively.Additionally, it was observed that the adsorption mass of 2-methylpiperazine on SiO 2 and Si 3 N 4 surfaces is higher than that of piperazine.This is consistent with   the stronger inhibitory effect of 2-methylpiperazine on the removal of SiO 2 and Si 3 N 4 under pH 4 conditions.

Adsorption behavior of piperazine and 2-methylpiperazine on CeO 2 particles
The thermogravimetric analysis results of the CeO 2 polishing slurry with two surfactants are shown in Figure 18.Both mixed slurries reach the maximum weight loss rate before 100 degrees Celsius.
It is known that the boiling points of water, piperazine, and 2-methylpiperazine are 100 degrees Celsius, 148.5 degrees Celsius, and 140 degrees Celsius, respectively.Despite the lower boiling point of 2-methylpiperazine compared to piperazine, piperazine reaches the maximum weight loss rate earlier than 2-methylpiperazine.This suggests that 2-methylpiperazine has a higher    adsorption degree on CeO 2 particles than piperazine, thus explaining why the inhibitory effect of 2-methylpiperazine is stronger than that of piperazine.

Zeta potential analysis of SiO 2 and CeO 2 slurries with piperazine and 2-methylpiperazine
To further investigate the removal mechanism, we conducted zeta potential analysis of SiO 2 surface and CeO 2 slurry with piperazine and 2-methylpiperazine at different pH values, as shown in Figure 19.Throughout the pH range, both the SiO 2 surface and CeO 2 slurry exhibited negative zeta potentials, indicating the presence of electrostatic repulsion between the abrasive particles and the wafer surface.When the pH values were 4, 6, and 7, the absolute zeta potential of CeO 2 increased after the addition of 2-methylpiperazine compared to piperazine.It can be inferred that the surface charge of the particles is larger when 2-methylpiperazine is added, resulting in stronger repulsive forces between the particles and the surface.This further suggests that 2-methylpiperazine has a stronger inhibitory effect on the removal of Si 3 N 4 and SiO 2 compared to piperazine.

Conclusions
In this study, We aim to achieve high-speed, low-defect, highly selective, and contamination-free chemical mechanical polishing (CMP) of the shallow trench isolation (STI) pattern arrays for integrated circuits, a comprehensive investigation was conducted on the STI CMP process by introducing different ionic surfactants into CeO 2 slurry.Several aspects were examined to reveal the polishing and removal mechanisms of ionic surfactants on Si 3 N 4 and SiO 2 .The main conclusions are as follows: (1) Pyrazine and its short-chain derivative (2-methylpyrazine) have a higher probability of binding to CeO 2 particles, thereby limiting precipitation phenomena and improving the dispersion of the polishing slurry.
(2) Piperazine and 2-methylpiperazine have similar effects on improving particle morphology and size, increasing the distance and gap between abrasive particles, thereby dispersing the CeO 2 abrasive particles to some extent.
(3) At different pH values, the stability of the slurry and the changes in particle size can be attributed to the diverse decomposition of the applied ionic surfactants.The peak width of the CeO 2 particle size distribution in piperazine and 2-methylpiperazine is significantly larger at acidic and neutral environments compared to alkaline environments, indicating that the particle state is more stable and uniform at higher pH values.
(4) After polishing with CeO 2 slurries containing piperazine and 2-methylpiperazine, the surfaces of SiO 2 and Si 3 N 4 can meet high-quality surface requirements.Additionally, these two ionic surfactants reduce the number and depth of scratches on the polished SiO 2 and Si3N4 surfaces, with better overall performance in alkaline environments compared to acidic environments.
(5) The inhibitory effect of these two ionic surfactants on the removal of SiO 2 and Si 3 N 4 follows the order of 2-methylpiperazine > piperazine.The main reason is that the adsorption quantity of 2-methylpiperazine on the SiO 2 and Si 3 N 4 surfaces, as well as the absolute zeta potential between CeO 2 particles and the surfaces, are greater than that of piperazine.

Figure
Figure 1.Comparison test of CeO 2 bonded abrasive with different ionic surfactants after ball milling and settling for 0, 1, 3 and 7 days.

Figures 6
Figures 6(a-b) show the influence of the addition of piperazine or 2-methylpiperazine in CeO 2based slurry on the removal rate (MRR) of Si 3 N 4 and SiO 2 at different pH values.It is evident that both modified slurries exhibit significant inhibition on the removal of Si 3 N 4 at pH 4, 10, and 12, and the introduction of piperazine and 2-methylpiperazine can suppress the MRR of Si 3 N 4 to less than 50 Å/min.Furthermore, the inhibition effect of these two ionic surfactants on the removal of Si 3 N 4 follows the order of 2-methylpiperazine > piperazine.Compared to Si 3 N 4 without surfactant addition, the removal rate of Si 3 N 4 is reduced by 53% with the addition of piperazine at pH 4 (~38 Å/min).Additionally, at pH 12, the lowest MRR of Si 3 N 4 in the mixture of 2-methylpiperazine and CeO 2 is

Figure
Figure 3. Particle size distribution of CeO 2 abrasive with the treatment of (a) piperazine and (b) 2-methylpiperazine at different pH conditions.

Figure 5 .
Figure 5. Decomposition species of piperazine at various pH values.

Figure 8
Figure 8 displays the surface roughness of polished SiO 2 and Si 3 N 4 after adding piperazine or 2-methylpiperazine in CeO 2 -based slurry at different pH values.Although the improvement in the polishing surface quality by these two ionic surfactants seems minimal, they both meet high requirements (Ra <1 nm), with the surface roughness of SiO 2 polished with piperazine and 2-methylpiperazine ranging from 0.22 to 0.32 nm, and that of Si 3 N 4 ranging from 0.11 to 0.19 nm.

Figure
Figure 6.Mrrs of (a) Si 3 N 4 and (b) SiO 2 in CeO 2 slurries with the addition of piperazine or 2-methylpiperazine at different pH values.

Figure 9 .
Figure 9. Number of scratches on (a) SiO 2 and (b) Si 3 N 4 surfaces after polishing by CeO 2 slurries mixed with piperazine or 2-methylpiperazine at different pH values.

FigureFigure
Figure 8. Surface roughness of (a) SiO 2 and (b) Si 3 N 4 after polishing by CeO 2 based slurries with the addition of piperazine or 2-methylpiperazine at various pH levels.

Figure
Figure 14.(a) vibration frequency and (b) surface dissipation of SiO 2 surfaces with the treatment of CeO 2 slurries with piperazine at pH 4.

Figure
Figure 16.(a) vibration frequency and (b) surface dissipation of Si 3 N 4 surfaces with the treatment of CeO 2 slurries with piperazine at pH 4.

Figure
Figure 17.(a) vibration frequency and (b) surface dissipation of Si 3 N 4 surfaces with the treatment of CeO 2 slurries with 2-methylpiperazine at pH 4.