Information on PPA, the base polymer of Phoenix 81

Anionically polymerized polyphalaldehyde (PPA), the base polymer for the thermo-structurable resist Phoenix 81, is a white powder which is storage-stable for at least 4 weeks at room temperature. PPA can thus be shipped with standard shipping and does not require the highly cost-intensive refrigerated transport of PPA solutions.
When stored in a refrigerator (8 – 12 °C), the PPA polymer is stable for at least 6 months. For long-term-storage, we recommend temperatures of -18 °C since this ensures that PPA remains stable for at least one year.
The polymer PPA dissolves easily in anisole. By varying the polymer concentration, solutions can be produced flexibly to realize different film thicknesses values. The film thicknesses can furthermore be varied by changing the spin speeds. This is exemplarily shown for AR-P 8100.03 (solids content: 2.5 %) and AR-P 8100.06 (solids content: 5.5%) in the table “Properties I“.

聚苯二甲酸由苯二甲醛經陰離子聚合法聚合而成，為白色粉末狀。可做為熱掃描微影的基礎成份，例如調配成Phoenix 81 (AR-P 8100)可立即使用的阻劑(ready-to-use tSPL resist)。
PPA polymer在室溫下可維持穩定約４週的時間，可以常溫運輸而無需像配製好的溶液需以昂貴的低溫運輸（-18℃)。
PPA polymer的儲存建議為:
8 – 12 ℃ – 可維持穩定至６個月
-18℃ –  可維持穩定至少１年。長期存放建議以此溫度。
PPA polymer可於甲氧苯溶解。以其為溶劑可依需要的厚度調配溶液濃度。厚度範圍可以塗佈轉速進一步微調。
範例可參考第四頁AR-P 8100.03 (固型份 2.5%)及AR-P 8100.06 (固型份 5.5%)的物性表.

PPA溶液濃度與塗佈厚度對應曲線

The following graph shows the relationship between polymer concentration and resulting film thickness

Dependency of film thickness on solids content (anionic PPA in anisole, 4000 rpm, 30 s, open chuck)

Preparation and use of PPA solutions溶液配製與應用

1. To prepare the PPA solutions, the weighed quantity of PPA is given into a suitable vial before the calculated amount of anisole is added. The mixture is stirred or shaken at room temperature until after about 10 minutes a clear, homogeneous solution results.
   先將秤重後的PPA置入適當玻璃瓶後再加入計量的甲氧苯溶液。於室溫下攪伴或搖晃混合物約10分鐘至混合成微清徹均勻的溶液。
2. The PPA solution is transferred into a syringe of appropriate size and filtered using a 0.2 μm syringe filter into a dry and clean bottle.
   將溶液置入類似容量針筒並以規格0.2um過濾至乾淨瓶裝容器

Formulation example for 2.5 % solution 配製2.5%溶液範例
2.5 g PPA + 97.5 g anisole = 100.0 g of 2.5 % 0.2 μm filtrated polymer solution. This corresponds to AR-P 8100.03 with respect to the properties. Please note: 2.5 % is rounded up to 3 % in the product designation.

2.5克 PPA + 97.5克的甲氧苯=100克的2.5%過濾溶液。過濾規格為0.2um。此為與AR-P 8100.03類似特性的溶液。附註: 3%為2.5%四捨五入後的值。

Storge 儲存
PPA solutions are only stable at low temperatures; we thus recommend storage at -18 °C. In this case, solutions are stable for at least 6 months. Prior to use, solution should be warmed to room temperature to avoid condensation of moisture. Short-term use of PPA solutions at room temperature (up to 3 hours) will still ensure the high product quality.
配製好的溶液建議儲存於零下18度的溫度（-18℃)。溶液於此溫度可維持穩定約６個月
使用前需回溫以避免濕氣凝結。
為維持品值穩定，溶液於室溫下建議於３小時之內使用完畢。

PPA solution-AR-P 8100 brief 原廠配製溶液說明

AR-P 8100為原廠配製好的即用型(ready-to-use)熱掃描微影阻劑。優點是無需自行調配。缺點是需在低溫-18℃下運輸,成本較高。在此低溫下儲存，有效期約６個月。目前配製的濃度有:
AR-P 8100.03
AR-P 8100.06

PPA solution Properties 產品物性

PPA solution spin curve 塗佈曲線

Two-layer process 雙層製程參數

Patterning of PPA with e-beam lithographyPPA以電子束微影

PPA layers can also be positively patterned directly by electron irradiation. Similar to the irradiation of commonly used e-beam resists like CSAR 62 or PMMA, electron beam exposure induces fragmentation of the polymer chains.
Polymer fragments resulting from PPA are however unstable and decompose into the volatile ortho-phthalaldehyde. Only very small amounts of monomeric phthalaldehyde are directly released in the device during e-beam exposure; only the subsequent PEB leads to an almost complete thermal development. But even in the range of the dose to clear (approx. 30 – 40 μC/cm²), a resist layer with a thickness of a few nanometers will remain. A residue-free substrate surface can nevertheless be obtained if a short plasma etching step is added.
The gradation passes through a minimum, but with increasing dose, also the concurrent cross-linking processes become increasingly important. This undesirable side reaction is due to radicals which are generated during electron irradiation and stabilize the layer by cross-linking. These effects also occur in PMMA layers, but only at much higher exposure doses, and are here used to produce negative PMMA architectures.
To determine the resolution limits of AR-P 8100, line patterns were examined in detail at the company Raith. Lines of different width were written into the PPA layer. After PEB and subsequent platinum metallization, metal bridges of < 20 nm width were obtained. The highest resolution that could be achieved was 16 nm.

與電子束微影阻劑，例如CSAR 62或是PMMA類似，PPA阻劑的塗層也可直接以電子束曝光。電子束能量會使高分子斷鏈而解離。解離的碎片不穩定而進一步分解成揮發性的鄰苯二甲醛。在電子束曝光下僅有少量的苯二甲醛單體會釋出，後續的曝後烤(PEB)會使其完全分解(熱顯影)。
但是，就算在安全劑量範圍(約30-40uC/cm²)，阻劑層仍會殘留幾奈米的厚度。短時間的電漿蝕刻可清除殘留。
電子束曝光的另一副作用則是存在的自由基產生的架橋反應會影響解析度。類似於PMMA阻劑於高劑量電子束曝光時變成負型阻劑的製程。
為測試PPA解析度能力，Raith公司寫入不同線寬的PPA層，曝後烤後接著沉積白金。可獲得<20nm的線寬。實驗的最佳解析度可達16nm。

Lines written in PPA (resist AR-P 8100)

bridge with width of 16 nm, obtained after sputter coating with platinum (film thickness: 4 nm)

Adding PAGs (photo acid generator) to PPA (sample SX AR-P 8100/5) can increase the sensitivity and allow a better control of the gradation. The exposure causes a release of acid in situ which decomposes the PPA layer at 95 – 100 °C during the following PEB (positive development). The thermally induced, solvent-free development proceeds almost completely. Despite the addition of PAGs, a very thin residual resist layer however remains.
If PAG-containing resists are used together with AR-P 617 in two-layer process, the thin remaining resist layer will not disturb the further process sequence since it is dissolved during the subsequent development. After e-beam exposure and PEB, bottom resist AR-P 617 is selectively developed with developer AR 600-50. The undercut is adjusted specifically by varying the duration of the development step. Reliably processable lift-off resist architectures can thus be produced. This method allows the realization of metal bridges (platinum):
The process window is however quite narrow; already small variations of the dose affect the obtained line width considerably.
實驗樣品（SX AR-P 8100/5)是在PPA溶液中添加光酸產生劑（PAG），以增加其敏感度可有較好的階度控制。機制是曝光時產生的光酸促使PPA層在隨後的95 – 100℃曝後烤時分解。這種無溶劑的熱顯影方式還是會有極薄的殘留層。（左下圖階度曲線）。
如果與AR-P 617應用於雙層製程則無殘留層問題。殘留層會在AR-P 617電子束曝光，曝後烤及以AR 600-50顯影時完全溶解。下切結構可以顯影時間控制。右下圖白金金屬橋結構為範例。只是製程寬容度不高，曝光劑量稍為變動就會影響線寬。

Gradation of SX AR-P 8100/5 after PEB at 98 °C

Platinum bridges width:58nm

Platinum bridges width:70nm

Patterning of PPA with photolithographyPPA以UV波長曝光

PPA layers can also be structured directly by means of photolithography. Irradiation with UV-light of a wavelength of < 300 nm results in a cleavage of the polymer chains to form volatile components.
By adding PAGs (photo acid generators), the photosensitivity can be significantly increased. The exposure releases acid in situ which then decomposes the PPA layer at 95 – 100 °C during the subsequent PEB (positive development).
The thermally induced, solvent-free development step proceeds almost completely. Cross-linking processes which are also induced by UV-exposure may however cause a thin, only a few nanometres thick residual resist layer. A residue-free substrate surface is obtained after addition of a short plasma etching step.
PPA層亦可以波<300nm的紫外線曝光，此波段會導致PPA斷鏈而形成揮發成份。
PPA溶液中加入光酸產生劑（PAG）會明顯的增加感光性。曝光產生的光酸會促使PPA層在95 – 100℃的曝後烤中分解。此為無溶劑熱顯影製程。
在曝光中,亦有可能產生架橋反應形成幾奈米厚的殘留，以電漿蝕刻可去除殘留層。

Structuring with laser (pulse)PPA以雷射直寫

PPA layers can also be structured by laser ablation. Substrates coated with AR-P 8100 were patterned at the IOM Leipzig with pulsed laser light at different wavelengths. This enabled the realization of architectures with very low edge roughness. In the absorption range of PPA (at 248 nm), complete ablation was achieved without damage of the silicon substrate.
PPA層亦可以雷射直寫。在德國的萊布尼茲表面工程研究所以不同波長的脈衝雷射直寫塗佈AR-P 8100的基板，驗證雷射直寫的圖案邊緣粗燥度低。下圖為以248nm直寫的圖案

Even though PPA shows only a very low absorption at a wavelength of 355 nm, a selective ablation with comparatively high sensitivity is nevertheless possible. The structures realised here are again characterized by very smooth edges.
PPA層對波長355nm的吸收較低，選擇性的直寫仍可有不錯的敏感度。如下圖範例仍可獲得平滑的邊緣。

0.1 J/cm2, 355 nm ps-laser, single-pulse exposure, 700 nm PPA on Si-wafer

The laser beam can also be used to generate 3D structures. Interference projection through a phase mask allows the production of lattice structures with sinusoidal shape and very low surface roughness..
透過干涉投影及相轉移光罩的機構，雷射直寫可以用來產生立體結構。下圖為說明及鎖產生的格子圖案。

Experimental setup of interference projection

SEM-image of PPA lattice with sinusoidal progression (period ~750 nm);
248 nm, 20 ns pulses, number of pulses: 10; 700 nm PPA on Si-wafer