What is a Software Engineer at Celestial AI?

At Celestial AI, the role of a Software Engineer is inextricably linked to the physical reality of our hardware. We are not a traditional SaaS company; we are architecting the Photonic Fabric™, a technology that fundamentally breaks the "Memory Wall" in accelerated computing. As a Software Engineer here, you are likely working in domains such as Verification (UVM), CAD/EDA Automation, or SoC Integration. You are building the critical software infrastructure that validates, designs, and enables our optical interconnect technology to function at the tens of terabits per second required by next-generation AI.

Your work directly impacts how hyperscalers and data centers manage the massive bandwidth and latency demands of Generative AI. Whether you are developing Python scripts to automate complex analog/mixed-signal (AMS) design flows, writing SystemVerilog to verify our optical chiplets, or architecting the firmware that manages data traffic across our Optical Multi-chip Interconnect Bridges (OMIB), your code ensures that our hardware is manufacturable, reliable, and performant. You are the bridge between theoretical physics and mass-production silicon.

Getting Ready for Your Interviews

Preparing for an interview at Celestial AI requires a shift in mindset from pure software to hardware-software co-design. You will be evaluated not just on your coding ability, but on your understanding of the underlying engineering constraints.

Technical Proficiency & Domain Knowledge Interviews here are technically rigorous. Depending on your specific focus (Verification, CAD, or Design), you must demonstrate deep expertise in your toolset—whether that is SystemVerilog/UVM for verification, Python/Tcl/SKILL for EDA automation, or C++ for system modeling. You must show that you understand the semiconductor design lifecycle, from RTL to GDSII.

Problem-Solving in Constrained Environments We value engineers who can solve problems where the constraints are physical—thermal limits, signal integrity, and manufacturing yield. You will be asked to solve scenarios where "perfect" code is not enough; the solution must be efficient, scalable across massive clusters, and robust against the realities of silicon manufacturing.

Cross-Functional Collaboration Our technology relies on heterogeneous integration—combining photonics, electronics, and advanced packaging. You will be evaluated on your ability to communicate complex software concepts to analog designers, packaging engineers, and physicists. You must demonstrate that you can work effectively in a multidisciplinary environment where your software is a key enabler for other engineering teams.

Interview Process Overview

The interview process at Celestial AI is designed to assess technical depth and your ability to thrive in a fast-paced, Series B deep-tech startup environment. The process typically begins with a recruiter screen to align on your background and interest in the semiconductor/AI space. This is followed by a technical phone screen, usually with a hiring manager or a senior engineer, focusing on the specific methodologies relevant to the role (e.g., UVM concepts for verification, scripting for CAD).

If you pass the initial screens, you will move to the onsite stage (often conducted virtually or in-person at our Santa Clara or Irvine offices). This stage consists of a panel of interviews covering technical domain knowledge, problem-solving, and behavioral fit. You should expect deep-dive questions regarding your past projects, specifically how you handled technical roadblocks in chip design or verification. The team looks for hands-on experience; they want to know how you implemented a solution, not just the theory behind it.

This timeline reflects a standard engagement for engineering roles. Note that for specialized positions like UVM Verification or CAD Engineering, the "Technical Assessment" phase often involves detailed discussions on specific industry-standard tools (like Cadence Virtuoso or Synopsys tools) rather than generic algorithmic coding tests.

Deep Dive into Evaluation Areas

Based on the technical demands of our open roles, you must be prepared to demonstrate expertise in the following areas.

Verification Methodologies (UVM & SystemVerilog)

For verification-focused software roles, this is the core evaluation area. You must demonstrate mastery of Universal Verification Methodology (UVM). We do not just look for test execution; we look for the ability to architect robust testbenches from scratch.

Be ready to go over:

• Testbench Architecture – Building UVM environments, agents, drivers, monitors, and scoreboards.
• Constrained Random Verification – Writing effective constraints to hit corner cases in complex SoCs.
• Coverage Closure – Strategies for achieving 100% functional and code coverage.
• Advanced concepts – Register abstraction layer (RAL), TLM transactions, and UVM phases.

Example questions or scenarios:

• "How would you architect a UVM testbench for a high-speed SerDes interface?"
• "Describe a difficult bug you found using constrained random stimulus and how you debugged it."
• "How do you handle reuse of verification components from block level to SoC level?"

EDA Automation & Scripting

For CAD and infrastructure roles, your ability to automate design flows is critical. We rely heavily on Python, Tcl, and SKILL to glue together various tools in the ASIC flow.

Be ready to go over:

• Flow Automation – Automating tasks in tools like Cadence Virtuoso, Innovus, or PrimeTime.
• API Interaction – Interfacing with EDA tool APIs to extract data or drive simulations.
• Infrastructure Management – Managing compute resources (e.g., SLURM clusters) and license servers (FlexLM).
• Version Control – Managing design data with Git or IC Manage (gdpxl).

Example questions or scenarios:

• "Write a Python script to parse a large log file from a synthesis run and extract timing violations."
• "How would you automate the regression testing flow for a mixed-signal design?"
• "Explain how you would troubleshoot a license checkout failure in a distributed compute environment."

Semiconductor Design Flow Knowledge

Even if your role is purely software, you cannot treat the hardware as a black box. You need to understand the ASIC design flow to be effective.

Be ready to go over:

• RTL to GDSII – Understanding the steps: Synthesis, Floorplanning, Place & Route, CTS, and Signoff.
• Timing Analysis – Concepts of Setup/Hold time, Static Timing Analysis (STA), and timing closure.
• Physical Constraints – Understanding power, area, and thermal constraints in 2.5D/3D packaging.

Example questions or scenarios:

• "Explain the difference between a functional ECO and a timing ECO."
• "How does your code account for process variations in deep technology nodes (like 5nm)?"

Key Responsibilities

As a Software Engineer at Celestial AI, your daily work is centered on enabling the creation of the Photonic Fabric. You will often be responsible for developing and maintaining the verification infrastructure, ensuring that our complex SoCs and optical bridges are functionally correct before they go to fabrication. This involves writing extensive SystemVerilog code, creating test plans, and reviewing coverage reports to sign off on designs.

In CAD or infrastructure roles, you will own the design environment. This means you are the architect of the workflows that Analog and Digital designers use every day. You will integrate Process Design Kits (PDKs) from foundries like TSMC, write scripts to automate repetitive layout tasks, and ensure that our compute clusters are optimized for massive simulation loads. You are also responsible for data integrity, managing the version control systems that house our most valuable IP.

Collaboration is a daily necessity. You will work closely with packaging engineers to understand the physical constraints of 2.5D integration and with architects to define the requirements for the next generation of interconnects. You are expected to be proactive in identifying bottlenecks in the design flow and proposing software-driven solutions to speed up our time-to-market.

Role Requirements & Qualifications

Successful candidates at Celestial AI typically possess a strong blend of software engineering skills and semiconductor industry experience.

• Technical Skills

  • Must-have: Proficiency in Python or Tcl for scripting and automation.
  • Must-have (for Verification): Expert-level knowledge of SystemVerilog and UVM.
  • Must-have (for CAD): Experience with EDA tools from Cadence (Virtuoso, Innovus, Xcelium) or Synopsys.
  • Nice-to-have: Experience with C++ for modeling, knowledge of CI/CD pipelines, and familiarity with job schedulers like SLURM.

• Experience Level

  • We generally look for 5-10+ years of relevant industry experience. Deep familiarity with advanced nodes (5nm/4nm) and complex heterogeneous integration is highly valued.
  • A Bachelor’s degree in EE, CS, or Computer Engineering is required, though a Master’s or PhD is preferred for many of our specialized roles.

• Soft Skills

  • Strong analytical and debugging skills are non-negotiable.
  • Ability to work in a "startup mode"—high ownership, ability to navigate ambiguity, and a drive to deliver results without hand-holding.

Common Interview Questions

The following questions are representative of the technical depth expected in our interviews. They are designed to test your practical knowledge of the tools and methodologies used in modern semiconductor design.

Verification & UVM

• "Explain the UVM phasing mechanism. How do you synchronize sequences with the DUT reset?"
• "How would you model a scoreboard for a protocol that supports out-of-order transactions?"
• "What is the difference between new() and create() in UVM, and why does it matter?"
• "How do you approach functional coverage definition to ensure you aren't over-covering or under-covering?"
• "Write a SystemVerilog constraint to generate a random address that is 64-bit aligned and within a specific memory range."

Scripting & Automation (Python/Tcl)

• "Given a directory of thousands of log files, write a script to identify which tests failed and categorize the error types."
• "How would you use Python to interact with a Cadence Virtuoso database to extract netlist information?"
• "Describe how you would implement a check to ensure that all design files in a release match the version control tag."
• "How do you handle exception handling in Tcl when automating a critical synthesis flow?"

Domain & System Knowledge

• "How does a Network-on-Chip (NoC) interconnect differ from a standard bus architecture?"
• "What are the specific verification challenges associated with 2.5D packaging compared to monolithic SoCs?"
• "Explain the concept of 'Clock Domain Crossing' (CDC) and how you verify it."
• "In a high-speed SerDes design, what are the critical parameters you need to track during simulation?"

These questions are based on real interview experiences from candidates who interviewed at this company. You can practice answering them interactively on Dataford to better prepare for your interview.

Frequently Asked Questions

Q: Do I need a background in photonics to apply? While familiarity with photonics is a plus, it is not a strict requirement for most software/verification roles. However, you must be willing to learn the system-level concepts of our Photonic Fabric to understand what you are building and testing.

Q: What is the work culture like at Celestial AI? We are an early-stage but rapidly growing startup. The culture is collaborative, fast-paced, and technically demanding. We value "smart and dedicated people" who are self-starters. Expect a high-bandwidth environment where you have significant autonomy and responsibility.

Q: Is this a remote role?

Due to the hardware-centric nature of our work, enabling close collaboration with design and lab teams, our engineering roles are predominantly onsite in Santa Clara, CA, or Irvine, CA.

Q: What is the typical interview timeline? The process can move quickly for strong candidates. From the initial recruiter screen to the final offer, the timeline can be as short as 2-3 weeks, depending on scheduling alignment.

Q: How does Celestial AI handle compensation? We offer a competitive total compensation package that includes base salary, a bonus, and significant early-stage equity. We view every hire as a partner in our long-term success.

Other General Tips

Know the "Memory Wall": Before your interview, ensure you understand the industry problem we are solving. Read up on why data center infrastructure is shifting from Systems-on-Chip (SoCs) to Systems of Chips, and how optical interconnects address bandwidth and latency bottlenecks.

Brush up on Industry Standards: Depending on your role, be fluent in the standards relevant to our work, such as PCIe, CXL, UCIe, or IEEE 1687 (IJTAG). Mentioning your experience with these specific protocols shows you can hit the ground running.

Prepare for Whiteboard Scripting: You may be asked to write pseudocode or actual script logic on a whiteboard (or virtual equivalent). Don't just memorize syntax; focus on showing how you structure your code for maintainability and error handling, which is crucial for our internal tools.

Summary & Next Steps

Joining Celestial AI as a Software Engineer means positioning yourself at the forefront of the Generative AI hardware revolution. You are not just writing code; you are enabling the Photonic Fabric™, a technology that is critical for the future of high-performance computing. The work is challenging, requiring a deep understanding of both software methodologies and silicon constraints, but the impact of your work will be felt across the entire data center industry.

To succeed, focus your preparation on your core technical domain—whether that is UVM verification, EDA automation, or SoC integration. Be ready to discuss how you solve complex, constrained problems and how you collaborate in a multidisciplinary hardware environment. We are looking for engineers who are ready to own their domain and drive innovation.

The salary data above reflects the base salary ranges for engineering roles in our California locations. Actual offers will vary based on your experience, interview performance, and the specific scope of the role. In addition to base salary, our compensation package includes bonuses and generous early-stage equity, aligning your success with the company's growth.

For further insights and to track new interview experiences, visit Dataford. Good luck with your preparation—we look forward to seeing what you can build with us.